8th Solar & 17th Wind Integration Workshop // E-Mobility Integration Symposium 2018 (SIW / WIW / E-Mobility 2018 // Stockholm, Sweden)
Stockholm / Sweden, 15 to 19 October 2018

Wind Integration Workshop 2018

Modeling the arrangement of turbines for onshore wind power plants under varying wind conditions
Submission-ID 004
Maja Celeska 1, Krste Najdenkoski 1, Vladimir Dimchev 1, Vlatko Stoilkov 1, Lothar Fickert 2, Robert Schuerhuber 2
1 Ss. Cyril and Methodius University in Skopje, Faculty of Electrical Engineering and Information Technologies, Macedonia
2 Institute of Electrical Power Systems Technical University of Graz, Austria, Austria
Optimization of wind farm layout concerning various parameters is a major point in planning and will influence the revenue for the whole life of the installation. Besides the obvious impact of wind distribution also other parameters like connection costs and levelized costs of energy influence the optimum layout and have to be included in a realistic optimization algorithm. In this paper the sophisticated optimization of wind farm layout with of two fundamentally different heuristic algorithms is investigated. To do so, detailed real-world data from an existing wind farm in Bogdanci, Macedonia is utilized by employing real wind farm data we are able to calibrate model adequacy and ascertain a model that will serve as a referent guidance in the planning of future onshore wind farms. The major unique feature of the research is the simultaneous optimization taking into account all major technical influence and cost factors, including: (i) detailed and advanced models for power modeling due to bivariate distribution of wind speed and direction; (ii) accurate estimation of levelized cost of energy (LCOE); (iii) analysis of the shortest electrical interconnections among wind turbines and (iv) correction of hub height on each wind turbine in the wind farm with taking also the wake effect into consideration. Different layouts were designed using sophisticated algorithms for handling the resulting high-dimensional, highly non-linear optimization problem. In particular, a non-dominated sorting genetic algorithm (NSGA) and a mixed‑discrete particle swarm optimization algorithm (MD-PSO) were applied. Both optimization algorithms established bi-objective fitness functions, in particular- minimizing the levelized cost of energy and maximizing the capacity factor. By comparing the results obtained with the existing layout, it is established that both optimization algorithms are adequate in determination of wind power plant layouts. Results show also a remarkable improvement of 4.6 % and 23.4 % for levelized costs and capacity factor, respectively, compared to the as built wind farm layout. So, it is proven that the implementation of sophisticated optimization methods can results in essential savings during the whole lifetime of the wind farm.

Keywords—Onshore wind power plant layout optimization, Non-dominated Genetic Algorithm, Mixed-discrete Particle Swarm, Bogdanci wind farm

Impact of Inertia Emulation on Damping of Inter-Area Power Oscillations during Under-Frequency Events in the Nordic Power System
Submission-ID 005
Tuire Kujansuu 1, Antti Harjula 1, Richard Ogiewa 2, Yuwa Chompoobutrgool 3
1 Fingrid Oyj, Finland
2 ENERCON GmbH, Sweden
3 ENERCON GmbH, Germany
This paper reviews the impact of Inertia Emulation control provided by converter-connected wind power plants on the damping of inter-area power oscillations during under-frequency events in the Nordic power system. For this purpose the paper summarises, the Inertia Emulation function of Type 4 wind power plants and the current state of the Nordic power system. Based on a future scenario a model of the Nordic power system is set up, containing a certain amount of wind power plants with Inertia Emulation function. Then, the effect of Inertia Emulation on the course of the frequency is reviewed for this future scenario. The presented simulations examine the effect of the delay between the frequency measurement and Inertia Emulation control response on the damping of the power and frequency oscillations. This issue is especially important to consider when using Inertia Emulation function with a power increase dependent on the frequency measurement. Namely, with a power increase dependent on the measured frequency and with a reasonable delay, Inertia Emulation control either does not affect the oscillations or can even dampen the oscillations during the under-frequency event. This leads to the conclusion that the delay between the frequency measurement of the Inertia Emulation function and the actual active power response is crucial to the damping of the inter-area power oscillations but also to the Inertia Emulation performance to support the power system.

Japan's R&D Project of Ramp Forecasting Technology: Deterministic Forecast with Post-processing Using Real-time Monitoring Data
Submission-ID 006
Shinji Kadokura, Daisuke Nohara, Masamichi Ohba, Atsushi Hashimoto, Keisuke Nakao, Yasuo Hattori, Takeshi Watanabe, Hiromaru Hirakuchi
Central Research Institute of Electric Power Industry, Japan
Deployment of power generation by renewable energy such as photovoltaics (PV) and wind turbine (WT) is progressing worldwide. Since PV and WT power generation is markedly fluctuated due to the change of the weather, there is concern about the stability of the electric power system.

Therefore, we have been developing a prediction method of wind power generation output to contribute for the stabilization of electric power system. In this method, the numerical forecast value provided operationally by the Japan Meteorological Agency is downscaled using the Weather Research and Forecasting model (WRF) and CFD model to predict the wind speed at the wind turbine, and the wind power output is projected using the power curve. As the summation of the wind power output projected for each wind turbine, the total output of the area is estimated.

In this study, we have improved the prediction of wind speed and power generation output by using the monitoring data for each wind turbine, by correction based on multiple regression and by using empirical power curve, which are derived by the relationship between the wind speed and wind power output observed at each wind turbine. By this improvement, the prediction error of the wind power generation output area total value is reduced by about 30%.

Demonstration of Low Voltage IGBT-based active power factor correction on Medium Voltage
Submission-ID 010
Dan Liljengren
Comsys AB, Sweden
Comsys AB and Ellevio (ex Fortum). The Kville wind park had specific requirement on reactive power and supply power to the 130 kV grid in western Sweden. A 2,5 MVA Comsys STATCOM solution based on liquid cooled low voltage active filters and step-up transformer was implemented. The projects requirements and technical solution is presented as well as the results. The results are discussed as well as specific challenges the project team met during the implementation. Furthermore a discussion regarding potential additional values this type of technology offer in terms of compensating other power quality issues as well.

Short term forecasting of wind turbine production whith Machine Learning methods : direct approach and integrated approach.
Submission-ID 011
Mamadou Dione 1, Eric Matzner-Lober 2, Philippe Alexandre 3
1 Engie Green France- CREST (ENSAE ParisTech), France
2 CREST (ENSAE ParisTech), France
3 Engie Green France, France
In this paper, we compared the bi-variate distributions of the component pairs (U, V) of the ECMWF and GFS model wind forecasts taking as a reference the measurements of a pylon on a wind farm. Then, we applied some machine learning algorithms for the short-term forecast (24h to 48h) of wind generation. We do two approaches: a direct approach that predicts wind generation directly from weather data, and an integrated approach that estimates wind from weather data and converts it into wind power by power curves. The models tested are random forests, CART + Bagging, CART + Boosting, SVM (Support Vector Machine). The application is made on a wind farm of 22MW (11 wind turbines) of the La Compagnie du Vent (that became Engie Green France). Our results are very conclusive compared to the literature.

Passive houses as power and heat storage in the smart grid
Submission-ID 013
Axel Bretzke, Richard Höfer
Univ. of appl. Sc. Biberach VDI, Germany
Due to the expansion of wind and solar power, there are major fluctuations in power generation. In order to make better use of this electricity, the consumption must be adapted to the generation of electricity. For this purpose, it is possible to use the thermal storage mass of massive, well-insulated buildings. For this purpose, the building is, in excess of electricity before a foreseeable winter low, e.g. heated by a heat pump to over 20 degrees, in the context of individual comfort. This energy is stored due to the insulation over several days in the building and lost slowly. Therefore there is in the following period of power shortage and despite lack of solar radiation no need for heating. For this purpose, the heating demand was simulated on the basis of real weather data and the reheating demand was assessed.


The most critical weather with the highest heating demand is the winter low, since the outside temperatures are only moderately low but there are hardly any solar and wind gains. If the heating days are considered, it should be noted that, due to the solar radiation, even in winter often occur days without heating demand. For days without solar radiation but with wind surplus (wind D> 23 GW and electricity export), the building can be heated with regenerative electricity to 2-3 Kelvin over 20 degrees, if subsequently a winter low with covered and wind poor weather is announced. Then, the building only needs heating after 4-5 days again by the heat pump. The building therefore acts as heat and electricity storage for the network for a period of 4-5 days. Overall, the heating effort with this driving with 407 kWh / a power consumption is slightly higher than at even 20 degrees. Depending on the need for comfort, the building can so be heated to almost 100% with regenerative electricity from surplus wind (wind D> 23 GW and electricity export) and passively with the sun. The demand for non-renewable electricity in the example is very low at 73 kWh / a or 0.34 kWh / a m². This also makes it possible to relieve the local networks by passive house buildings and their storage options, which could be used differently within the scope of smart meters and corresponding electricity price offers, depending on the users' need for comfort. In-depth studies on the possibilities based on nationwide and local generation and grid situations are underway or completed by various parties (including ibp, network-reactive buildings). The inclusion of a building's own PV would also make sense. Conclusion: It is possible to use the heat storage mass of buildings with PH standard as variable power consumers to permanently relieve the power grid.

A Fault Handling Current Control Strategy for Offshore Wind Power Plants with Diode Rectifier HVDC Transmission
Submission-ID 016
Christian Neumann 1, Hans-Günter Eckel 1, Sven Achenbach 2
1 University of Rostock Institute of Electrical Power Engineering, Germany
2 Siemens AG Energy Management Division, Germany
This paper presents an extension of the Fixed Reference Frame (FixReF) current control strategy for wind turbine converters in diode rectifier based HVDC links. By adding a negative sequence FixReF current control loop, the line-side converters can cope with symmetrical as well as asymmetrical faults within the offshore AC grid. The implemented fault ride through (FRT) control strategy, based on this decoupled double FixReF current control scheme, is capable of low voltage and high voltage FRT which is validated at different operation points using PSCADTM/EMTDCTM simulations. Furthermore, the paper illustrates a possible solution for the auxiliary power supply during calm wind situations utilizing the existing DC cable together with a paralleled medium-voltage modular multilevel converter (MMC). The mentioned FRT strategy is used for the transition between HVDC power export and MVDC auxiliary power supply mode.

Keywords-HVDC; rectifier; wind power plant; FixReF; grid-forming control; FRT; asymmetrical fault; auxiliary power

An Impedance-Based Active Filter for Harmonic Damping by Type-IV Wind Turbines
Submission-ID 020
Emerson Guest 1, 2, Tonny Rasmussen 2, Kim Jensen 1
1 Siemens Gamesa Renewable Energy, Denmark
2 Technical University of Denmark, Denmark
In this paper, an impedance-based active filter is developed for the three-phase grid-connected converter used in a Type-IV wind turbine. The active filter utilizes the voltage and current measurement to provide programmable narrowband shaping of the converter impedance. This has application in mitigating voltage or current amplification in wind power plants by increasing the harmonic damping applied by the wind turbines. Moreover, the active filter inherently compensates converter generated voltage harmonics, ensuring the voltage quality of the wind turbine. The performance of the active filter is verified through experiments on a 0.6MW grid-connected converter.

The effect of large scale renewable projects integration into Jordanian power system in 2025
Submission-ID 021
Ahmad abu dyak, Ahmad Harb
Transmission planning engineer at NEPCO National electric power company, Jordan
Abstract The Hashemite Kingdom of Jordan has globally one of the highest dependency on the foreign energy sources with an import ratio of around 92 %. Furthermore, energy demand in Jordan is growing rapidly with a percentage that exceeds 6% due to different reasons such as high growth in population, hosting large number of refugees from neighboring countries [1]. Jordan faces considerable challenges inconsistently and securely covering the demand for electricity. This is due to, for example, limited locally available fossil fuel resources, insufficient available conversion capacities and the financial weakness of the energy entities. One option for a secure and climatically sound improved electricity generation system is the increased use of renewable energy sources. This is especially possible in Jordan, due to the availability of favorable solar and wind resources. This paper aims at studying the effect of integrating large scale renewable energy projects on the stability behavior of Jordanian transmission grid in the current year 2017 scenario and compare it with the grid situation in 2025. The Power analysis program Digital Simulation and Electrical Network Calculation Program, DIGSILENT, along with the current and planned system model of the Network will be used, the proposed renewable and conventional projects that are planned to be connected up to the year 2025 [2]. It is found that for the studied year 2017 the grid will be stable in the studied cases, while the system will face unstable situations in simulation of the National grid in year 2025 load especially in loss of wind events.

Analysis of Harmonic Resonance Stability in Power System with Renewable Generations
Submission-ID 026
Lijun Cai 1, Ulas Karaagac 2, Jean Mahseredjian 3, Hans-Günter Eckel 1, Harald Weber 1
1 University of Rostock, Germany
2 Hong Kong Polytechnic University, Hong Kong
3 Polytechnique Montréal, Canada
Key words: renewable generation; harmonic impedance; subsynchronous resonance; supersynchronous resonance; harmonic resonance stability.


With the development of renewable energy in the power systems, converters that interface with renewable generations are becoming new potential sources of harmonics. Harmonics may lead to problems such as damage of capacitors due to overheat, increased mechanical vibration of inductors, wrong trig of power system controllers and unintended shutdown of renewable generations.

Moreover, the renewable generations are increasingly connected to the distribution weak systems and voltage source converter (VSC) based HVDC systems, where the whole system harmonic resonance behaviors should be considered in detail. Therefore, this paper concerns harmonic resonance stability analysis in power system with renewable generations.

First, an overview of harmonic impedance modeling and harmonic resonance analysis will be given.

Then harmonic impedance of both doubly fed induction generator (DFIG) and full converter (FC) based wind turbines are analyzed in detail: the impedance scan of DFIG and FC wind turbines are performed, where the detailed generator model, rotor side converter (RSC) control, grid side converter (GSC) control and filters are considered.

Concerning the harmonic impedances of wind turbines and power systems, traditional resonance stability (Nyquist) criteria will be improved by concerning the harmonic resistance (damping) and harmonic reactance separately.

Both subsynchronous and supersynchronous resonance scenarios between wind farm and compensated power system are simulated to verify the proposed modified harmonic stability criteria.

Finally, measures for damping the harmonic resonances are given.

Comparison of Connection Policies for the Planning of Generation Integration into a Distribution Network
Submission-ID 027
Anthony Chabrol, John Wallace, Conor Molloy
ESB Networks, Ireland
Comparison of Connection Policies for the Planning of Generation Integration into a Distribution Network

Over the past two decades the Irish distribution system has experienced large-scale integration of renewable generators, such as Wind Farms and small scale synchronous generators. This has posed a challenge for distribution system operator (DSO) planners, with the distribution network originally designed generally as a one-way feed to meet load downstream, this is no longer the case.

The role of the planner when assessing proposed generator connections, such as wind or solar, is to ensure that the connections can be made at least cost whilst keeping the system reliable and capable of operating within the SO limits. In order to connect these generators in a timely fashion, a process must be used. There are two different approaches that are used. This paper will compare and contrast these two different methods, allowing for others to determine which method would work best on their system if required.

Understanding the Role of Short-term Energy Storage and Large Motor Loads for Active Power Controls by Wind Power
Submission-ID 032
Vahan Gevorgian
NREL, United States
Many studies identified the importance of active power control (APC) and other service by wind power at higher levels of penetration for reliable grid operation. Several ancillary service market design efforts are under way in different areas of the U.S and abroad. During the past decade there has been an increasing level of interest from both the industry and academia in researching wind power’s provision of Active Power Control (APC). Numerous wind turbine manufacturers have developed their own versions of APC functionality that de-rate the turbine or wind plant to specified power set-points and include provisions for inertial emulation, primary frequency control (PFC), and wind power participation in automatic generation control (AGC). Short-term power storage (at the single turbine, plant or system levels) can directly support these capabilities, and also provide for power variability smoothing, which in turn can also be used to smooth transient power fluctuations and thereby reduce wind turbine drivetrain loads. Co-optimizing wind plant control to meet both power system reliability and turbine system reliability (e.g., drivetrain loads) is a critical need for understanding grid system LCOE reductions for wind plants. Short-term storage for wind power can be realized by different technologies including electrochemical lithium-ion batteries, mechanical flywheel systems, etc. In this work we developped controls and demonstrated them at real multi-MW scale for using various short-term energy storage systems (Li-ion batteries, mechanical inertia stored in multi-MW undustrial motor drives). In particular, we show unique results on how the inertial response by wind power can be enhanced by these technologies. In addtion, we demonstrate the benefits of short-term energys storage for other active power controls by wind power such as primary frequency response (PFR), paricipation in autoratic generation control (AGC), and wide-area reliability services.

Validation and Assessment of the High Definition Modular Multilevel Converter for Offshore Wind Turbines and Other Medium Voltage Applications
Submission-ID 035
Michael Smailes 1, Raymundo Torres-Olguin 2, Chong Ng 1, Angel Perez 3, Jose Luis Dominguez 4
1 Offshore Renewable Energy Catapult, United Kingdom
2 SINTEF, Norway
3 Tecnalia, Spain
4 IREC, Spain
The Conventional Modular Multilevel Converter (MMC) is well suited to offshore wind power applications due to its modularity, efficiency and low harmonic generation. The converter is constructed from a stack of identical cells, each capable of creating one voltage level. Hence, with many modules, the AC waveform has a low Total Harmonic Distortion (THD).

While a low THD can be achieved with the MMC, many modules are required, limiting its use to low voltage applications. This paper presents an experimental validation under realistic electrical conditions, of a novel control methodology that significantly reduces the THD generated by a given MMC design without adding additional modules. Therefore, the MMC can be used in lower voltage applications such as within wind turbines without sacrificing the generated THD.

This represents the latest step in the continued development of a Hybrid HVDC Transformer. Located within each wind turbine’s tower, the Hybrid HVDC Transformer connects each turbine directly to an HVDC grid. The offshore HVDC substation can therefore be eliminated/significantly reduced and hence lower the wind plant’s capital cost by around 15 %.

This paper presents the results of the experimental performance assessment of four HD-MMC configurations based on the THD produced, efficiency and their dynamic stability. Furthermore, a weighting factor into the cell balancing algorithm is introduced. This weighting factor permits the priority the balancing algorithm places, on either reducing SM voltage ripple or switching frequency to be dynamically altered, to maximise efficiency.

A three-phase, 18-cell, 600 V MMC was used to evaluate the four HD-MMC configurations over a range of weighting factors (based on the THD produced, efficiency and dynamic stability). The results were compared to those of a MMC using both Nearest Level Modulation (NLM) and Pulse Width Modulation (PWM). A Grid Emulation System (GES) generated the AC and DC buses for each converter.

The static performance of the converter configurations with each weighting factor were tested. Testing was carried out using a stable AC voltage and the cell voltages, number of switching events and THD were recorded. The dynamic responses to step input changes in voltage, frequency and power were also tested. Time to reach the new equilibrium and converter stability were also recorded.

The results of these tests demonstrate the HD-MMC algorithm to be an attractive alternative to PWM for MMCs in Medium and Low Voltage applications such as for the next generation offshore wind turbine. The number of switching operations and hence losses was significantly reduced while the THD was also reduced compared to the NLM MMC. Furthermore, the HD-MMC performed well to step changes in frequency, voltage and power requirements, matching or improving on the MMC’s response.

Modelling of Large Size Electrolyser for Electrical Grid Stability Studies - A Hierarchical Control Approach
Submission-ID 037
P.K.S Ayivor 1, J.L. Rueda Torres 1, M.A.M.M. van der Meijden 1, 2
1 Delft University of Technology, Netherlands
2 TenneT TSO B.V, Netherlands
This paper proposes a control approach which extends the capabilities of large scale commercial electrolysers (>1MW) to support power system stability. The search for new sources of ancillary services and the projected demand for hydrogen as a medium of energy storage has aroused considerable interest in the use of large scale electrolysers for power system ancillary services. In order to fully leverage the potential of large scale electrolysers to support power system stability, effective control schemes are needed. To illustrate the feasibility of the proposed approach, case studies are built on a model of a 1 MW plant augmented with a high level controller. Simulations are performed using the Real-Time Digital Simulator (RTDS) platform, to investigate the response of the high level control scheme to external command signals and power system disturbances. The results show that the proposed control scheme can enable an optimal operation of the electrolyser load on the basis of market and power system conditions.

Global Geospatial Optimization of the Locations of Wind Farms and the Configuration of Transmission Networks
Submission-ID 040
Kazuaki Iwamura, Ryo Kobayashi, Kohei Nishiyama, Yosuke Nakanishi
Graduate School of Environment and Energy Engineering, Waseda University, Japan
1. Introduction

Herein, we present a global geospatial optimization for the location and transmission network (TN) configuration of wind farms (WFs). We aim to minimize construction costs by considering geographical conditions using a cost surface. To select the best WF locations and TNs in large areas, penetration Voronoi division (PVD) is applied to group the WFs. Then, a genetic algorithm (GA) is applied with a relaxation process on the minimum spanning tree (MST) to minimize the construction cost and avoid overloads. Moreover, we simulated a large area in Japan to test the proposed technique.

2. 4D-GIS and Cost Surface

A geographic information platform, 4D-GIS, is used to process 2D, 3D, and time-series data and manage the cost surface. The cost surface data is a digital elevation model (DEM) that included topographical factors (e.g., height constraints), environmental factors (e.g., construction-prohibited areas), and economic factors (e.g., power transmission losses).

3. Extraction of feasible areas for WFs

To identify the feasible areas for building WFs, we proposed a method to merge WF areas. Originally, small candidate areas in the DEM are selected based on the wind speed and cost surface constraints. These areas are merged as WF-feasible areas by merging small neighboring areas with high wind speeds.

4.Configuration Design of Transmission Networks

Challenges associated with designing the configuration of a TN in a large area exist:

[1] Selecting WFs that exceed their target capacities.

[2] Configuring TNs among WFs and access points (APs).

PVD and a GA are applied to address these challenges.

4.1 PVD

In large areas, WFs should be classified into small groups to avoid long-distance connections. PVD is used to classify WFs based on the cost surface. The areas on the cost surface that are reachable from an AP are merged by an area-range expansion process called penetration.

4.2 GA with a MST Relaxation Process

All transmission lines among the WFs and an AP in each group are found using a shortest-route search via PVD. The WFs whose total capacities exceed their targets are selected and a TN among these WFs and APs is configured. With 100 WFs (practically), 2100 combinations of these WFs exist; thus, a GA is used. In the GA, WFs are treated as genes in a chromosome. The objective function is the sum of the construction costs of WFs, transmission lines, and transformers. The GA is executed iteratively to minimize the objective function. At the optimal point, the TNs are configured with the MST, which is a tree graph structure without loops. Then, MST relaxation is applied to partially release and reorganize the MST topology to avoid overloads.

5. Simulation Results

A simulation of a 9,600 km2 area in Japan is performed. In this area, 110 feasible WF areas are identified. The target capacity is 2.0 GW; thus, the introduction capacity is 2.09 GW. This result demonstrates that the proposed method yields an optimal result.

Enabling 80+ percent share of wind and solar in Lithuanian power sector: role of storage technologies
Submission-ID 048
Justinas Jasiūnas, Lennart Söder
KTH Royal Institute of Technology, Stockholm, Sweden, Sweden
For wind and solar to play a major role in deep decarbonization of power sector costs of systems with very high shares of variable supply need to become lower than that of existing systems. Balancing challenges for systems with high shares of variable supply is expected to differ significantly from systems with intermediate levels of supply variability. To get a better understanding on the former study is performed on insulated, 100% renewable Lithuanian power sector with pumped hydro and hydrogen energy storage systems. The study is performed using deterministic optimization model with linear formulation, which simultaneously varies installed capacity and its dispatch. Model is run at hourly resolution for a single year with existing technologies.

Among the aspects considered in the study none were found to be prohibiting feasibility of the system analysed. Performed analysis shows seasonal energy shortages to account for more than 5% of annual consumption in non-oversized system. System balancing in the model was achieved with oversizing supply, use of hydrogen and pumped hydro storage systems at 3-5 times of electricity costs in today's wholesale market. The same scenarios show wind and solar curtailment levels of 11% to 46%. Given limited number of technologies included in optimization, large cost reduction potential remains. Among included technologies wind power capacity cost reductions have the largest impact to overall system costs. However, wind cost reductions alone are highly unlikely to achieve overall system competitiveness.

Submission-ID 061
Markus Lehmann 1, Dr. Martin Pieschel 1, Dr. Łukasz Kocewiak 2, Miguel Juamperez 2, Sridhar Sahukari 2, Karsten Kabel 2
1 Siemens AG Freyeslebenstr. 1-3 91058 Erlangen, Germany
2 Ørsted A/S Kraftværksvej 53 – Skærbæk 7000 Fredericia, Denmark
The application of active filtering (AF) in a large-scale STATCOM for offshore wind power integration is presented in this paper. The project topology and its main parameters are introduced to give an understanding of the basic layout of an offshore wind power plant (WPP) integration using an AC connection. The impact of the AF functionality on the measurement hardware of the STATCOM system is described. The methodology and operating principle of the AF within the STATCOM is shown and explained. Harmonic propagation studies illustrate that low-frequency resonances in WPP components , such as HVAC export power cables, reactive power compensating devices and park transformers may introduce amplification of harmonic distortion (HD) at the point of connection (POC). The impact of the AF functionality during operation is shown in real-time simulation cases.

Wind generation in adequacy calculations and capacity markets in different power system control zones
Submission-ID 063
Lennart Söder 1, Ana Estanqueiro 2, Damian Flynn 3, Bri-Mathias Hodge 4, Juha Kiviluoma 5, Magnus Korpås 6, Emmanuel Neau 7, Kaushik Das 8, Danny Pudjianto 9, Goran Strbac 9, Daniel Burke 10, Tomás Gómez 11, Antonio Couto 2
1 Department of Electric Power and Energy Systems, KTH, Sweden
2 Laboratório Nacional de Energia e Geologia, Portugal
3 Electrical and Electronic Engineering University College Dublin, Ireland
4 National Renewable Energy Laboratory, United States
5 VTT Technical Research Centre of Finland, Finland
6 NTNU, Norway
7 EDF, France
8 Wind Energy, DTU, Denmark
9 Imperial College London, United Kingdom
10 National Grid, United Kingdom
11 Universidad Pontificia Comillas, Spain
Generation capacity adequacy is a major issue in most power systems, but there are many approaches which can be assessed. Power system planners often define target values for the capacity adequacy, which may be achieved through capacity markets/auctions, capacity reserves, or capacity purchases. Wind power contributes to the generation capacity adequacy of the power system since there is a possibility that wind power will generate in high load situations and thereby decreases the risk of generation capacity deficit compared to the system without this source. The contribution is probabilistic – as it is with any other source, since nothing is 100% reliable - but the capacity value of wind power is significantly smaller compared to the capacity value of conventional fossil-fueled plants.

In this article, an overview of the fundamental challenges in the regulation of capacity adequacy as well as how wind power is treated in some selected existing jurisdictions is presented. The jurisdictions that are included are Sweden, Great Britain , France, Ireland, United States (PJM), Finland, Portugal, Spain Norway and Denmark.

Approach to design and review the system defence plan for over-frequency Ensurance of frequency stability in the ENTSO-E Continental Europe Synchronous Area
Submission-ID 064
Joachim Lehner 1, Johannes Weidner 2, Tobias Hennig 3, Georg Deiml 4
1 TransnetBW GmbH, Germany
2 50Hertz Transmission GmbH, Germany
3 Amprion GmbH, Germany
4 TenneT TSO GmbH, Germany
Power systems all over the world are affected by decreasing power system inertia caused by increasing infeed of non-synchronous renewable generation, in particular wind and photovoltaics. In small synchronous areas temporary low system inertia already affects system operation during normal operation. In contrast this is not the case in big synchronous areas like the ENTSO-E Continental Europe Synchronous Area. Low inertia in parts of the synchronous area is not an issue during normal interconnected system operation where imbalances typically do not exceed the design incident of 3 GW.

However, in case of an unintentional system split, dividing the system into two or more asynchronous islands, low inertia combined with high imbalances (much bigger than the design incident) can have a big impact on frequency stability due to the resulting high rate of change of frequency (RoCoF).

Electricity market simulation results of the German grid development plan predict that parts of the ENTSO-E Continental Europe Synchronous Area will temporarily be faced with very low inertia in the future, caused by the predicted further increase of non-synchronous renewable generation (in particular wind power in northern Germany) replacing synchronous conventional generation providing system inertia. Furthermore, the power transits over the extra-high voltage transmission system to transport the infeed from the renewable generation to the load centres during these hours increases accordingly.

The analysis of potential system split scenarios for the ENTSO-E Continental Europe Synchronous Area identifies the risk of very high RoCoFs in case of a system split with a high impact on frequency stability. These high RoCoFs occurring directly after a system split can only be diminished by additional inertia. Different emergency control schemes (power-frequency control) as part of the system defence plan are designed to prevent the power system from blackout by balancing the active power imbalance, the under-frequency load-shedding (UFLS) in case of generation deficit (under-frequency island) and the limited frequency sensitive mode over-frequency (LFSM-O) in case of generation surplus (over-frequency island).

The evaluation of potential over-frequency islands show, that today’s implementations of LFSM-O as well as the remaining system inertia are not adequate to prevent the power system from blackout for all relevant potential future system split scenarios. On the one hand the remaining system inertia is not enough and on the other hand the time response of LFSM-O, amongst others of today´s wind power plants, realised by the pitch-control, is not satisfactory.

For that reason, a two-step approach is recommended for dimensioning of the system defence plan for over-frequency in this paper. In a first step the necessary additional inertia has to be assessed, based on maximum allowed RoCoF. In a second step the necessary time response of LFSM-O has to be determined.

Experiences of state forecasting using wind generation forecast in real, smart distribution grids
Submission-ID 065
Frederik Paulat, Kamil Korotkiewicz, Marcel Ludwig, Markus Zdrallek
Bergische Univesität Wuppertal, Germany
In the course of the installation of smart grid systems in the distribution grids, it becomes more important to establish Smart Markets as a new actuator of the overall energy distribution system. The Smart Market is important to constitute flexibility if needed. To identify a demand of flexibility in the grid due to a component overload or voltage range violation, the Grid State Forecasting (GSF) becomes much more important. To achieve an accurate grid state forecast, a modular approach to a generic GSF concept was developed. In this concept the GSF is not realized directly by forecasting the voltages depending on the outside influences. Instead, the GSF is computed by a power flow calculation considering predicted nodal powers (feeds and loads) in the grid.

For a precise GSF it is necessary to develop a generation forecast of the volatile decentral power plants such as wind- and photovoltaic power plants. The generation forecast, in combination with a load forecast, forms the base of the GSF implemented as a bottom-up approach. While existing smart grid systems are capable of reacting to an already occurred violation only; GSF provides a prevision of critical grid situations, before they occur. If the GSF predicts a critical situation, it is possible to request a flexibility, which is able to prevent this situation. This advance is based on the BDEW capacity traffic light concept. In this concept, a smart market is the central platform, where the grid prevention is done. This market and its trades are activated in the yellow light cycle triggered by the GSF.

The presented topic is part of a research project, supported by the Federal Ministry for Economic Affairs and Energy, aiming for the development of an integrated smart grid system with modelling a new local smart market as a component to avoid critical situations in the distribution grid. Based on the detection of critical grid states in advance by the GSF, the Smart Market determines and constitutes “free” flexibilities in the grid for physical prevention. The developed system will be evaluated during a field test as part of the research project.

In this contribution, the authors focus on the influence of wind power forecast as an advanced module of the GSF. Therefore, three wind parks were chosen and the power forecast analyzed in relation to real measured values. Therefore, a distribution grid located in Hesse was consulted. In this grid, wind power systems with nominal powers of 12MW, 9MW and 3MW are located. With the power forecast and measured values several simulations (Power Flow Calculation) of the GSF in comparison to the real grid state were done. To evaluate the results, there were some additional load scenarios chosen. Subsequently, it was possible to evaluate the influence of a wind power generation forecast on the GSF as a function of the grid topology and load distribution.

System Services by Wind Power Plants Supporting 75% Wind Penetration in Ireland
Submission-ID 067
Mark Gilsenan 1, David McMullin 1, Sönke Engelken 2
1 ENERCON GmbH - Irish Branch, Ireland
2 WRD GmbH, Germany
This paper briefly reviews a number of markets where system services by wind are active or in the early stages of development, before turning its focus to Ireland's DS3 System Services market. The paper includes a review of the technical requirements of relevant system services in Ireland, while providing context as to why the System Operators, EirGrid and SONI, have developed such requirements. Finally, this paper presents and assesses the outcomes of system services trials conducted in 2017 on Wind Power Plants connected to the Irish transmission system. Since the trials' conclusion, the system operator has commenced issuing contracts to Wind Power Plants of operating reserve services.

Requirements for control strategies of grid connected converters in the future power system
Submission-ID 068
Hanna Emanuel, Roberto Rosso, Konstantinos Pierros, Johannes Brombach
ENERCON GmbH, Germany
The ongoing increase in penetration of converter-based generation in many power systems around the world has sparked a discussion about how to operate these power systems with the usual levels of efficiency, reliability and cost-effectiveness. Current grid-feeding converter-based generators have proven to run stably in parallel to one another, even if there are thousands of them connected in a power system, and even in very small isolated power systems with extremely low system inertia. Discussions around the necessity of additional converter performance, usually under the “grid-forming” and “Virtual Synchronous Machines” concepts, have recently been transferred from the academic sphere to national and international industry fora. Formal discussions have started in Great Britain, in Germany and at ENTSO-E level. However, there is still a lot of uncertainty about the real and not simulated performance of grid-forming converters and the needs case for requiring this radically different control method has not been adequately justified. With the present paper we raise key questions that will serve towards an objective discussion about power system needs, grid infeed technologies, and their interaction.

Potential for Provision of Ancillary Services by Renewable Energy Resources in ERCOT
Submission-ID 071
Julia Matevosyan, Connor Anderson
ERCOT, United States
ERCOT serves as the independent system operator (ISO) for about 90% of the electric load in the state of Texas. The Texas Interconnection is not synchronously connected with any other grids, so ERCOT is solely responsible for maintaining reliability at the interconnection level.

In 2017, wind generation capacity reached 20% of the ERCOT’s total installed generation capacity and produced 17% of the total system energy for the year. As of the end of April 2018, installed wind capacity was almost 21 GW and solar capacity was around 1.1 GW.

The all-time summer peak load in ERCOT is around 71 GW, while the minimum system load can be as low as 24 GW. On October 27, 2017 wind generation served 54 % of system load (28.4 GW) at one point in time. There is over 5 GW of additional wind generation and 1.5 GW of additional solar generation that is planned to come online between now and 2020.

With renewable generation routinely serving a large portion of ERCOT load, production from conventional generation, which traditionally provides reserves, is declining. It is expected that more synchronous generation may start de-committing at night during the spring and fall seasons when load is low and wind power production is high. Additionally wind and solar power curtailment are common in ERCOT due to transmission congestion issues or, at times, due to too much generation on the grid.

In the past few years, however, it has been proven in a number of tests and studies from research labs, developers and equipment manufacturers, that wind and solar resources are capable providing various ancillary services with performance similar to or even superior to traditional synchronous generation.

Since March 2012, new renewable energy resources in ERCOT are required to have governor-like response capability with set deadbands and droops and respond to frequency events if they have available headroom. Similar requirements apply to all generators that are interconnected to the ERCOT grid. This requirement ensures that the capability to provide frequency containment reserve (also called primary frequency reserve) is already built into most existing renewable energy plants.

Based on analysis of recent historic data, this paper aims to investigate the potential from existing renewable energy resources to provide Ancillary Services for the ERCOT grid. We will also study the uncertainty associated with provision of AS from renewable energy resource, since in ERCOT reserves are traded in the day ahead market and reserve responsibility, if awarded, is for at least one hour. Finally, the paper will investigate and highlight any administrative and technical barriers to the provision of AS from renewable energy resources that currently exist in ERCOT grid codes and market rules.

Operation of Hydrothermal System with Increased Wind Generation
Submission-ID 074
Luke Schwartfeger 1, Alan Wood 1, Gari Bickers 2
1 University of Canterbury, Christchurch, New Zealand, New Zealand
2 Transpower New Zealand, New Zealand
New Zealand, like many countries, aspires to replace fossil fuel based generation with renewable resource based generation. Wind generation is a likely candidate for this in New Zealand. This paper describes a case study of New Zealand’s hydrothermal electricity system in response to additional wind generation. A modelling tool using implicit stochastic optimization with dynamic programming was developed along with a model of New Zealand hydrological and electricity systems to conduct this analysis.

Frequency Response of Energy Storage Systems in Grids with High Level of Wind Power Penetration- Gotland Case Study
Submission-ID 075
Firas Daraiseh, Vincent Gliniewicz, Erica Lidström
Vattenfall Research and Development, Sweden
Gotland is a Swedish island that is connected to the synchronous grid only by two HVDC cables. While being known for its high level of wind power penetration, the growth of installed wind power on Gotland has been temporarily stopped due to concerns regarding the reliability of the electrical grid operations. This paper investigate the capability of a centralized energy storage system along with or without wind curtailment to support the growth of installed wind power capacity. The energy storage system is tested for maintaining the frequency stability during an unintentional islanding through dynamic studies using PSS/E. The results assess the ability of an energy storage to prevent frequency instabilities and provide primary frequency response albeit of the absence of any form of rotating inertia in highly wind power penetrated electrical grid. The analysis determines the requirements for the power and energy capacity of the energy storage system in relation to the exported power from the HVDC cables at the instant of fault, which eventually relates to the installed wind power capacity. Moreover, the study examines wind power curtailment as primary and/or secondary frequency response technology and the impact on the energy capacity of the energy storage system. Keywords- HVDC; Energy Storage System; Frequency Response; Gotland; Wind Curtailment

Environmental friendly high-voltage switchgear for an emerging renewable energy market.
Submission-ID 078
Cem Buetuener
Siemens AG Energy Management High Voltage Products Gas-Insulated Switchgears, Germany
Gas-insulated switchgears (GIS) in wind turbines were chosen for quite some time up to 33 kilovolt (kV) operating voltage. Because of increasing power capacity of wind turbines the need for 66kV as operating voltage is required. Nowadays in high-voltage (HV) switchgear sulfur hexafluoride (SF6) is by far the most applied medium as arc quenching and insulating gas. SF6 requires a careful handling during the entire lifecycle to avoid gas leakage to the atmosphere as it is a greenhouse gas with a high Global Warming Potential (GWP). At this stage the decision was taken to develop a SF6-free environmental friendly 66kV GIS for wind turbine applications. Two main challenges were tackled: substitute SF6 by a fluorine-free and climate-neutral insulation gas (Clean Air) and introduce vacuum switching technology into a HV GIS. Actual available SF6 alternatives were investigated within a basic development in regard to technical, environmental, health and safety aspects. A further technological area of work was the short-circuit breaking and switching concept. Here the focus was on implementation of vacuum interrupting (VI) technology – which is state of the art and well-known from medium-voltage (MV) GIS applications – like in 33kV array operating voltage applications. Further on an early cooperation and exchange with wind farm developer and wind-turbine-manufacturer was carried out in regard to electrical & mechanical properties, installation & commissioning, operation and maintenance and health & safety. The selection of the gas was conducted to the selection of short-circuit breaking and switching technology. Selection of VI technology led to separation of two functionalities of gas in GIS: dielectric insulation and switching. The switching is performed in the hermetically tight and sealed for life vacuum chamber of the VI and the dielectric insulation is realized with Clean-air a composition of 80% nitrogen and 20% oxygen – the natural gas breathed by men and animals on earth but it is purified and free from humidity for the technical use in HV GIS. Due to the hermetically tight VI the influence of decomposition products caused by power switching operations is eliminated and it was possible to implement all wind farm specific required electrical functions of the GIS in one gas compartment. Finally the product development was completed by successfully type testing the new 66kV compact GIS according to the relevant standards and introduced into the market for wind turbine applications. Vacuum interrupter and clean air insulation gas realizes the fluorinate-gas-free technology and supports the demand for a full environmental compatible wind power. The overall GWP impact can be considerable reduced for the GIS as a part of the electric system device of o wind turbine compared to a conventional SF6 GIS. First practical applications of this new GIS showed acknowledge the positive characteristics in regard to manufacturing, on-site handling, installation and operation.

Frequency Support Provision to Power Systems from HVDC-Connected Offshore Wind Power Plants
Submission-ID 081
Ali Bidadfar 1, Oscar Saborío-Romano 1, Eduardo Prieto-Araujo 2, Oriol Gomis-Bellmunt 2, Müfit Altin 1, Nicolaos A. Cutululis 1, Poul E. Sørensen 1
1 Wind Energy Department, DTU, Denmark
2 CITCEA-UPC, Barcelona, Spain
High penetration of power electronic devices in generation and transmission networks results in lower inertia in power systems, which can increase the risk of frequency instability. Thereby, the fast frequency support is anticipated to be the most important ancillary service provided by renewable generation systems in the near future. Frequency support from offshore wind power plants (WPPs) connected to high-voltage dc (HVDC) links is a challenging issue because of the complexities in their control and operation. In this paper, some technical challenges regarding fast frequency support from HVDC-grid-connected offshore WPPs are addressed. Two different methods–communication-based and droop based–of implementing such support are explained first, then the advantages and disadvantages of each are detailed by means of mathematical and numerical (simulations) analyses. A four terminal HVDC grid, including two offshore WPPs and two onshore ac systems, is used for analyses and simulations. Moreover, a point-to-point HVDC connection is also studied to understand how in an HVDC grid converters interactions can impact the frequency control.

Frequency services for Grid Support: International Experiences from On-shore and Off-shore Wind Farms
Submission-ID 083
Sigrid Bolik
Senvion, United Kingdom
Senvion turbines have been operating worldwide with different Frequency Service (FS) options in both on-shore and off-shore wind farms. Throughout several grid events, this control provided valuable contribution to the key importance to the stability and recoverability of the grid following a load-generation mismatch (frequency event). Real-time monitoring and disturbance recording was employed to evaluate performance, optimize turbine configuration and verify compliance with grid code requirements i.e. Inertia response, frequency sensitive mode, high frequency response. The purpose of this presentation is to present the lessons learned, using field data from some of the on-shore and off-shore wind farms. We will present and compare the performance of the different control solutions available and discuss the technical challenges and the opportunities from Type 3 and Type 4 wind turbines to contribute to FS.

Grid Code Certification in Germany – A recipe for Europe?
Submission-ID 087
Bernhard Schowe, Christian Scheefer, Alina Hoppmann, Christoph Lütke-Lengerich
FGH Zertifizierungsgesellschaft mbH (FGH Certification Body), Germany
The liberalization of the energy markets demanded for guidelines regulating the access to the market as well as defining technical requirements to guarantee a safe and reliable operation of the network grids. Following the harmonization inside the European Union, the Network Code on Requirements for Grid Connection Applicable to all Generators (RfG) defines the technical requirements for the connection of power generating units (PGU), plants (PGP) and their components. In Germany, certification processes for power generating units and plants are in place for about 14 years. Europe, why don’t you take lesson’s learned besides starting from scratch?

Systemic issues of converter-based generation and transmission equipment
Submission-ID 092
Tobias Hennig 2, Joachim Lehner 1, Georg Deiml 3, Johannes Weidner 4
1 TransnetBW GmbH, Germany
2 Amprion GmbH, Germany
3 TenneT TSO GmbH, Germany
4 50Hertz Transmission GmbH, Germany
Considering the growing share of renewable energy sources of the total installed generation capacity in the Central Europe power system, especially in the German sub-system, conventional power plants are getting gradually replaced. While conventional units are relying on energy conversion based on synchronous generators directly coupled with the grid, most of the renewable conversion systems utilize power electronic converters. Thus, this development gives rise to several open questions on system level in terms of how converter-based generation units are affecting power system stability with its associated stability classes, power quality and protection schemes. Familiar concepts like system inertia und short circuit power can only be applied if synchronous generators are the dominant generation technology. Furthermore, the increasing demand of transport capacity accounts for modern transmission technology such as HVDC and FACTS, which are also based on converter technology with comparable impact on system behaviour.

At the same time, increasing distances between generation and consumption as well as the extension of transport capacities provided to the European electricity market increases the impact of potential extreme contingencies such as an unintentional system split. Both the transformation of generation portfolio as well as the change in transport patterns faced by TSOs, require a methodical approach in order to assess their influence on system performance in case of disturbances.

In this paper, results of an on-going research project of German TSOs with different partners in academics and industry will be reported, focussing on several systemic issues in the prescribed context. Different studies in the framework of the project have dealt with (a) the performance of under-frequency load-shedding schemes considering an increasing share of decentralized generation, (b) the performance of power system protection schemes taking into account an increasing share of power electronic devices, (c) the impact of converter-based generation on power system stability with focus on voltage and transient stability and (d) the robustness of the system against system split contingency scenarios. The paper will provide an overview of the results of the project and the methodology applied including data acquisition, simulation studies in different time-scales (emt and rms domain) as well as model verification. In addition, requirements for converter control are derived from project results to ensure power system stability taking into account the above-mentioned topics.

Considering curtailments in wind power forecasting
Submission-ID 093
Jonas Koch, Dominik Jost, Axel Braun, Jan Dobschinski
Fraunhofer Institute for Energy Economics and Energy System Technology IEE, Germany
Curtailments (often called EinsMan or Eisman) are not only a hot topic for grid operators but also for providers of wind power forecasts. Regarding different use cases curtailments and other reductions have to be considered within the forecast model or not. The difference between the possible power output and the actual feed-in can be of significant magnitude and makes a differentiation between available and actual feed-in power in forecasts necessary. In categorizing reductions of wind power generation output first, we suggest a clearer definition of the differentiation between possible, available and actual power forecasts (for further discussions). With these definitions and with focus on curtailments an algorithm will be presented allowing the detection of curtailment events from power data and providing the ability to improve two hour short-term power forecasts by nearly 30 %.

Wind Farm Grid Code Compliance Testing: Different approaches, biggest hurdles and the foreseeable future
Submission-ID 096
Tobias Rösner, Jonas Fleischhauer, Manuel Fernandez
Nordex Energy GmbH, Germany
Many markets require grid code compliance testing for wind farms to prove their electrical capabilities and control behaviour. For a global acting wind turbine manufacturer it can be difficult to deal with the different approaches in different markets for performing a Grid Code Compliance Test (GCCT). In dependence of the grid code and network operator such testing can be done within a few hours or needs a few weeks. This paper gives an idea of what keeps a test simple and what makes it extensive. Topics like scripted tests compared to manual testing and compliance testing performed due to a third party consultant are being discussed.

A further objective of this paper is to give an idea of common hurdles on the way to a grid code compliance certificate for a wind farm project. It explains what the usual mistakes are when it comes to evaluate the performance of different control functionalities like reactive power, voltage or frequency control. The main purpose is a discussion regarding accuracy requirements related to set point change or target values which often leads to technically not achievable tolerances. It reflects simple written requirements within grid codes and gives an example of how the same sentence used in two different grid codes could lead to two complete different requirements.

GCCT will grow and it will grow big. It is the best way to prove grid code compliance and to demonstrate the contribution to a sustainable network. Some of the reasons of greater importance of GCCT in the future are the rising continuous monitoring of wind farms by network operators and the approach of repetitive testing every few years. Additionally costumers ordering their own GCCT for the project acceptance even if not required within the grid code.

Control solutions for Blackstart capability and Islanding operation of Offshore wind power plants
Submission-ID 097
Anubhav Jain, Kaushik Das, Ömer Göksu, Nicolaos Antonio Cutululis
Department of Wind Energy, Denmark Technical University (DTU), Denmark
Environmental sustainability concerns make renewable energy systems (RES) integrated into the grid crucial for future power systems. Amongst RES, wind energy especially offshore wind power plants (OWPP) show huge promise. Increasing penetration of RES requires re-thinking of critical operation states that could lead to an increased risk of generation tripping that ultimately triggers blackouts. Thus maintaining reliability, robustness and stability of grid operation has become more complex and so blackstart (BS) and islanded (Is) operation requirements are being considered as options for WPPs in the grid codes. Additionally, advanced control functionalities provided by modern wind turbines (WT) owing to their power electronics converter (PEC) interface, enables them to provide fast, high power environment-friendly BS capability that facilitates grid recovery & reduces the impact of a blackout. In this paper, the motivation for BS capabilities in OWPPs has been presented, and the different stages of restoration using OWPPs identified. Finally the existing control solutions and potential challenges for BS&Is using OWPPs have also been investigated.

How large wind parks contribute to harmonic waveform distortion
Submission-ID 101
Daphne Schwanz, Math Bollen, Sarah Rönnberg, Anders Larsson
Electric Power Engineering, Luleå University of Technology, Sweden
This paper summarizes the important issues related to waveform distortion and large wind parks. The emission at the classical frequencies (low-order odd harmonics) is low. Instead emission is higher than for most other installations at even harmonics, especially at high order even harmonics, at interharmonics and at supraharmonics. Theoretical studies have shown that secondary emission (emission driven by sources outside of the turbine or outside of the park) can be a substantial part of the harmonic voltages and currents with a large wind park. Other issues to be considered are aggregation and resonances..

Importance of voltage-dip knowledge for improving fault-ride-through of wind-power installations
Submission-ID 102
Azam Bagheri 1, Math Bollen 1, massimo Bongiorno 2, Chen Cheng 3, Sarah Rönnberg 1
1 Electric Power Engineering, Luleå University of Technology, 931 87 Skellefteå, Sweden
2 Dept of Electrical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden
3 KTH Royal Institute of Technology Stockholm, Sweden
This paper verifies the dynamic behavior of DFIG-based wind turbines in presence of voltage dips or transients due to various origins: electrical faults, transformer energizing, capacitor energizing, and cable energizing. These different origins result in different characteristics of voltage dips in term of, residual voltage, duration, dip type, harmonic distortion, and so on. Each of these characteristics has different impacts on electrical parameters of the wind turbines. The voltage dip ride-through capability of the wind turbine installations is provided by grid codes which are derived based on electrical fault origin voltage dips, considering only two dip characteristics: residual voltage and duration.

The double-fed induction generator (DFIG)-based wind turbine is simulated in MATLAB/Simulink environment to investigate the dynamic behavior of the critical electrical parameters including DC-link voltage and rotor current. The simulation results show that some shallow dips which are aligned with grid-code requirements may lead to unwanted trips because of heating problem and crowbar system activation.

Balancing by Wind and Energy Storage
Submission-ID 103
Andreas Linder, Ana Kosareva, Bettina Lenz, David McMullin, Konstantin Kunz
ENERCON GmbH, Germany
Modern Wind Power Plants (WPPs) may provide additional System Services beyond complying with today’s Grid Code requirements. Nevertheless today System Services are mostly provided by conventional power plants as the market rules and technical requirements for System Services have historically evolved to suit such power plants.

WPPs, being based on a variable primary energy source and non-synchronous inverters, differ fundamentally from conventional power plants in their System Services capabilities. Existing frameworks for such services thus need to be adapted if the power system is to take full advantage of the performance of WPPs.

In this paper, the functionality of modern WPPs, and battery energy storage systems (BESS), both based on full inverter technology, will be demonstrated. When equipped with appropriate technologies, modern WPPs and BESS can be utilised to provide System Services such as Voltage Support and Frequency Response. The latter, in the context of Balancing, will be the focus of the present paper.

Key features in this respect include fast-acting wind farm controllers, emulated inertia, and the Available Active Power Signal (AAP). Within the paper, examples for projects which have demonstrated such capabilities will be presented according to the current ENTSO-E product types.

Frequency Containment Reserve (FCR)

BESS are increasingly used for the provision of Balancing Services. One such example in Germany is a 10MW Li-ion BESS being operational since 2016, providing FCR as an asset within a pool. The paper will give a technical overview of the BESS hardware and control strategies. The process of planning, commissioning and operating the BESS will also be outlined. Notably this BESS features inverters and control systems which are also deployed in WPPs. The paper will discuss how these originally WPP-based systems were adapted for BESS applications, and the practical benefits of this arrangement.

Automatic Frequency Restoration Reserve (aFRR)

In 2015, a pilot project for the provision of R2 down (aFRR) by a WPP in cooperation with the Belgian TSO ELIA has been completed. The paper will present the project, and how the WPP contributed, for a period of about 2 months, to the delivery of aFRR to the Belgian grid by continuously changing the active power output of the turbines according to a setpoint defined by ELIA.

Manual Frequency Restoration Reserve (mFRR)

The German TSOs set up a pilot project in 2015 for the supply of negative mFRR by WPPs. A pool of WPPs was integrated into a dispatch system which is connecting the TSOs to the WPPs and enables the aggregator to offer mFRR by WPPs. The paper will outline which new technical features such as an AAP-based control mechanism and the possibility of a WPP to follow mFRR setpoints were implemented to enable WPPs to participate in the Balancing Market.

Experiences in the NEM: Practical considerations for the successful integration of utility-scale renewable storage solutions
Submission-ID 105
Harley Mackenzie, Jonathon Dyson
Dispatch Solutions, Australia
Many existing and proposed wind and solar plant generators in Australia and internationally are presently considering the integration of storage solutions with their generation assets to provide a means of arbitraging energy spot prices and potentially participating in market ancillary services, or capturing energy that might be lost due to curtailment. This paper will document and discuss in practical detail some of the recent experiences in the competitive Australian National Electricity Market (NEM). Whilst the regulator and system operator have all been supportive of the recent developments, a number of challenges have been encountered that test many of the existing specifications and requirements that were formulated during market design discussions 20 to 25 years ago. During recent project developments and implementation, a number of practical constraints and limitations have been encountered such as:

* The NEM Dispatch Engine (NEMDE) required the battery to be registered in the market, bid and operated as a separate load and generator, as a storage model had not been implemented,

* The ancillary service performance has exceeded expectations but effective payment for those services appears less than desired, and

* The half hour trading interval settlements (that average the 5 minute dispatch periods) potentially lead to economic incentives not matching the system requirements, which in turn lead to very challenging system implementation challenges. Similarly, given storage devices as low as 5MW are registered in the market as dispatched generators, many of the inherent control and trading controls required by large scale fossil fuel or hydro generators are now required by renewable generators. The considerable costs of running 24/7 operations and trading centres are not well suited to the prevailing economic thinking of small scale operations that have been a significant contributor to developing and constructing renewable generation in Australia.

This paper will also discuss a number of the information and dispatch system challenges that occur with utility-scale storage implementation in 5-minute dispatch competitive markets.

Steady-state characteristics of substation-free wind power plant composed of series-connected wind turbine generators and current-source thyristor inverter
Submission-ID 106
Shoji Nishikata, Fujio Tatsuta
Tokyo Denki University, Japan
For offshore wind power plants it is desirable not to equip offshore substations because of initial costs and maintenance costs. From this point of view, we have already proposed a substation-free wind power plant consisting of series-connected wind turbine generators, HVdc transmission line, current-source thyristor inverter and synchronous compensator with duplex reactor. The basic studies on the series-connected wind power plant have been reported in the previous papers.

In this paper the steady-state characteristics of the series-connected wind power plant are investigated in some detail.

In order to clarify the performances of the whole system as accurately as possible, the performances of the system should be investigated based on the equations established on the system, and a set of such equations is first derived. Based on these equations the steady-state characteristics of the series-connected wind power plant are discussed in this paper.

In this system since the commutation of the inverter thyristors is accomplished by the electro-motive-forces induced in the armature windings of the synchronous compensator, the margin angle of commutation for the thyristors (which is the difference of leading angle of commutation of the inverter and overlapping angle of inverter output currents) must be always positive to secure safe commutation of the inverter thyristors. Hence, the steady-state characteristics of these angles have to be discussed to clarify the operation limit of the system. In addition, the characteristics of the field current and the armature current of synchronous compensator, as well as the system output current, have to be discussed.

To confirm not only the usefulness of the proposed system but also the validity of the system equations derived in this paper we have developed an experimental set-up (system output: three-phase 200 V, 50 Hz, 4 kVA) composed of two simulated wind power generators (three-phase 200 V, 100 Hz, 2 kVA, 8 poles x 2 sets) connected in series. The experimental results of the effective value and THD of the output current, the DC link voltage, the DC link current, and the margin angle of the inverter will be included when the power factor of the system output changes. The value of the power factor was chosen as specified by the Danish grid code.

Finally, steady-state characteristics of the system when dc input voltage is changed due to the changes in the wind speed are discussed using the equations introduced in this paper, and the operation limit of the system is revealed for various power factors of the system output.

This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Japan's R&D Project of Ramp Forecasting Technology: Meteorological Pattern Analysis Method
Submission-ID 107
Maki OKADA 1, Tomohiro ICHIZAWA 1, Yuko NAKAMURA 1, Koji YAMAGUCHI 1, Ryo KODAMA 1, Hisashi KATO 2, Yoshinori NAGANO 2, Ryosaku IKEDA 3, Van Q. DOAN 3, Hiroyuki KUSAKA 3, Takamitsu ARAKI 3, Noriko N. ISHIZAKI 3, Norimitsu OGASAWARA 1
1 Japan Weather Association, Japan
2 Nihon University, Japan
3 University of Tsukuba, Japan
Abrupt wind power output fluctuation, which is known as wind power ramp, tends to negatively influence electric grid operations. To understand and counteract these influences, a national research and development (R&D) project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) is underway to develop forecasting technology for wind power ramps. In this project, authors have been developing forecasting technology and systems for wind power ramps from the viewpoint of meteorological pattern analysis. The developed forecast system used the Weather Research and Forecasting Model (WRF) as a numerical prediction model and selected the different setup WRF model depending on the weather patterns. Firstly, the forecast system evaluated the analogy between sea level pressure (SLP) patterns in the latest global-scale NWP model and past SLP patterns associated with abrupt wind speed changes. According to the latest SLP patterns, secondly, the forecast system selected the optimal WRF setup. Wind forecast data from the WRF model were statistically corrected and converted into wind power output using an empirical power curve. This power curve was constructed using past wind forecast and power output monitoring data that were collected during the NEDO R&D project. The developed forecast system also outputted the probability of ramp occurrence. The probability was calculated from the means and standard deviations of wind power output on and around two transit locations (the start and after six hours) in the principal component space related with SLP forecast data. Ramp alert, which informed the occurrence of wind power ramp in advance, was defined to occur when the ramp probability exceeded 20%. We will explain our forecasting system and its performance for the three target areas (Hokkaido, Tohoku, and Tokyo) of Japan over a set timeframe.

Active Power Control for Mitigation of Very-Short-term and Short-term Fluctuation of Wind Power
Submission-ID 108
Chiyori T. Urabe, Tetsuo Saitou, Kazuhiko Ogimoto
The University of Tokyo, Japan
At the 21st session of the Conference of the Parties (COP21), participating countries agreed to achieve a balance between sources and sinks of greenhouse gases in the second half of this century. The countries are required to meet strict targets to reduce carbon emissions. In electric sector, for reduction of carbon emission, deployment of renewable energies (RE) can contribute. While variable renewable energy (VRE) sources, such as wind and solar power, occupy a large part of deployed RE, output of them can change rapidly depending on wind speed and insolation. Therefore, challenges of VRE penetration have emerged depending on the level of VRE share [1].

One of the challenges is large fluctuations of VRE. To compensate fluctuation, flexibility which conventional thermal and hydroelectric power supply is increasingly required depending on integration level of VRE in the electric system. From operational perspective on electrical system, fluctuations of demand and VRE can be classified according time scale: very-short-term, short-term, and long-term fluctuations, or primary, secondary, and tertiary control. For short-term fluctuation, power plants changes their generation by control signal from a central supply center or an aggregator. To provide such flexibility, some operations sacrifices economic efficiency for providing flexibility.

We focus on very-short-term and short-term fluctuations wind power output. Using high-resolution data of wind power output in FY2012 in Hokkaido, the northern Japanese island where large wind capacity is expected to be deployed, first we separated the very-short-term fluctuations and the short-term fluctuations from time series of the original wind power output. Fluctuations in Hokkaido is relatively large because the capacity of wind power is unevenly distributed and is a half of Tohoku which is the nearest balancing area.

To mitigate fluctuations in Hokkaido, we proposed a fluctuation mitigation control method that active power ramp rate limitation and setpoint control which the most modern wind turbines are equipped are incorporated to mitigate the very-short-term and short-term fluctuations respectively. There are two parameters related the active power ramp rate limitation and the setpoint control. In this study, we limited energy loss with the control, because there is tradeoff between the mitigating fluctuations and energy loss. Under energy loss is less than a few percent, we seek effective combination of the parameters for two types of fluctuation mitigation.

[1] International Energy Agency (IEA): “System Integration of Renewables”, IEA (2018)

Unconventional High-voltage Ride-Through Technical Retrofitting Scheme for Certain Imported Old Generating Units of the Northeast China Power Grid
Submission-ID 109
Qi Lv 1, Hong Peng Zhang 2, Guang Hui Shao 2, Jia Qing Liu 2, Yang Liu 2, Shi Bo Du 2
1 Columbia University, United States
2 Northeast Power Dispatching Center of SGCC, China
Since 2017, the ±800kV extra-high-voltage DC transmission project extending from Tongliao in Inner Mongolia to Qingzhou in Shandong Province, with a transmission capacity up to 10000MW, has been put into operation. Over the past year, the high-voltage ride-through retrofit of the wind power generating units has been carried out in the adjacent region of the EHV convertor station in the Grid. Generally, as for most of wind power generating units manufactured by China and put into operation in recent years, the conventional retrofitting approaches are applied. The control strategy of the converter of the wind power generating units needs to be retrofitted; and then, the retrofit of control logic of the wind power generating units is also required, so as to realize the grid-connected controlled operation of the wind power generating units during the overvoltage period of the Grid through coordination between the main control system and the wind turbine converter. As for a few of old generating units imported from Vestas in Denmark and Suzlon in India in the last decade, and those wind turbines manufactured by China in the early period using the imported frequency converters supplied by Converteam in France, ABB in Switzerland and SEG in Germany, if the high-voltage ride-through retrofit is to be conducted by the original equipment suppliers, the hardware upgrading and software optimization of the main control system, variable pitch system and converter are also required, which will not only result in a high cost and a long period, and in particular, when these three systems are not supplied by the same supplier, there will be some technical communication problems between different manufacturers. To solve this problem economically and efficiently, some wind power enterprises of the Grid have adopted the high-voltage ride-through retrofitting technical route with parallel connection of SCR controlled shunt reactor at the wind turbine terminal, which can realize the high-voltage ride-through capability without necessarily changing the equipment of the wind power generating units. Through the field test, it is proven that after using the retrofit scheme, the wind power generating units are able to meet the standard requirements of the high-voltage ride-through. This technical scheme is a kind of technically and economically feasible solution for the high-voltage ride-through technology retrofit for a great number of complete wind power generating units imported in the early period of the wind power development, and those wind power generating units manufactured by China using the imported frequency converters in the early period. This paper will present in detail the unconventional high-voltage ride-through technology retrofit scheme for the old generating units as well as the results of field tests.

Japan's R&D Project of Ramp Forecasting Technology: Probabilistic Forecast Based on Dynamical and Statistical Ensemble Methods
Submission-ID 112
Daisuke Nohara, Shiji Kadokura, Masamichi Ohba, Takeshi Watanabe
Central Research Institute of Electric Power Industry, Japan
Abrupt change in wind power generation, known as ramp events, due to fluctuating wind speeds present challenges to the stability of the electric power supply. In Japan, the ramp events are generally induced by extratropical cyclones along their track. Since prediction for the behavior of the cyclones entails uncertainty caused by nonlinear dynamics, probabilistic prediction is more effective. For this study, we developed a regional ensemble prediction method using the Weather Research and Forecasting model (WRF). To obtain dynamically consistent perturbations with a synoptic weather pattern, both initial and lateral boundary perturbations were determined by differences between the control and an ensemble member of the Japan Meteorological Agency (JMA)'s operational one-week ensemble forecast. This method provides 11 ensemble members with a horizontal resolution of 15 km for 75 hours at 30 minutes interval outputs by downscaling JMA's operational global forecast along with the perturbations. Wind power is projected using power curve which is estimated by relationship between area averaged wind speed and wind power generation. In addition to the dynamical ensemble, we expanded the ensemble member using Monte Carlo method to take account of the impact on uncertainty in the power curve. The expanded ensemble has 100 ensemble members. Forecast for the ramp events using the expanded ensemble exhibits high statistical consistency and reliability of ability to capture the ramp events compared with the dynamical ensemble prediction.

Case Study: Reliability of the Summation Method to Assess the Harmonic Current due to a Wind Power Plant
Submission-ID 116
Koldo Redondo 1, Izaskun Azcarate 1, José Julio Gutierrez 1, Purificación Saiz 1, Luis Alberto Leturiondo 1, Stefano Lodetti 2
1 Department of Communications Engineering University of the Basque Country (UPV/EHU), Spain
2 CIRCE Universidad de Zaragoza, Spain
The measurement of Wind Power Plant (WPP) Harmonic Current (HC) is an important issue in terms of power quality. The IEC 61400-21 standard specifies the procedure to measure the HC produced by a grid connected Wind Turbine (WT). The standard also proposes an aggregation method to estimate HC from all the WTs connected to the Point of Common Coupling of the WPP. The aim of this work is to assess the reliability of the summation method proposed by the standard, based on current signals recorded at a WPP in Spain. The results show that the summation method does not accurately estimate the HC of the WPP. The obtained deviations between the measured and estimated HC values are significant.

Operational options to integrate decentralized generation into restoration processes after severe system black outs
Submission-ID 118
Holger Becker 1, Denis Mende 1, Udo Spanel 2, Alexander Bernhart 2, Sascha Bendzko 2, Johannes Brombach 3
1 Fraunhofer Institute for Energy Economics and Energy System Technology (IEE), Germany
2 DUtrain GmbH, Germany
3 I4E: Innovation for Enercon, Germany
System restoration after a severe black out is an exceptional challenge to the control center operators of all network- and system operators involved. While during 'normal state' operation power/frequency balancing and reserves are preplanned to cope with occurring imbalances, the balancing process during restoration has to be arranged ad hoc taking the then available resources into account.

The available resources are depending on the scheduled generation before the blackout occurs. The more decentralized generation (DG), mainly wind-farms and pv-units, are in operation before the blackout happens, the less conventional generation is scheduled. As the restoration scenario is dominated by the preconditions before the black out, the most critical scenario lies in a high penetration of DG and a minimum of conventional generation units. Compared to them, which are usually equipped with adequate control possibilities, DG in most cases is optimized to generate their maximum available power leading to a fluctuating power infeed with a severe impact onto the network’s frequency in the early stage of restoration. DG usually has less control functions to support the grid during restoration. Furthermore in case of sufficient control functions the operational control possibilities from a control center are oftentimes limited. Therefore DG is not actively considered during system restoration today.

In order to prevent under frequency load shedding, allowable load connection during system restoration is determined by the amount of inertia provided by the rotating generators connected to the grid. As this functionality is not inherently provided by DG today, connection of them shows no advantage in terms of speeding up the process like connection of conventional power plants would do. The integration of DG into the restoration process under these circumstances is a crucial task which results in deployment of DG as late as possible and necessary to continue the restoration process. Especially in situations with high penetration of DG the restoration therefore becomes very time consuming.

To depict possible improvements of existing restoration plans, different scenarios have been simulated, of which no support of neighboring systems is the most critical. In these cases a black-start unit was used to start the restoration process from inside. The operational control center view is realized using a training simulator to demonstrate the overall restoration process on the one hand side, and to be limited to operators’ possibilities on the other. Different possibilities to integrate DG in the system restoration process are shown. Since the demonstration was not scripted in advance, also specific difficulties and setbacks using DG in the process are presented. Nevertheless the system restoration with explicit consideration of DG could successfully be executed.

Comparison of different estimation methods for the grid frequency using the example of a system split in the interconnected electrical power system
Submission-ID 119
Holger Becker 1, Hendrik Sölter 1, Lutz Hofmann 1, 2
1 Fraunhofer Institute for Energy Economics and Energy System Technology (IEE), Germany
2 Institut for Electrical Energy Systems, Leibniz University Hannover, Germany
According to experience, system splits in the European electrical system happen every few years (as examples see Italy in 2003, Europe in 2006 or Turkey in 2015). Compared to normal operation, high frequency gradients occur due to spontaneous occurance of disturbances causing a system split and the power imbalances within the different network islands. The chance for surviving of these islands rises with a fast and reliable detection of the situation and an appropriate reaction of the generating units. Due to direct coupling of generator’s rotor and its mechanical inertia, synchronous machines provide instantaneous frequency support to the frequency, while inverter based generators do not do so automatically. Additionally, the grid frequency can be easily determined from generator’s speed of rotation, while inverters need to determine the frequency by analysing the voltage progress. In the event of a system split with different separated island grids, it is important to stabilize them by balancing active power prompt. Quick frequency estimation is important for fast adjustment of the feed-in power in relation to the frequency.

According to today’s grid codes in continental Europe, wind energy converter (WEC) currently do not provide fast frequency response functionalities. With an increasing proportion of WEC, it is important for grid stabilization during the system split that these generation plants also contribute frequency support rapidly. For being able to do so, WEC control unit need to detect high frequency gradients while being robust against local disturbances like phase angle jumps. Different frequency estimation methods like Zero Crossing, Clarke-Transformation (αβ-transformation) and discrete Fourier-Transformation based methods are described in detail and analysed by investigation of synthetic test signals showing voltage progress during different realistic and extreme situations. The created synthetic test signals represent voltage progress during a system split as well as during other disturbance phenomena. The goal of the frequency estimation algorithm is to detect a system split reliable by being robust against other disturbances like phase angle jumps caused by line tripping or short circuits. In comparison to the Zero Crossing method and the Clarke-Transformation it has been shown, that the discrete Fourier-Transformation is a robust and reliable frequency estimation method under realistic conditions. In contrast to the other methods, the discrete Fourier-Transformation is robust against harmonics and noise. With this method it is possible to clearly distinguish a frequency gradient from a phase jump. Under ideal conditions and without any measuring filters, the Clarke-Transformation could be an appropriate estimation method in consideration of speed and measuring error.

Reducing operational costs of offshore HVDC energy export systems through optimized maintenance
Submission-ID 121
Jan Frederick Unnewehr 1, Hans-Peter (Igor) Waldl 2, Thomas Pahlke 2, Iván Herráez 3
1 Department of Sustainable Systems Engineering, University of Freiburg, Freiburg im Breisgau, Germany
2 Overspeed GmbH & CO. KG, Oldenburg, Germany
3 University of Applied Science Emden/Leer, Emden, Germany
For the grid connection of offshore wind farms, today, in most cases an HVDC connection to the shore is implemented. The scheduled maintenance of the offshore and onshore HVDC stations makes up a significant part of the operational costs of the connected wind farms. The main factor for the maintenance cost is the lost income from the missing energy export. In this study, we show an in-depth analysis of the used components, maintenance cycles, maintenance work and the risks assigned to prolong the maintenance cycle of the Modular Multilevel Converter (MMC). In addition, we investigate the potential, to shift the start date of the maintenance work based on a forecast of the energy production. The results presented in this contribution indicate that an optimized maintenance of an HVDC-export-systems can decrease the maintenance-related losses for an offshore wind farm to almost the half.

Japan's R&D Project of Ramp Forecasting Technology: Correction method with additive model for NWP-based wind speed forecast
Submission-ID 122
Takamitsu Araki, Ryosaku Ikeda, Van Q. Doan, Noriko N. Ishizaki, Hiroyuki Kusaka
Center for Computational Sciences, University of Tsukuba, Japan
To further spread the wind generation, an accurate wind forecast is an essential process. To improve the accuracy of a short term wind forecasts given by a numerical weather prediction (NWP) model, nonlinear regression methods using observed wind speed data and the NWP outputs have been applied. In this study, we introduce an additive model based on B-spline basis expansion into the NWP wind forecast correction, which consists of nonlinear and flexible coordinate regression functions of input variables. We also estimate parameters of the functions using the maximum penalized likelihood method to obtain smooth and flexible regression functions. We compared the proposed method with two nonlinear regression methods that have been widely used, an artificial neural network (ANN) and analog ensemble (AnEn) method through the correction of the wind forecasts of the Weather Research and Forecasting (WRF) model. As the results, our method corrected the WRF wind speed forecasts more accurately than the ANN and AnEn method and revealed the relationships between the WRF wind speed forecasts and the observed wind speed. The relationship was not able to be obtained by the ANN and AnEn method.

Method for Harmonic and TOV Connection Impact Assessment of Offshore Wind Power Plants - Part I: Harmonic distortion
Submission-ID 124
Richard de Groot 1, Frans van Erp 1, Kees Jansen 1, Jeroen van Waes 1, Manon Hap 2, Luc Thielman 2
1 TenneT TSO BV, Netherlands
2 Tractebel Engineering, Belgium
This paper presents the method used by TenneT, the Dutch transmission system operator, to assess the risk of high harmonic distortion due to amplification of background distortion (Part I) and high temporary overvoltages (Part II) in transmission networks that connect (large) offshore wind power plants to the onshore transmission system using long high-voltage AC cables. Studies have shown that the impact of the wind power plant, in particular the capacitance of the long high-voltage AC cable, on the resonance frequencies of the post‑connection network is a potential source for amplification of the pre‑connection harmonic distortion observed at the point of common coupling. This could cause harmonic distortion to exceed compatibility levels, forming a potential threat against the system’s reliability and normal operation. The policy, assessment method and a case study are presented, including the design of a harmonic passive filter for mitigating the risk of too high harmonic distortion.

Keywords-harmonic analysis, harmonic passive filter, connection assessment, offshore wind power, power grid

Analysis and mitigation of storm and ramping risks from offshore wind power in Belgium
Submission-ID 125
Ruben Baetens 1, Simon De Clerq 1, Achim Woyte 1, Carlos Guerrero 1
1 3E, Belgium
2 Elia, Belgium
Once the current concessions will be operational, the total installed capacity of wind power in the Belgian exclusive economic zone will increase from 0.7 GW in 2017 to 2.3 GW in 2020 (and up to 4 GW before 2030), this in an area of only 3447 km² and a control area with a peak load of 13.4 GW and a low of 5.8 GW. By then, unexpected variations in wind power generation may caused by high wind speeds or sudden changes in wind speed or direction could trigger a substantial imbalance in the control area.

To understand the possible impact of such events, ELIA and 3E assessed the observed events of 2015 and 2016, and extrapolated them towards the expected installed capacity in 2020.

It is concluded that by 2020 and in the most realistic scenarios, the power loss caused by a storm event (i.e. cut-out) often goes beyond 1000 MW, while a major event with wind speeds above 30 m/s will always cause a power deviation of more than 2000 MW. In addition, when looking at the maximal ramps observed in both cut-out and cut-in phases, it is observed that deviations around 1000 MW can happen in both directions (up and down) within a period of 30 minutes. Also ramping becomes more significant: power variation of 150 MW within 15 min will happen during approximately 3% of all 15-min time steps.

Method for Harmonic and TOV Connection Impact Assessment of Offshore Wind Power Plants – Part II: TOV Impact Assessment
Submission-ID 129
Kees Jansen 1, Richard de Groot 1, Bart van Hulst 1, Konstantinos Velitsikakis 2, Christiaan Engelbrecht 2
1 TenneT TSO BV, Netherlands
2 DNV GL, Netherlands
Various methods exist to assess the risk of high harmonic distortion due to amplification of background distortion (Part I) and high temporary overvoltages (Part II) in transmission networks that connect (large) offshore wind power plants to the onshore transmission system using long HVAC power cables. Studies have shown that resonance frequencies between the 2nd and 3rd harmonic might occur at the Point-of-Common-Coupling (PCC). Transient conditions around the PCC, either scheduled switching events (e.g. energizing a power transformer) or fault conditions, could be a potential source for exciting these resonances. Once excited, the resonances might result in Temporary Overvoltages (TOVs), which could form a potential threat against the system’s reliability and normal operation. This paper presents the assessment method for TOVs used by TenneT TSO BV, the Dutch Transmission System Operator (hereafter TenneT). The method is based on the evaluation of simulation results, as obtained via detailed Electromagnetic Transient (EMT) studies and its outcome indicates whether remedial measures against excessive TOVs need to be further considered. Last but not least, the results of a case study are presented, including the application and effectiveness of harmonic passive filters for mitigating TOVs.

Keywords / Indexing terms

Temporary overvoltages, TOV impact assessment, harmonic passive filter, C-type filter, connection assessment, offshore wind power, power grid

Submission-ID 133
Corinna Möhrlen
WEPROG, Denmark
In this paper, the progress of the first part of a recommended practices guideline “Forecast Solution Selection Process” is presented that deals with the selection and background information necessary to collect and evaluate when developing or renewing a wind power forecasting solution for the power market. This is part of the IEA Wind Task 36 project 2016-2018. The work aims to provide a series of three recommended practices that deal with the development and operation of forecasting solutions of wind (and solar power) in the power market.

The effectiveness of forecasts in reducing the variability management costs of power generation from wind and solar resources is dependent upon both the accuracy of the forecasts and the ability to effectively use the forecast information in the operational decision-making processes. With increasing amounts of forecasting methods and vendors, it has become more difficult to obtain forecasts of high quality with a fit-for-purpose that can effectively be used as input to operational processes in system operation, trading, market management, unit commitment etc. The recommended practices guideline is intended to provide guidance to forecast users who are seeking a forecasting solution that fits their purpose and enables them to work efficient and economically responsible. In this paper we summarize some of the important aspects in this respect from the document under review and explain, how the decision support tool to establish procedures for the selection process, can be applied.

Lyapunov-Based Control for Grid Side Inverters of Wind Turbine Systems
Submission-ID 134
Alexander Schöley, Magdalena Gierschner, Wolfgang Drewelow, Torsten Jeinsch
University of Rostock, Germany
The control of the grid side inverter of a wind turbine system is addressed in this paper. The main objective of the grid side converter is to feed an appropriate amount of electrical power into the grid in order to keep the DC-link voltage constant. At first, a nonlinear model of the inverter is developed in the ABC-frame consisting of the DC-link circuit, the LCL output filter and the phase-locked loop (PLL). Then a control strategy is proposed that is based on the Lyapunov stability theory. Contrary to the well-known VOC control scheme, the new strategy does not rely on Park's Transformation. Two superordinated PI-controllers are added to the Lyapunov control law to track the reference values for active and reactive power as well as to cope for uncertainties. The designed controller is tested in simulations with varying references and measurement noise.

Synchronous Condensers Applications in Transmission Network with Power Electronics Based Devices
Submission-ID 135
Adham Atallah
Siemens AG Power Technologies International, Germany
Keywords: transmission networks, synchronous condensers, FACTS, short-circuit power, rate of change of frequency, Classic HVDC

In the last couple of decades transmission systems around the world have been undergoing a significant change in their structure and constituents fueled mainly by the increasing penetration of renewable energy sources (RES). RES generation in most of its forms is based on power electronics devices that electrically decouple the mechanical component from the transmission system. Simultaneously RES power plants are located in remote areas far from the existing transmission network infrastructure where transferring the generated power to load centers dictates the use of direct current power electronics based (DC) technologies.

The increasing number of power electronics based generation and transmission devices is depriving transmission networks from the support needed from conventional generation and transmission networks. Short-circuit power levels and rate-of-change-of-frequency are two metrics for transmission network strength which are deteriorating as the number of connected power electronics based devices increases.

Synchronous condensers (SynCons) were previously utilized in the dawn of alternate current (AC) transmission systems as a variable reactive power source prior to the introduction of power electronics FACTS devices such as SVCs and STATCOM.

Nowadays SynCons are back in focus as they provide a much needed support to transmission networks with an outlook of decreasing conventional generation and transmission, increasing RES integration and power electronics devices.

In this paper an overview on the real life applications of SynCons in transmission networks with power electronics based devices is presented. The applications range from supporting transmission networks with high penetration level of RES with increasing the short-circuit level as well as stiffening the frequency volatility. For transmission networks with classical DC transmission technology transmission in weak networks SynCons provide the required support to avoid commutation failure.

Meteorological Categorization of Wind Power Ramp Events - Case Study of Three Areas of Japan -
Submission-ID 136
Maki OKADA 1, Koji YAMAGUCHI 1, Ryo KODAMA 1, Norimitsu OGASAWARA 1, Kazuhiko OGIMOTO 2
1 Japan Weather Association, Japan
2 University of Tokyo, Japan
Although renewable energies such as wind power continue to spread globally, electrical output from wind power fluctuates constantly with wind changes. Thus, the installation of large wind power capacities tends to result in negative impacts on grid stability. In particular, sudden wind power changes (wind power ramps) have significant effects, and making the forecasting of wind power ramps is one of the major issues in wind power usage. Since a wind power ramp is most commonly associated with abrupt wind changes, knowledge about the connections between power output changes and weather conditions has the potential to contribute to the development of wind power ramp forecasting. Accordingly, this study meteorologically categorized wind power ramp events occurring in Japan. Occurrence tendency of ramp events and weather patterns that cause ramp events will be introduced. We used total power output, which interval was 10-s, in three electrical power areas (Hokkaido, Tohoku, and Tokyo) collected by a project of the New Energy and Industrial Technology Development Organization (NEDO). The total power output data were normalized by the total output capacity (PU) because the total output for each area were different. A wind power ramp was defined as a 30% or more fluctuation in the total output capacity between the start and end times, and an average fluctuation rate of 5% / h or more during that time period [Kataoka et al., 2013, Proc. of 2013 Annual Conference of Power & Energy Society, IEE Japan]. Weather patterns for each power areas in 2016 was categorized by weather forecasters, who focused on the track, location, and strength of low- and high-pressure systems in the surface weather chart. For example, a weather pattern similar to “winter monsoon pattern” occurred frequently in Hokkaido from October to April. Here, “winter monsoon pattern” means a distribution of sea level pressure in which the high-pressure system is to the west and the low-pressure system is to the east of Japan. In this categorization type, as the low-pressure system developed and was moving to the east of the Hokkaido, the interval between constant-pressure bars narrowed. Then, the north wind gradually strengthened, and resulted in a sudden increase of wind power (ramp-up). Other categorized results will be presented.

Ancillary Services and its role in grid balancing in India
Submission-ID 138
Sanjay Bhagat
Ministry of New and Renewable Energy, Government of India, India
Ancillary Services and its Role in Grid Balancing in India

As India plans big for the integration of renewable energy in the electricity grid, there is an opportunity for the development of Ancillary Services market in India. Currently, India has approximately 14% of total installed capacity from Renewable Energy. Target is set for approximately 40% of clean energy by 2030 as per Intended Nationally Determined Contributions (INDCs) of Paris Climate Pact. This target will require a whole new level of electricity services moving from just a market for power trading to a market of electricity services, capacity market, and demand management. If this target of the Indian government is to be realised, the development of ancillary services is the most crucial. This paper highlights the efforts of Indian Government in terms of making Ancillary Services operational for grid balancing in India, it also provides policy interventions required for development of Ancillary market in India.

Japan's R&D Project of Ramp Forecasting Technology: Metrics for Evaluating Ramp Forecast
Submission-ID 140
Kenji Yoshida 1, Nobuyuki Hayasaki 1, Naoto Ushigami 1, Norimitsu Ogasawara 2, Maki Okada 2, Yuko Nakamura 2
1 ITOCHU Techno-Solutions Corporation, Japan
2 Japan Weather Association, Japan
In the R&D project of the New Energy and Industrial Technology Development Organization in Japan, the authors participate in the group focused on forecasting of wind generation ramps in eastern Japan. In the group, an enhanced technology to forecast wind power output ramps is being developed. To evaluate the ramp forecasting performance of developed technologies, we select various metrics suitable for each forecast output (wind power time series, prediction intervals, ramp forecast and ramp probability forecast). Critical Success Index (CSI) for ramp forecast is defined as the primary metric in the project. We conduct ramp forecasting by the existing and developed technologies over the past period and compare their ramp forecasting performance. The developed technologies show better ramp forecasting performance than the existing technologies.

North Sea offshore Grid development: Combined optimization of grid and generation investments towards 2050
Submission-ID 142
Matti Koivisto 1, Juan Gea-Bermudez 2, Poul Sørensen 1
1 DTU Wind Energy, Denmark
2 DTU Management Engineering, Denmark
The North Sea area has already significant offshore wind generation, and it offers large amounts of investable generation with high capacity factors (CFs) in the future. In addition, several transmission lines are located in the area. This paper models optimal transmission and generation investments in scenarios towards 2050, and investigates the viability of connecting future transmission and offshore wind generation investments to create an offshore grid.

A baseline scenario is created using the traditional solution of connecting offshore wind power plants (OWPPs) directly to shore and using country-to-country lines for transmission (radial case). Results from the integrated approach of combining offshore wind generation and transmission (meshed case) are then compared to the radial case. The modelling is carried out using the Balmorel energy system model. The countries with investment optimisation are Denmark, Norway, Germany, UK, Netherlands and Belgium. Surrounding countries participate in the electricity dispatch optimisation.

The benefits of a meshed North Sea offshore grid have been shown in previous research. In many studies, the generation investments were fixed parameters. This study contributes to the literature by allowing both generation and transmission investments to be optimized by Balmorel. In addition, scenario years 2030 and 2050 are optimized concurrently, which allows optimal investments in 2030 considering planned investments in the future.

To compare offshore wind to other renewables, onshore wind and solar photovoltaic (PV) investments are optimized simultaneously in the model. Other generation types are also part of the optimization, but only some gas turbine investments are seen necessary as back-up capacity.

DTU Wind Energy’s CorRES tool is used to simulate the wind and solar PV generation time series used in Balmorel modelling. CorRES models the varying CFs depending on installation locations, and the spatiotemporal dependencies in VRE generation. Especially offshore wind is modelled in detail, starting from the planned locations of individual OWPPs.

All analysed scenarios show significant transmission investments, with increased connection of Norwegian hydro generation to the other countries. The results indicate that going to a meshed solution can increase the total offshore wind investments by several GW. In addition to offshore wind, the scenarios include large amounts of onshore wind and solar PV. A large share of country-to-country transmission is provided by radial transmission lines also in the meshed case. However, the presented analyses show that integrating the offshore hubs as part of the transmission system can be beneficial.

Modeling the dynamics and control of power systems with high share of renewable energies
Submission-ID 143
Sabine Auer, Tim Kittel
Potsdam Institute for Climate Impact Research (PIK), Germany
A challenge for renewable and hybrid power systems is the dynamically stable integration of Renewable Energy Sources (RES). This paper specifically investigates the influence of intermittent RES and measurement delays from power electronic resources on frequency stability. In addition it presents an Open-Source framework to undertake dynamic RES modeling.

First, for local intermittent fluctuations in lossy distribution grids we find a remarkable and subtle but robust interplay of dynamical and topological properties, which is largely absent for lossless grids.

Second, we show how delays from measurement and reaction times of power electronic devices may induce resonance catastrophes in power networks.

Third, the above research topics illustrate the necessity to transition to a new open-source software framework for dynamic power grid modeling. For this we want to present PowerDynamics.jl, which is in the process of being published in the programming language Julia. It will cover the rich novel dynamics caused by the integration of RES.

Altogether, this paper investigates the stability of future power grids moving towards integrating more aspects of renewable energy dynamics and presents an adequate modeling framework for RES integration studies.

Onshore Grid Frequency Control using DC Capacitor in Full-Scale Converter for Offshore Wind Generator and Adjustable Speed Motor for Offshore Plant Connected by Multi-Terminal HVDC
Submission-ID 144
Hiroshi Matsuda, Yutaka Ota, Tatsuhito Nakajima
Tokyo City University, Japan
Renewable energy resources (RERs) are being widely introduced recently. A number of offshore wind farms are being commissioned in the world. R&Ds on multi-terminal HVDC (MT-HVDC) links for connecting between onshore power grids and offshore wind farms are being conducted in research projects. The offshore MT-HVDC will be applied not only for power transfer from the wind farms, but also for power supply to offshore plants such as oil or natural gas platforms. With increase in the RERs, on the other hand, conventional thermal power stations using synchronous generators will be out of operation in the future. Power system inertia will be decreased, and that will make the grid frequency more fluctuate consequently. Effective countermeasures must be established for keeping power grid stability.

This paper proposes a new control method for supplementary supporting the onshore grid frequency, using both the dc capacitor in a full-scale converter (FSC) for the offshore wind generator, and an adjustable-speed induction motor drive (ASMD) for the offshore plant connected to the HVDC link. In the proposed method, when a disturbance occurs in the onshore grid, in accordance to the frequency signal transmitted from the onshore grid, the generated power of wind generator system is controlled by making the FSC dc capacitor charge or discharge. The consumed power of the ASMD is also controlled by changing the motor speed using the motor-driving inverter.

In the FSC for the wind generator, a frequency droop for the grid frequency deviation is added to the dc voltage control block. When the grid frequency is decreased, the dc voltage is changed slightly lower, and the dc capacitor is discharged for the frequency support. In the ASMD system, the grid frequency deviation is converted to a change in the motor consuming power reference, based on the grid power frequency constant. It is assumed in the paper that the motor has a pump load, of which torque is proportional to the square of the motor speed, and thus the motor consuming power is approximately proportional to the third power of the motor speed. The required motor speed and the required driving-inverter frequency are obtained. When the grid frequency is decreased, the motor speed is regulated slightly lower, and the motor inertia energy is released for the frequency support.

The proposed control method was validated by PSCAD simulation case studies. The simulation model consists of a four-machine grid, a three-terminal HVDC link, one-machine wind generator with one FSC, and a ASMD with a inverter. The grid disturbance is made by connecting a load to the grid. The simulation results show that, during the grid disturbance, power is released or absorbed by both the dc capacitor and induction motor, without any control interference problem, through HVDC. The results also prove that the grid frequency nadir is improved to be higher, and the oscillated grid frequency is damped effectively.

Japan's R&D Project of Ramp Forecasting Technology: A Forecast Integration Method
Submission-ID 145
Tomoya Takeuchi 1, Yoshito Hirata 2, Shunsuke Horai 3, Kazuyuki Aihara 3
1 Graduate School of Mathematical Sciences, the University of Tokyo, Japan
2 Mathematics and Informatics Center, the University of Tokyo, Japan
3 Institute of Industrial Science, the University of Tokyo, Japan
We need to reduce the CO2 emission for overcoming the global warming. One of its solutions is to introduce more renewable energy. This trend is now worldwide. Japan is not an exception. Introducing more renewable energy into power grids is, however, a hard task because renewable energy such as wind powers intensively fluctuates due to the weather conditions. To mitigate the fluctuations, many researchers and practitioners are trying to predict the natural fluctuations of wind powers, especially ramps, which are sudden increases or decreases of wind powers. Our project titled “Grid Integration of Variable Renewable Energy: Mitigation Technologies on Output Fluctuations of Renewable Energy Generations in Power Grid,” supported by the NEDO, Japan, is one of such projects focusing on forecasting of wind generation ramps. However, we believe that our project is unique because (i) we prepare a set of individual forecasts produced by different methods and (ii) combine and integrate them so that we can enjoy the merits for preparing multiple forecasts simultaneously. Specifically, we developed an ensemble method to integrate a collection of time series predictions with different structures such as the time of delivery, the time resolution, the updating frequency and the lead time. The proposed method is an extension of the expert advice, which is an online ensemble technique that produces a weighted average of multiple forecasts, where the weights are computed by the normalized exponential function of a constant multiple of a user-defined cumulative loss function with a discounting parameter. The standard expert advice selects these parameters based on the regret analysis which imposes a strong condition on the loss function, while our method determines the weights by an optimization method without imposing any restriction on the loss function. The approach enables us to introduce a customized loss function that captures the nature of the target time series. We show that our integration method produces a single prediction that results in better performance over other individual forecasts.

A dispatch methodology to secure power system inertia in future power systems
Submission-ID 146
Henning Thiesen, Clemens Jauch
Flensburg University of Applied Sciences - Wind Energy Technology Institute, Germany
Power system inertia is a fundamental part of grid frequency control. Present power markets do not take account of a sufficiently high amount of power system inertia. In this paper an extension for the conventional economical dispatch is presented, which can be applied once the power system falls of a certain level of power system inertia. Non-inertia providing generators of the merit order are replaced with synchronous generators. A new market equilibrium is achieved under the condition of sufficient inertia. Additionally, this novel approach allows for a quantification of system costs for additional inertia due to the application of the market extension. Certain impacts on the instrument like external costs, inertia from loads and synthetic inertia are discussed.

Eigenvalue-based Stability Analysis of Sub-synchronous Oscillation in an Offshore WPP
Submission-ID 148
Lei Shuai 1, Ranjan Sharma 1, Syed Pirzada 2
1 Siemens Gamesa Renewable Energy A/S, Denmark
2 Siemens Gamesa Renewable Energy Limited, United Kingdom
With increasing penetration of renewable energy such as wind and solar as well as widely spread application of HDVC and FACTS system, the conventional power system has been undergoing a transition towards a complex power converter-based power system. This introduces opportunities but also challenges for safe and reliable operation of today’s power systems, as the stability operation greatly depends on the control and electrical design of both wind turbines (WTs) and wind power plants (WPPs).

A typical WPP today consists of power converter-based WTs, MV array cables, park transformers and HVAC cables as well as various shunt units for harmonic filtering and reactive power compensation etc. The resonances created by such complex electrical systems may be interacting with feedback control of power converter-based WTs, which could lead to instability at worst. Especially in a weak-grid system, the interaction between WTs and grid system are so strong that the stability of such a system shall be carefully evaluated to ensure reliable operation under all circumstances.

A sub-synchronous oscillation occurred in one of offshore WPPs when one of the 2 exporting offshore cables was taken out. Under such a contingency operation it was observed that a sub-synchronous oscillation with frequency of around 8.5Hz gradually built up when total active power production was increased to certain level. An investigation was initiated to replicate this oscillation case in the simulation environment by modelling the whole system from WTs to external grid based on small-signal state-space matrix approach. The results show precise replication of this sub-synchronous oscillation in simulations when similar active power level was reached. An eigenvalue-based small-signal approach clearly shows trajectory of critical poles moving from left-half-plane towards imaginary axis when the total active power and the number of connected WTs is increasing. With the proven validity of modelling, a number of power converter control parameters were tuned to stabilize the system in such a contingency operation. Later on site tests were carried out to confirm the validity of new control parameters.

This paper starts to describe sub-synchronous oscillation event in this offshore WPP. Following, the modelling of WTs and WPPs is introduced to replicate the oscillations in simulations based on small-signal state-space matrix approach. The eigenvalue-based stability analysis is employed to graphically show critical poles movement. PSCAD simulations were further taken to cross check the results. Similar conclusions can also be drawn. Based on the eigenvalues approach the critical poles were ‘relocated’ to a safe distance from imaginary axis by tuning a number of converter control parameters. Site tests results were shown to confirm the stability of the WPP under such a contingency situation.

Investment Analysis on Transmission Lines using TIMES-JMRT Grid Model under a Scenario with large amount of Renewable
Submission-ID 149
Yoh Yasuda 1, Hiroshi Hamasaki 2
1 Kyoto University, Japan
2 Fujitsu Research Institute, Japan
"The fifth energy master plan" is now under discussion in Japan after relese of the forth plan in 2014. The renewable target in 2030 is being kept in 21-23% as low as the previous plan, in which the target of VRE (Variable Renewable Energy) is only 9% (7% for PV and 1.7% for wind).

The low trather than ambitious arget by Japan Government is virtually a "cap" of renewable promotion in Jpan. Very few reserch and analysis on more "ambitoius" target than that cannot seen in Japan. Cost-benefit analysis on renewables and grid expansion has not active very much.

This paper discuss investment analysis on grid expasion in Japan under a ambitious scenario with 50% share of wind and PV in 2050. JMRT-gird model based on TIMES, which was develped by one of authors(HH) are chosen to describe future power grid and renewable deployment in Japan.

After the optimal analysis by the JMRT-grid model, it became clear that investment to grid expecially in Northn Japan (Hokkaido and Tohoku areas) can be befitable with 0.030 Yen/kWh of grid investiment cost and –0.383 Yen/kWh reduction in wholesale market price in 2030 and 0.092 Yen/kWh of cast and –1.597Yen/kWh reduction.

Analysis of Power System Oscillation Stability with Large Integration of Renewable Generations
Submission-ID 150
Lijun Cai 1, Harald Weber 1, Hans-Günter Eckel 1, Ulas Karaagac 2, Jean Mahseredjian 3
1 Institute of Electrical Power Engineering, University of Rostock, Germany
2 Department of Electrical Engineering, Hong Kong Polytechnic University, Hong Kong
3 Department of Electrical Engineering, Polytechnique Montréal, Canada
Key words: renewable generation; electro-mechanical oscillation; angle stability.


Electromechanical oscillations in power systems are classified by the system components that they affect and they are mainly the result of the interaction of generation units. Different modes of electromechanical oscillations could occur in power systems: i.e. intra-plant mode, local mode, inter-area mode, control model and torsional mode. The oscillation modes are strongly dependent on the characteristics of the generation units, the network topology and the power flow scenario. To ensure stabile operation of power systems, oscillation modes must present an acceptable damping, so that those oscillations are well damped after small perturbations. The classical method for analyzing oscillation stability is the modal analysis and it is well approved in the large power systems with synchronous generators.

Recently, with the rapid development of renewable energies in the power systems, converters that interface with renewable generations are becoming more popular. Due to the new characteristics of these renewable generations, power systems will experience changes in oscillatory dynamics due to the following reasons:

- Changes in the architecture of transmission system to connect wind farms;

- Replacement part of the synchronous generators by large renewable generations;

- Alteration in the dispatch of synchronous generators in order to meet the strongly varying renewable generations and

- The reduction of the power system inertia due to the large amounts of renewable generations.

All these changes will result in the change of the synchronizing and damping behavior of the power system. Furthermore, the modal analysis could not be directly applicable to the renewable generations, since these generations partially (i.e. doubly-fed induction generator wind turbines) or fully (i.e. full converter wind turbines) decouple the mechanical side of generators to the grid by the converters. Therefore, the dynamic interaction between renewable generations and synchronous generators should be considered in detail.

This paper analyzes electro-mechanical oscillations in a power system with large integration of renewable generations. After system disturbances, oscillatory modes are analyzed. Also, the main factors that influence the damping of the oscillation modes are given.

Furthermore, different scenarios are considered (i.e. different active and reactive power exchange between wind farms and synchronous generator) to analyze their effects on oscillatory behavior.

Finally, measures for damping of the electro-mechanical oscillations will be given.

Assessment of a potential hybrid system between offshore wind and tidal park
Submission-ID 151
Christina Merkai 1, 2, Lara Pérez Andrés 1, Per Holmberg 1
1 Vattenfall, Sweden
2 KTH Royal Institute of Technology, Sweden
Assessment of a potential hybrid system between offshore wind and tidal park

Christina Merkai, Lara Pérez Andrés, Per Holmberg

Vattenfall R&D, Sweden

KTH Royal Institute of Technology

Offshore wind has proven to be one of the most reliable and cleanest energy sources over the last few years. The industry has experienced a significant growth, with an increase of 101% only in 2017 compared to 2016. This raises the importance of the need for more secure power supply systems, which are used for controlling the offshore wind farms during network disconnections. Nowadays, diesel generators are employed to feed auxiliary services of the offshore wind turbines in situations of disconnections from the grid. However, this solution is not environmentally friendly and it is highly dependent on the diesel price fluctuations. Therefore, other alternatives are being investigated.

As the marine renewable energy industry evolves, tidal devices have the ability to replace diesel generators and provide a more sustainable and eco-friendly solution for a long-term back-up energy. Due to its high predictability, tidal power output can be calculated beforehand and used respectively. Moreover, the tidal devices, independent of their size, have the potential to produce large amounts of power due to the water’s high density, which can be either stored in a battery for future use or linked directly to distribution. The paper presents a technical, financial and environmental assessment for a potential hybrid system between offshore wind farms and tidal parks. The different device parameters, maturity levels and connection possibilities are taken into consideration. A comparison with alternative sources for emergency power supply is also performed for more accurate results. Possible locations for the implementation of this solution are areas with high wind speeds and optimal tidal conditions. Such areas are identified in North-West Europe, and specifically the U.K. and are evaluated with the use of ArcGIS maps and other accessible marine data.

Grid Code Development for Wind Power Integration in China and Electrical Simulation Model Validation
Submission-ID 153
Lei Shuai 1, Yongning Chi 2, Frank Martin 1, Junwei Shi 3, Zhiquan Zhu 3, Haiyan Tang 2
1 Siemens Gamesa Renewable Energy A/S, Denmark
2 Renewable Energy Research Center, China Electric Power Research Institute, China
3 Shanghai Electric Wind Power Group Co., Ltd., China
Cumulative wind power in China has reached unprecedented 35% market share globally even with reduced pace of installation in 2017 -- ‘only’ 19.5GW. Along with impressive growing capacity of wind power grid curtailment still remains a big challenge in the wind power market in China. With the national average of grid curtailment at 17% in 2016, the curtailed electricity nearly cancels out the new installation in 2016. To address the grid curtailment challenge there are a number of solutions which has been proposed, including establishing long-distance HV DC/AC transmission line, promoting distributed renewable energy and increasing share of natural gas and storage etc. As a consequence the Chinese power grid is facing new challenges from a centralized power grid towards more distributed power grid with fast-increasing share of renewable energy.

Fault-Ride-Through (FRT) capability has become a basic requirement for Wind Power Plant (WPP) in China since 2011. But it has been mainly addressing Under-Voltage-Ride-Through (UVRT) requirement. However the application of HVDC solution together with long AC cables in WPPs has imposed higher requirement for Over-Voltage-Ride-Through capability (OVRT). Another challenge brought by distributed wind power is how to ensure stability of WPP in a relative weak grid system as well as maintaining power quality for end-users. Besides others, challenges like frequency support including inertial response, have also been discussed and addressed in the upcoming new revision of grid code of China.

During the planning stage of a new WPP project power system simulation studies are often carried out to ensure the fulfillment of requirements of grid code as well as system stability and robustness under contingency circumstances. The validity of electrical simulation model provided by OEM’s is essential to trusted simulation results. In China NB/T 31066-2015 is the technical guideline for electrical simulation model development and validation. The validation process is to compare simulation results and field measurement results in pre-defined FRT cases by taking active power level, type of faults, and severity of fault voltage, fault duration etc., into considerations. SGRE has partnered with SEwind to provide large offshore wind turbines for Chinese offshore market. The offshore turbine product SG-4MW electrical simulation model has been validated against field measurement based on NB/T 31066-2015. The results are presented in this paper.

This Paper is to firstly address the challenges in Chinese grid integration with fast-growing wind power, and discuss the upcoming new requirements in the new version of grid code. A practical case of model validation with SG-4MW electrical simulation model is provided. Process and procedure for electrical simulation model validation in China are introduced and discussed. The results show relative good match between simulation results against field measurement.

Understanding Uncertainty: the difficult move from a deterministic to a probabilistic world
Submission-ID 155
Corinna Möhrlen 1, Ricardo Bessa 2
1 WEPROG, Germany
2 INESC TEC, Germany
Forecast methodologies have advanced over the past 20 years along side the needs of system operation and trading of energy at the power exchange markets. Like every discipline in development, also forecasting of renewable generation has evolved disruptive and chaotic at times when new ways to handle these variable sources were sought.

The forecasts of these sources inherit an uncertainty in their operation due to the uncertainty of the underlying weather forecast. Once these uncertainties are understood the future outcome at the time scale required to operate our electric grids and trade the energy on our power exchanges can be forecasted much more efficient than with deterministic methods. Uncertainty forecasts are filling a gap of information missing in deterministic approaches and are gradually moving into the control rooms and trading floors.

Nevertheless, there are a number of barriers in the industrial adaptation of uncertainty forecasts that have their root in a lack of understanding of the methodologies and their respective applicability. There is a complication level that needs to be overcome in order to move forward. The IEA Wind Task 36 has been carrying out a number of expert round discussions picking up a number of the loose ends of integration and application issues. The applications presently used in industry, suggestions how to apply and integrate uncertainty forecasts into operation and an outlook from this discussion are presented and discussed in this paper.

Japan's R&D Project of Ramp Forecasting Technology: A Machine Learning Scheme for Ramp Forecast
Submission-ID 156
Yu Fujimoto, Kazutoshi Higashiyama, Yasuhiro Hayashi
Waseda University, Japan
In Japan, the R&D project “Grid Integration of Variable Renewable Energy: Mitigation Technologies on Output Fluctuations of Renewable Energy Generations in Power Grid” promoted by the New Energy and Industrial Technology Development Organization (NEDO) has been started since 2014. Forecast of ramp events in wind generation is one of important topics of technical development in this project; this topic aims to break through difficulties in forecast of largescale changes within a short time in wind generation. The authors study an applicability of machine learning methodology to the ramp forecast of wind generation and implemented a hierarchical forecast scheme composed of the wind power predictor and the ramp event predictor in this project. This paper shows supervised setups for learning predictors and discuss the difficulties in these learning schemes.

Submission-ID 160
Corinna Möhrlen 1, Jeffrey Lerner 2, Jethro Browell 4, Craig Collier 7, Gregor Giebel 8, John Zack 6, Jakob Messner 3, Aidan Tuohy 5
1 WEPROG, Denmark
2 Vaisala, United States
3 Anemos Analytics, Denmark
4 University of Strathclyde, United Kingdom
5 EPRI, United States
6 AWS Truepower, United States
7 DNVGL, United States
8 DTU Wind Energy, Denmark
In this paper, we summarize the second and third part of a series of three recommended practice documents for the power industry that deal with how to setup and run a trial or benchmark as well as verifying the goodness of forecast solutions.

The Recommended Practice is intended to serve as a set of standards that provide guidance for private industry, academics and government for the process of obtaining an optimal forecast solution for specific applications as well as the ongoing evaluation of the performance of the solution to increase the probability that it continues to be an optimal solution as forecast technology evolves. The work is part of the IEA Wind Task 36 on Wind Power Forecasting.

The guideline provides an overview of the factors that should be addressed when conducting a benchmark or trial and present the key issues that should be considered in the design as well as describe the characteristics of a successful trial/benchmark. We also discuss how to execute an effective benchmark or trial and specify common pitfalls that a Forecast User should try to avoid.

Part 3 of the recommended practices guideline deals with the effective evaluation and verification of forecasting solutions, benchmarks and trials. The core of any effective evaluation and verification is ``fairness'', ``repeatability'' and ``representativeness.'' The evaluation paradigm is another aspect that needs consideration. Accuracy metrics need to be weighed versus the value of a solution, benefits of blended forecasts versus strategic forecasts, and how to verify complex solutions that feed into various processes inside an organisation. Recommendations on the design and execution of incentive schemes, their pros and cons for the development and improvement of forecast solutions is also part of the guideline and will be presented and discussed briefly.

On-line Markov Chain Based Thermal Risk Estimation for Offshore Wind Farm Cables
Submission-ID 163
Maria Angelica Hernandez Colin, James Pilgrim
Electronics and Computer Science (ECS), University of Southampton, United Kingdom
This paper proposes the use of a Markov Chain and Monte Carlo Analysis to estimate likely thermal risk of cable overheating in submarine export cables 6 hours ahead. The methodology can account for the uncertainty in the load current transferred by offshore cables which is the product of wind variations. The estimated conductor temperature and thermal risks calculated by the methodology shown a close resemblance to the real conductor temperature and thus a high percentage of successful thermal risk estimations 6 hours ahead of up 98% over one year of testing was estimated.

Analysis of Harmonic Aggregation in Wind Power Plants Based on Phase Angle Measurements and Modeling
Submission-ID 164
Tonny Rasmussen 1, Emerson Guest 2, Lei Shuai 2, Łukasz Kocewiak 3
1 The Technical University of Denmark, Denmark
2 Siemens Gamesa Renewable Energy, Denmark
3 Ørsted, Denmark
Nowadays, the number of wind turbines in offshore wind power plants is growing. Furthermore, the complexity off offshore electrical infrastructure is also increasing. This creates challenges in relation to harmonic generation from wind turbines, propagation through resonance circuits and distortion at the point of evaluation. Therefore, the assessment of harmonic disturbance from offshore wind power plants is becoming an important task.

The estimation of harmonic current and voltage levels at the grid connection point is typically performed in harmonic studies. This requires detailed and accurate modelling of each wind power plant component such as cables, transformers, converters. The studies are done to control the harmonic levels to acceptable levels and to ensure harmonic stability within the whole electrical infrastructure as well as grid code compliance at the connection point. Therefore, it is important to predict the harmonic emission as accurate as possible to avoid harmonic underestimation leading to power quality and grid compliance issues as well as harmonic overestimation leading to unnecessary filtering and capital expenditure increase.

Typically, the harmonic summation is performed based on the summation law given in IEC 61000-3-6, where assumptions are made about the phase angle through a summation factor coefficient. However, the actual harmonic phase angle is not considered in the standard, which is necessary to be included for accurate assessment of the harmonic emission. The state-of-the-art knowledge of harmonic phase in wind turbines is quite limited. Therefore, extensive phase-aligned measurements using GPS-disciplined timebase were done at Avedøre Holme offshore wind power plant in Denmark. This creates a foundation to investigate harmonic emission aggregation based on phase angle measurements and modeling in wind power plants.

This paper presents the state-of-the-art results and review on harmonic aggregation considering type-4 wind turbines. The harmonic phase angle measurement procedure is not well specified in existing standards. The paper provides additional recommendations how to measure and process harmonic phase to fill the gap in standards. The harmonic phase data from GPS-synchronized measurements is compared with the summation law given in IEC 61000-3-6. Finally, the paper suggests guidelines for harmonics summation procedures to further improve the IEC summation method.

Analysis of the Flicker Estimation at PCC of a Wind Power Plant
Submission-ID 166
Koldo Redondo, Izaskun Azcarate, José Julio Gutierrez, Luis Alberto Leturiondo, Purificación Saiz
Department of Communications Engineering University of the Basque Country (UPV/EHU), Spain
In terms of power quality, the measurement of the flicker emission of a wind power plant is an important issue. The procedure to measure the flicker produce exclusively by a grid-connected wind turbine is specified in the IEC 61400-21 standard. The standard also proposes a summation method to estimate the flicker produced by a wind power plant at the point of common coupling. This work studies the reliability of the summation method, based on actual current and voltage waveforms recorded at a wind power plant in Spain. The results show that the summation method significantly overestimates the flicker emission of the wind power plant.

Japan’s R&D Project of Ramp Forecasting Technology: Project Overview
Submission-ID 168
Takahiro Suga 1, Nobuyuki Hayasaki 1, Kazuhiko Ogimoto 2
1 ITOCHU Techno-Solutions Corporation, Japan
2 The University of Tokyo, Japan
Renewable generation has been increasingly deployed under the global trend of reduction of CO2 emission. It is necessary to use more enhanced forecast and control technologies to integrate them in power systems operation planning and real-time operation in order to increase the use of renewable energy. This is the objective of the R&D project “Grid Integration of Variable Renewable Energy: Mitigation Technologies on Output Fluctuations of Renewable Energy Generations in Power Grid”, which has been going on since 2014. We aim to break through the important issue regarding the forecast error of wind power output: that it is too large to properly estimate up/down ramps within short time. This project focused on wind power ramp forecasting have not tackled yet even though it is important issue to forecast wind power ramp phenomena. This paper shows an overview of the Project.

Grid Forming Control for Stable Power Systems with up to 100 % Inverter Based Generation: A Paradigm Scenario Using the IEEE 118-Bus System
Submission-ID 172
Mario Ndreko, Sven Rüberg, Wilhelm Winter
TenneT TSO GmbH, Germany
This paper presents a grid forming control for grid connected inverter based renewable-generation and grid connected storage units that could facilitate stable and robust operation of power systems with even up to 100 % inertia-less inverter based renewable generation. The proposed grid forming control directly regulates the voltage magnitude and the angle of the respective grid side inverter in the synchronous dq-reference frame. During severe three-phase grid faults, the control scheme provides current limitation capability and injection of reactive fault current using a modified current control block. EMT time domain simulations are performed on two different test systems. The first test case is on a simple inverter-based power plant connected to a small and weak grid (with very low short-circuit power ratio). The second test case is on the modified 118-bus system in which the penetration of inverter based generation connected to the 33-kV level may reach up to 100 %. For the latter case, grid faults are presented in order to demonstrate the stable response of the system. The paper concludes that bulk-transmission systems with high penetration of inverter based generation up to 100 % are stable when a portion of the inverter based active-power infeed units (renewable generation or grid connected storage) apply grid forming control scheme.

Methods to Aggregate Turbine and Network Impedance for Wind Farm System Resonance Analysis
Submission-ID 177
Haijiao Wang 1, Christoph Buchhagen 2, Marlien Greve 2, Jian Sun 3
1 China Electric Power Research Institute, China
2 TenneT, Germany
3 Rensselaer Polytechnic Institute, United States
Modeling of wind turbines and wind farms for system harmonic analysis has been discussed at the Wind Integration Workshop for a number of years. Initially, the attention focused on wind turbines as the “source” of harmonics and a number of papers presented methods to aggregate harmonic current emissions from individual turbines for the prediction of wind farm and power system harmonics. The effects of grid voltage background harmonics started to draw attention several years ago and new grid codes that take into account such background harmonics are under development. In more recent years, resonance has been identified as a major contributor to system harmonic problems: in addition to amplifying pre-existing harmonics, an under-damped resonance can also create new harmonics that otherwise don’t exist. Such resonance-created harmonics can reach very high level and are responsible for most harmonic incidents in practice.

We presented two papers at last year’s Wind Integration Workshop on the development of new grid codes and methods undertaken by two TSOs to support wind power system harmonic studies that focus on resonance. Since impedance is responsible for resonance in any electrical system, our emphasis was on practical methods to obtain impedance characteristics for wind turbines. The new grid code requires manufacturers to provide such impedance characteristics to the TSO for system resonance analysis. Control hardware-in-the-loop (CHIL) simulation was also presented as a practical method to obtain impedance characteristics for wind turbines and other types of grid-connected power electronic devices. This paper is a follow-up to those two papers and reports our joint effort to develop practical methods for system impedance modeling and resonance analysis at the farm level.

A typical wind farm involves a number of turbines and a medium voltage collection network using submarine cables or overhead distribution lines. Reactive power compensation devices such as reactors, capacitors, and STATCOM may also be used. There are a number of ways to build an impedance model that represents the entire wind farm seen from the grid interconnect point for system resonance analysis. The simplest method is to consider just one turbine (with its associated transformer and distribution lines/cables) and divide its impedance by the total number of turbines in operation; the most comprehensive method is to build a complete network model including each turbine represented by its own impedance and the impedances of different segments of distribution lines/cables; and there are a number of variations between these two with variable degree of accuracy and complexity. We will use detailed turbine and network circuit simulation as benchmark to compare the accuracy of different methods and the resulting system models. Different types of turbines will also be considered. The goal is identify a practical method that is simple to use while meeting accuracy requirements.

Optimal Allocation of Wind Power Considering its Contribution to Security of Supply
Submission-ID 178
Jakob Peter, Johannes Wagner
Institute of Energy Economics, Department of Economics, University of Cologne, Germany
Reliability of supply has due to its high economic value always been a major concern in electricity systems while ongoing climate change creates the need to switch to low-carbon electricity generation such as wind power with its fluctuating stochastically dependent electricity supply. Thus the following questions arise: What is the optimal allocation of wind capacity throughout Europe in order to benefit from balancing effects both in wind generation and equivalent firm capacity provision from wind, considering interconnection constraints between countries and resulting system effects?

Against this background, this paper builds on a new methodological approach which endogenously determines the contribution of wind power to security of supply in an optimization model for electricity markets. We deploy the capacity credit as a well-established measure to depict the contribution of fluctuating power generation to reliability of supply. In contrast to many existing modeling approaches, we use a capacity credit formulation which accounts for its dependence on the amount of installed wind capacity, its spatial distribution and the available interconnection capacity. Thereby, a more precise assessment of the contribution of wind power to security of supply is reached, allowing for an optimal system configuration to reach a required level of security of supply. In a next step, the proposed methodology builds on an iterative approach, which captures the non-linear dependency of the capacity credit of wind power on installed capacity and interconnection while keeping computational tractability in a large-scale application. We apply our methodology to the European electricity system to determine an optimal decarbonization pathway until 2050. We base the analysis on a new dataset which is based on meteorological reanalysis data and has a high spatial and temporal resolution, capturing the stochastic properties of wind power generation.

The analysis shows that wind power can substantially contribute to security of supply in a decarbonized European electricity system, with regional capacity credits ranging from 1 - 40%. Additionally the results show that the capacity credit of wind power depends on the specific wind properties in a country or region as well as the installed capacity of wind power and available interconnections to neighbouring countries. Consequently the capacity credit is heterogeneous across different regions and years. Existing modeling approaches, which typically assign constant values for the capacity credit of wind power therefore over- or underestimate back-up capacities, which are required to guarantee security of supply in an electricity systems with high shares of wind power. Especially if the potential of wind power to provide secure capacity is neglected in simulations, an inefficient allocation of wind capacity arises, leading to an overestimation of required back-up capacities and total system costs.

Energy Control of Modular Multilevel Converters in MTDC Grids for Wind Power Integration
Submission-ID 179
Kosei Shinoda 1, Ragavendran Ramachandran 1, Abdelkrim Benchaib 1, Jing Dai 1, 2, Bruno François 3, Seddik Bacha 1, 4, Xavier Guillaud 3
1 SuperGrid Institute SAS, France
2 Group of Electrical Engineering - Paris (GeePs), UMR CNRS 8507, CentraleSupélec, Univ. Paris-Sud, Université Paris-Saclay, Sorbonne Universites, UPMC Univ. Paris 06, France
3 Universite Lille, Centrale Lille, Arts et Metiers, HEI EA 2697 - L2EP, France
4 Universite Grenoble Alpes, CNRS, Grenoble INP, G2Elab, France

The role of Modular Multilevel Converters (MMCs) in HVDC grid greatly differs depending on whether it is an offshore or an onshore station. From the common point in their control schemes, an unexploited ability of the MMC—the controllability of the internally stored energy—is identified in both offshore and onshore applications. The virtual capacitor control, previously proposed by the authors, makes use of this degree of freedom to provide energy contribution to the DC grid. The impact of this control is demonstrated by time-domain simulations of a five-terminal HVDC grid.

Development of Wind Ramp Forecasting Technology in the National R&D Project (in Japan): Evaluation of Developed Forecasts by Power System Operation Simulation
Submission-ID 180
Yuki Nishitsuji 1, 2, Yusuke Udagawa 1, 2, Kazuhiko Ogimoto 1, Katsuyuki Ukegawa 2, Suguru Fukutome 3
1 the University of Tokyo, Japan
3 JP Business Service Corporation, Japan
Renewable Energy shows large grow in the global stream of reduction on CO2 emission. In Japan, a lot of deployment of renewable energy facilities has continued, boosted by the disastrous earthquake in 2011 damaging nuclear power plants, and FIT begun in 2012. Although penetration is dominated by photovoltaics at present, it is expected that wind power will raise its impact with the completion of environmental assessment and construction.

To ensure the security of efficient power system operation with high penetration of variable renewable energy, it is vital to enhance power generation forecast and control technology.

To tackle with this task, Japanese R&D project “Grid Integration of Variable Renewable Energy: Mitigation Technologies on Output Fluctuations of Renewable Energy Generations in Power Grid” was started in 2014. This project has four working groups, consisting of the group for development of forecast on wind power generation and on occurrence of steep ramp up/down of its output, mitigation technologies for power output fluctuation by energy storage, power supply-demand balancing simulation, and field experiment of developed technologies. This paper is written for reporting forecast evaluation using power system operation simulation in the working group of forecast development.

Developed forecasts are required to be beneficial to power system operation. To evaluate those forecasts from this viewpoint, we developed a simulation model of advanced power system operation which can utilize developed “ramp alert”, which is a binary signal, with developed wind power output forecast. Ramp alert can inform power system operators of the possibility of large total wind power output fluctuation exceeding a threshold in a balancing area. In addition to the conventional constraints such as supply-demand balance and reserve requirement for load frequency control (secondary reserve), our simulation has some constraints that require reserves to address steep ramp up/down of wind power output, when ramp alert is active.

Using this simulation, the investigation into the impact of developed ramp alerts and wind power output forecasts on power system operations is conducted, not by statistical criteria such as Root Mean Square Error, but by the economical indices such as operational costs and indices for supply security such as energy generated by the last resort generator. The details of simulation results, which describe how each generator deals with wind power forecast error, are also analyzed. Furthermore, we discuss the characteristics and qualities of developed ramp alerts and wind power output forecast associated with power system operations.

Experimental Results of a Wind Power Plant Scheduling Method Considering State-of-Charge Transition for an Electricity Market with the Compressed Air Energy Storage System
Submission-ID 181
Masakazu Ito 1, Aki Kikuchi 2, Yu Fujimoto 1, Masataka Mitsuoka 1, Hideo Ishii 1, Yasuhiro Hayashi 2
1 Advanced Collaborative Research Organization for Smart Society (ACROSS), Waseda University, Japan
2 Dept. of Electrical Engineering and Bioscience, Waseda University, Japan
Wind power plant (WPP) operators may have to consider selling electricity at an electricity market when the feed-in-tariff (FIT) term finish or the law of the FIT finish. In this situation, WPP operators may need to introduce energy storage systems (ESSs) to control amount of electricity to sell at the electricity market. The authors proposed the state-of-charge (SOC) control considering SOC transition (SCST) previously which enhance controllability of the SOC, and effectiveness of the ESS increase. To verify the effectiveness of the SCST, it has been mounted on an actual ESS. It is the Kawazu compressed air energy storage (CAES) system test site installed in Higashi-Izu beside of an actual WPP which is in the south of Tokyo. Then the experiment assuming an electricity market had been executed by using actual wind power output and the CAES system. As a result, imbalance between generation schedule and total output which is wind power output and CAES power output was less than 0.5% of the generation schedule while SOC kept between 0.0 to 1.0. Furthermore, post-experiment simulation had been executed to compare effectiveness of operation with and without the proposed method by using measured wind power output and wind generation forecast (WGF). In case of a variable wind speed day, imbalance without the proposed method was 6.9%, however the imbalance with the proposed SCST reduced to 6.0%. The result shows that the SCST can reduce imbalance and WPP operators can reduce imbalance fee, in consequence, the SCST increase WPP operator’s profit.

Evaluation of Recent Advancements in Machine Learning Methods in Very Short-term Time Series Forecasts of Wind Power Production
Submission-ID 184
John Zack
UL AWS Truepower, United States
The importance of very short-term (0-3 hrs ahead) forecasting of wind power production is growing as system operating procedures and market rules evolve to increase system flexibility and take advantage of the lower degree of variable generation uncertainty offered by very short look-ahead periods. Forecasts for very short-term look-ahead periods are characterized by a much heavier dependence on time series data from the generation facilities and perhaps nearby meteorological monitoring stations or atmospheric remote sensing devices than predictions for longer look-ahead periods, which typically employ data from physics-based Numerical Weather Prediction (NWP) models as the primary input information.

A time series modeling approach has been used for 0-3 hrs ahead wind power forecasts for many years. Traditional methods such as ARIMA schemes and linear regression have been the standard approach. In recent years, more sophisticated methods (often referred to as machine learning (ML) algorithms) based on decision-tree models, artificial neural networks and other modeling structures have come into widespread use. Data science technology is currently undergoing rapid development with more sophisticated and better performing methods appearing routinely. Research in the data science community has indicated that no single prediction algorithm has been demonstrated to achieve the best performance over a wide range of applications. Thus, it is left to those involved in each application to select and optimize the approach for that application.

A project is in progress at UL AWS Truepower to determine the range in performance of the newest machine learning methods and data pre-processing techniques for 0-3 hour ahead time-series-based wind power forecasting. The issues under investigation are the dependence of forecast performance on: (1) newest machine learning methods (e.g. XGBoost and CatBoost) vs. older ML methods and traditional time series models, (2) automated vs. manual meta-parameter optimization, (3) data sample size (length), (4) using new vs. traditional (e.g. PCA) data preprocessing techniques to condition the data for the training of the ML models, and (5) the magnitude and times scales of variability in the time series (i.e. location and seasonal dependence). The forecast performance is being evaluated from two perspectives: (1) typical performance over an entire test period as measured by traditional metrics (e.g. RMSE) and (2) performance for events of significance to grid operators such as large ramp events or outlier patterns (e.g. high volatility).

The presentation will include (1) an overview of the experimental design and the questions to be addressed, (2) the quantitative results that best address each key question and (3) a preliminary statement of what overall approaches are likely to yield optimal forecast results in different wind power forecasting settings.

Wind Farm Fault Ride Through – An Irish Context
Submission-ID 187
Aaron McDonnell, John Kelleher, Stephen Hunt, John Whelan
ESB International, Ireland
In 2017 Ireland installed 426 MW of new wind turbines and had an average power consumption of 3 GW. This made Ireland the European state with the highest level of installed wind capacity relative to its power consumption. Ireland had an annual penetration rate of 26 % from wind in 2017 and has a target of 40 % by 2020. Furthermore Ireland is trialling a System Non-Synchronous Penetration (SNSP) limit of 65 %. This has resulted in an increased focus on the ability of wind farms to contribute to system stability during system disturbances such as faults or other step changes in voltage. In this paper the key Fault Ride Through (FRT) requirements specified in the Irish Grid Code are outlined. The modelled performance of four wind farms connected to the Irish distribution system are compared against the clauses relevant to the FRT requirements. The wind farms in question comprise of turbines with different technologies and from different manufacturers, allowing for a comprehensive overview of the common clauses where compliance may not be fully achieved.

Sector-coupling eliminates the cost-benefit of transmission expansion in a highly renewable European energy scenario
Submission-ID 188
Tom Brown, Veit Hagenmeyer, Jonas Hörsch
Karlsruhe Institute of Technology, Germany
Coupling electricity to other energy demand sectors, such as heating and transport, and to other energy carriers, such as gas and heat, is a necessary part of reducing the use of fossil fuels in the energy system. This is driven partly by the large expandable potentials for wind and solar power, and partly by the high efficiency with which electricity can be used in, for example, electric vehicles and heat pumps. We consider the optimal investments and operation of an hourly, 200-node, sector-coupled model of the European energy system with zero net carbon dioxide emissions. The high spatial resolution of the model in comparison to previous studies allows us to assess in detail the cost-benefits of expansion of the electricity transmission network. While electricity-only studies typically show a high benefit from new transmission lines, the results in a sector-coupled model are quite different. We find that synthetic fuels produced with renewable electricity are required for some transport sectors, some industrial demand and in space heating to bridge cold spells in the winter when renewable generation is low. The large flexible demand of synthetic fuel production and its low-cost storage, coupled with demand-side management and cheap thermal energy storage in the heating sector, eliminates the economic benefit of grid expansion beyond today's capacities. This result has significant consequences for current debates surrounding the public acceptance of new transmission lines.

Dynamic Line Rating in Western Danish Transmission System: a Case Study
Submission-ID 189
Nicola Viafora 1, Joachim Holbøll 1, Rasmus Aabye Olsen 2, Anders Steen Kristensen 2
1 Technical University of Denmark (DTU), Denmark
2 Energinet, Denmark
Traditionally, overhead transmission lines are dimensioned in a very conservative way assuming worst case weather scenarios. However, during steady state operation of the power system, maximum temperature of components remains well below the allowed limits meaning that there is a substantial room for optimization [1]. In this regard, Dynamic Line Rating (DLR) offers an innovative method to operate the system by adjusting rating of components in real-time based on weather conditions. Since building new lines has become unpractical for environmental, social and economic reasons, DLR is an appealing solution for dealing with increased electricity demands, given its potential to increase power grid utilization. The value of DLR becomes apparent especially when considering the positive correlation with wind power generation as this implies that the extra transmission capacity is made available when needed. Studies on wind power integration such as [2] show how DLR could reduce the curtailed energy from wind power generation, increase the economic potential of wind farms and potentially postpone network reinforcement [3].

Faced by increasingly high wind power generation, the western Danish power system is expected to encounter many challenges in the upcoming years. Wind power infeeds from offshore installations would certainly stress the existing transmission network, unless remedial actions are planned in time. This has motivated the Danish TSO, Energinet, to investigate the value of DLR in facilitating wind power integration into the existing transmission network. Together with Ørsted and the Technical University of Denmark, Energinet has launched a joint research project aimed at exploring the use of DLR in the transmission system as a whole rather than on single lines.

This contribution will present the latest findings of the research project and compare them to the TSO experience with DLR. Several studies are being conducted in order to estimate the DLR potential on overhead lines throughout the seasons and accounting for different wind speed regimes. Simplified approaches developed by the TSO are compared with simulations based on Numerical Weather Prediction (NWP) models. The comparison highlights the potential that DLR has to unlock extra capacity and favour wind power integration on a wide scale. Simplified approaches under development at the TSO could help a gradual implementation of DLR and introduce more flexibility in line rating calculations.

[1] TWENTIES project, “Demonstration project 6 – Improving flexibility of the grid (FLEXGRID)”, October 2013 [2] C. J. Wallnerström, Y. Huang and L. Söder, "Impact From Dynamic Line Rating on Wind Power Integration," in IEEE Trans. on Smart Grid, vol. 6, no. 1, pp. 343-350, Jan. 2015. [3] A. McLaughlin, M. Alshamali, J. Colandairaj and S. Connor, "Application of Dynamic Line Rating to Defer Transmission Network Reinforcement due to Wind Generation,"UPEC, Soest, Germany, 2011, pp. 1-6.

Benchmark of Spatio-temporal Shortest-Term Wind Power Forecast Models
Submission-ID 190
Stephan Vogt, Axel Braun, Jan Dobschinski, Dominik Jost, Jonas Koch
Fraunhofer Institute for Energy Economics and Energy System Technology IEE, Germany
Many European energy supply systems are increasingly penetrated by wind energy. In order to be able to act optimally on the market or in the operation of electricity grids, it is necessary to have high-quality intraday forecasts of the expected wind power production. For this purpose, numerical weather forecasts and recent power measurements transmitted in real time are used. This provides a lot of information to the forecaster. It is, on the one hand, necessary to be able to decide which information are beneficial, and on the other hand, to be able to handle proper forecasting models. Suitable models to calculate wind power forecasts are power curve-based models and models from the field of statistics as well as machine learning models.

In this work we benchmark (first) different models ranging from power curve based to machine learning like random forests, artificial neural networks and extreme learning machines, and (second) the value of spatio-temporal information from surrounding wind parks .

A Machine Learning Approach to Low System Strength Grid Identification for Large Scale Power Systems
Submission-ID 191
Angel Clark 1, 2, Yang Zhang 3, Shun Hsien Huang 3, Le Xie 2
1 KTH, Sweden
2 Texas A&M University, United States
3 ERCOT, United States
This research proposes a screening method to identify low system strength, i.e. weak grid with low short circuit current level, portions of power systems where potential voltage instabilities could occur in a system with high penetration of inverter-based resources (IBRs), like wind and solar. The proposed method uses a random forest algorithm by clustering the transmission buses with interconnected IBRs based on features extracted from the short circuit current and electrical distance. This proposed screening method can be used as a screening tool to identify the system strength based on short circuit current and connected IBRs’ capacity without computationally intensive dynamic simulations. Therefore, the impact of system outages and different system scenarios can quickly be analyzed, with the resulting areas identified and quantified in each scenario. The developed method was applied to an ERCOT case under a developed long-term system condition and a Synthetic Texas Network, demonstrating the robustness of the screening tool and ability to identify the areas with low system strength challenges.

Validating wind turbine EMT model used for phase to earth fault UVRT simulation from field measurements and fulfilling grid connection requirements
Submission-ID 195
Pramod Ghimire, Frank Martin, Ranjan Sharma, Ireneusz Szczesny
Siemens Gamesa Renewable Energy, Denmark
One of the main requirements for the grid connection of a Wind Power Plant (WPP) is Under-Voltage Ride Through (UVRT) capability, wherein each WPP is obligated to stay connected and support active and reactive current at the Point of Connection (POC). As a proof of grid compliance, most of the grid codes require three phase and phase to phase fault tests to be conducted in the field and to be documented as a part of type test campaign. Some grid codes, such as the UK grid code, require additional phase to earth and two phases to earth test to be performed and documented as well.

Three phase and phase to phase UVRT tests are well known in comparison to earth fault tests. The phase to earth fault is dependent on zero sequence impedance of the network. If a network start point is isolated, meaning the zero sequence impedance is infinitely high, then 1 phase or 2 phases to earth fault events cause voltage on healthy phases to rise up to levels above 1.5 p.u. Such a situation might have negative impact on grid connected components; therefore many grid operators do not allow the earth fault tests to be conducted. The zero sequence current flowing through the ground during a test may cause step voltage around the tests equipment. In case of high resistivity of the ground, a person standing close to the test equipment could be at risk. Hence, the possibility to perform of earth fault tests is very limited.

An alternative way of demonstrating wind turbine UVRT response on earth faults is to provide simulation results of an EMT full order model, which consists of:

  • Wind Turbine
  • UVRT test unit
  • Grid

This paper introduces relevant aspects and information needed to be able to perform earth fault simulation using an EMT model (e.g. PSCAD). Simulation results are presented and compared against field tests results, performed on a Wind Turbine of the D8 platform at the Østerild Wind Turbine Test Center in Denmark. This article ends with an assessment of the model accuracy with respect to relevant national and international standards and outline the possible need for EMT model validation requirements as RMS model validation requirements as for example FGW TR4 and IEC 61400-27 may not be applicable. Conclusions and a future outlook are also provided.

Validation of an Experimental Test-bed System for Ancillary Services of Wind Power Plants
Submission-ID 196
Arne Kisser, Laila Rezai, Jens Fortmann
HTW Berlin - Universitiy of Applied Sciences, Germany
A test bed for the experimental verification of inverter control design of ancillary services for wind power plants is presented. Ancillary services get more important in a changing energy system with an increasing share of fluctuating renewable energies. Despite increasing cost pressures, new technologies must be developed to ensure a secure and stable power grid. Requirements that are set as default like fault ride-through capability, as well as new ones like reactive power supply, voltage controlled converter or new control algorithm need to be tested under realistic conditions. Simulation tools are used to design new features and show the basic functionality of the technology. Nevertheless, experimental investigations have to be done to validate the function.

The concept and first results of an experimental test-bed system, set up at the HTW Berlin has been presented in [1]. The set-up consists of three 11 kW wind turbines with fully rated converter and one 11 kW DFIG wind turbine. A 20 kW synchronous generator operates as a conventional power system. The producers are connected are through a 40 kVA power grid on 400 V level, with the ability to test different grid faults. The test-bed allows a flexible arrangement of network topologies, consumers and producers as well as fault cases.

This paper illustrates the possibilities of test scenarios and shows exemplary measurements. As the development and potential of new approaches is usually assessed through simulations, the simulation environment must represent the significant dynamics of the electrical grid in a detailed fashion. For that reason, the focus in this paper is the comparison between results from the simulation environment and test-bed measurements, such that the validity of the simulation tools can be verified.

[1] A. Kisser, M. Engel, L. Rezai, M. Andrejewski, J. Fortmann, H. Schulte. A Test-bed System for Validation of Ancillary Services of Wind Farms under Realistic Conditions. Wind Integration Workshop, 2017, Berlin

How to combine state-of-the-art multi-scale numerical wind power forecasts and benefits of a human meteorological expertise?
Submission-ID 197
Olivier VANNIER, Aurélien BEN DAOUD, Alexandre FALGON, Guillaume BONTRON
Compagnie Nationale du Rhône, France
As France’s leading producer of energy exclusively generated from renewable sources, Compagnie Nationale du Rhône (CNR) has the need to forecast intermittent energy production for its own assets as well as for those belonging to external producers. CNR has benefited from human expertise on hydrometeorological forecasts for a long time, and has now to face the challenge of keeping using its meteorological knowledge for wind power forecasting, in a context of constantly growing production capacity. This study presents the results of a live trial performed over three months, aiming at evaluating a potential new method of expertised wind power forecasts, based on daily confidence indices given by CNR meteorologists to different Numerical Weather Prediction (NWP) models for the meteorological situation at hand. During these three months, the method was not found to give better performances than automatic forecasts, but such results need to be confirmed on a longer evaluation period, with normal wind conditions. These results may possibly also highlight a potential expertise bias that would need to be further post-treated.

Type IV Wind Turbine System Impedance Modelling for Harmonic Analysis: On the Use of a Double Synchronous Reference Frame and Notch Filter
Submission-ID 198
Lucia Beloqui Larumbe, Zian Qin, Pavol Bauer
Delft University of Technology, Netherlands
Several efforts are being done nowadays to improve the modelling of Wind Turbine Systems (WTSs) for harmonic analysis in Offshore Wind Power Plants (OWPPs). Due to the high influence of the different control structures in the Power Electronic Converters (PECs) on the dynamic response of a WTS, each structure needs to be modelled specifically.

Following these lines, this paper addresses the impedance part of the harmonic model in the case of a double Synchronous Reference Frame control structure. The main focus lies on the correct modelling of one of the main elements of this structure: the notch filter tuned at twice the fundamental frequency.

The inclusion or disregard of this notch filter is very important because, as shown in works by other authors, this filter can have a big influence in the shaping of the WTS output impedance and such of the OWPP. However, the modelling procedure for the notch filter followed previously ignores the cross-couplings that this element creates in the αβ frame, which leads to a wrong calculation of the WTS impedance in the lower frequency range.

The proper modelling of this notch filter and its implications are detailed in the paper first theoretically and then by numerical simulations.

A new method of grid control of a wind turbine
Submission-ID 199
Laila Rezai, Norbert Klaes, Jens Fortmann, Marcel Engel
Hochschule für Technik und Wirtschaft (HTW) Berlin, Germany
The operational control of power inverter based energy generation is changing due to the ongoing removal of conventional power plants and the continuing increase of renewable power plants like photovoltaics and wind power systems. Nowadays the main task of renewable energy systems is to support the power grids and do not disturb the power flow. However, the change from conventional to renewable power generation makes it necessary to adjust typical control algorithms from wind power plants to stabilize the power grid. This paper describes an implementation of a voltage control algorithm for wind power systems. The control method is based on droop control, which commonly used in conventional power plants to control the synchronous machines. This type of control is adapted to run on a power inverter based energy system. The droop control and a state-of-the-art current control are compared to show in which working range the droop control is better than the current control. Both types of control algorithms are tested under realistic conditions like FRT and very weak grids to show how this system works under different conditions. In order to test and comparison of the functionality of this two control methods in case of grid disturbances, as a grid model an experimental setup system by a three-phase power system (230/400 V) with a maximum power of 44 kW are used. As test cases, the three-phase short circuit with different residual voltage, phase shift at the PCC and Islanding operation are considered. Test scenarios in time domain Simulink model as well as test-bed system are compared. The test system to test the mentioned control algorithm and generate measured values is described in [1]. [1] Arne Kisser, Marcel Engel, Laila Rezai, Moritz Andrejewski, Jens Fortmann, Horst Schulte, A Test-bed System for Validation of Ancillary Services of Wind Farms under Realistic Conditions, Wind Integration Workshop 2017

Investigation of Transient Energy Storage Sources for Support of Future Electrical Power Systems
Submission-ID 201
Yiheng Hu, Laolu O. Shobayo, Nan Zhao, Nigel Schofield
University of Huddersfield, United Kingdom
The connection of renewable energy sources to local low voltage networks is becoming more accepted as electrical power networks progress to higher renewable penetration. Renewable energy resources, for example, wind and solar, are highly dynamic and intermittent compared to more traditional generation sources, which imposes increasing challenges to the electrical network operator in terms of effectively managing the resource to maximize energy transfer and maintaining system stability. Therefore, batteries and supercapacitors, with fast charging/discharging capabilities, are suitable candidates to improve the reliability of connected renewable systems [1]. Most of the existing research in this area is focused on the power system load forecast and control algorithms of transient electrical energy storage systems [2], for example, frequency regulation including energy management strategies for enhanced frequency response [3], black start and voltage support services. Energy balancing management strategies for enhanced frequency response (EFR) to manage under frequency to a nominal value and improve system frequency post-fault. Engineering experiences from transient energy storage systems (TESSs) projects are reported in [3] that consider the frequency regulation, power quality and reliability improvement of power systems. However, the existing literature rarely discusses the system operating performance and detailed design of the TESSs encompassing system losses, thermal management, component mass and volume, and system dynamic operation etc.

In this paper, three typical TESSs technologies are presented, Lithium-ion and Sodium-nickel-chloride (NaNiCl) batteries and supercapacitors (SCs), together with combinations of batteries and SCs, are investigated to improve the power systems in term of power and frequency fluctuations. Models of the three energy storage devices are presented. For the systems studied, terminal voltage variation, peak current, power and thermal performance and efficiency are compared. TESS devices have different capability in terms of specific power and energy, system to the capacity and operating performance, component mass, volume and cost of TESSs will be determined. Moreover, this paper develops procedures for the design of transient energy storage systems that are shown to be different and independent of the optimization method chosen.

  1. H. Chen, et al, “ Progress in electrical energy storage system: A critical review ,” ScienceDirect, Natural Science, Vol. 19, no. 3, pp. 291-312, Mar. 2009.
  2. X. Xu, et al , “Application and Modeling of Battery Energy Storage in Power Systems,” CSEE Journal of Power and Energy Systems , vol. 2, no.3, pp. 82-90, Sep. 2016.
  3. S. W. Mohoda, et al , “Grid Support with Variable Speed Wind Energy Systems and Battery Storage for Power Quality,” ScienceDirect, Energy Procedia , Vol. 12, pp. 1032-1041, 2011.

Model-based Control of grid-side Converter: An LMI approach
Submission-ID 202
Horst Schulte, Nico Goldschmidt
University of Applied Sciences Berlin (HTW Berlin), Germany
This paper presents the basic approach of a generic method for the robust control of grid-side converters. For this purpose, the concept of an LMI-based representation of stability regions with pole specifications is used. A special feature of this approach is that the desired closed loop behavior is determined by the selection of a pole region. The pole regions are formulated via constraints as linear matrix inequality. The solving of this LMI guaranteed the stable solution of the control design.

As a model basis, a grid-side converter is represented in d-q coordinates. The resulting non-linear state space model is converted into a linear model structure with Taylor expansion. The linear model is the design basis for the pole region LMI approach. In order to decouple the influence of the disturbances on the control loop dynamics, suitable control laws are presented.

The design is supported and validated by simulation studies based on the various types of model uncertainties. Finally, the controller performance is compared with state-of-the-art d-q Inverter control using PI controller. The improved robustness and performance properties compared to the standard approach are presented.

Utilization of Battery Energy Storage to Assist Renewable Energy Networks
Submission-ID 204
Laolu O. Shobayo, Nigel Schofield, Yiheng Hu, Nan Zhao
University of Huddersfield, United Kingdom
The introduction of renewable energy sources (RES) such as wind, solar, and hydro, into modern power systems for generation is steadily increasing, resulting in a reduction in conventional hydrocarbon based generation. Thus transitioning society from an age of hydrocarbon based generation to renewable based generation, which is the cleaner option in a CO2 context [1].

However, the growth of renewable energy generation presents a number of problems to existing legacy utility networks. The major problems being associated with the transient nature of generation as opposed to the steadier generation from hydrocarbon based schemes. RES generation is dependent on natural resources such as wind speed, solar irradiance, and water flow. That can be highly transitory and unpredictable. Thus, it is envisaged that energy storage will form a key component in any future energy resource mix [2].

This paper studies the utilization of energy storage to support grid frequency and energy in future high percentage penetration renewable energy networks. Electrochemical battery systems (BESS) are considered here as the primary storage medium. In 2015, National Grid Electricity Transmissions (NGET) in the UK released an expression of interest for provision of grid support via the addition of BESS systems. NGET specified upper and lower limits for system frequency, which is used in the study to assess BESS energy conversion requirements [3]. The paper presents results from a study into the application of BESS systems driven via NGET frequency profiles from 2014 to 2016, when delivering transient and longer-term energy to the system. Typical power train components, their ratings and operational issues together with an assessment of longer-term energy support are discussed.

[1] G. Frey and D. Linke, "Hydropower as a renewable and sustainable energy resource meeting global energy challenges in a reasonable way", Energy Policy, vol. 30, no. 14, pp. 1261-1265, 2002 [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0301421502000861. [Accessed: 07-May-2018].

[2] A. Zahedi, "A review of drivers, benefits, and challenges in integrating renewable energy sources into electricity grid", Renewable and Sustainable Energy Reviews, vol. 15, no. 9, pp. 4775-4779, 2011 [Online]. Available: https://www.sciencedirect.com/science/article/pii/S1364032111003194. [Accessed: 02-May-2018].

The Impact of Renewable Energy Schemes on Electrical Power Networks.
Submission-ID 206
Nigel Schofield
University of Huddersfield, United Kingdom
The connection of renewable energy sources to electrical power networks is becoming more accepted as the electrical power network operators face the competing challenges of changing and increasing load demands, and continuing pressures to reduce net carbon footprint. Renewable energy resources are highly dynamic and intermittent compared to more traditional generation sources. Hence, they pose a challenge to the electrical network operator in terms of effectively managing their resources to maximise energy transfer while maintaining a stable interconnected network. A particular aspect of existing renewable energy resources is that they will be connected to the main transmission and distribution system via some form of power electronic interface.

This paper will discuss the impact on the wider electrical power system when solar and/or wind energy resources are connected or when the renewable energy resources are the dominant component in the electrical network – as may be the case for small systems, or the future grid scenario if the growth in renewables continues.

The quality of the power that is injected to the system must be managed. The paper will focus on the comparison between the impact of traditional schemes and the new renewable schemes. The balance and harmonics that each scheme would produce will be examined and appropriate mitigation / management schemes presented.

The paper can be summarised as:-

  1. presenting a technical benchmark to model an existing electrical power systems with light renewable penetration (reference system will be the UK). The model will be tested against readily available grid load demands and operational data. The model will be developed to reflect increasing / higher penetration of renewables as the share in energy mix in the near future;
  2. using published power system statistics, a vision for the future development of the UK electrical power network is proposed based on a number of renewable penetration scenarios.

Smart Energy Network Demonstrator - SEND
Submission-ID 209
Nigel Schofield
University of Huddersfield, United Kingdom
Keele University is working with businesses, graduates and academics to create Europe’s first ‘at scale’ Smart Energy Network Demonstrator (SEND) – a living laboratory where new energy-efficient technologies can be researched, developed and tested in a real world environment. The Smart Energy Network Demonstrator will be a world class demonstrator facility for smart energy research, development and innovation (RD&I), enabling businesses to develop, test and evaluate new energy technologies, and allied services, on a smart energy network demonstration system. This will enable them to assess their efficiencies in terms of system integration, energy reduction, cost and greenhouse gas emissions. The paper will discuss the main attributes of the SEND project including:

(1) Capital equipment, facilities and plant to convert an existing energy supply network into a smart energy network demonstrator (SEND) RD&I facility;

(2) A supply chain development programme for smart energy technologies and services;

(3) A collaborative Research, Development and Innovation (RD&I) product development programme with eligible companies and universities to support the development and commercialisation of new SMART energy products and services using the SEND RD&I facility.

The Demonstrator will build on Keele University’s privately-owned and managed infrastructure, comprising:

  • 600 acre site, making it the UK’s largest university campus
  • 341 buildings, ranging from academic, student residential, staff flats & houses, and Science & Innovation Park business accommodation
  • 204,000m² built environment
  • 80,000m² development-ready land
  • 12,000+ staff and students on site per day

With a campus energy demand of: 39.2GWh pa - Gas • 23.8GWh pa - Electricity

    • 10km+ of underground gas network (6 MP/ LP meter points)
    • 18km+ of electrical network (cable) with 22 sub-stations (11kV/400V)
    • 28km+ of fibre-optic cabling
    • 16km+ of surface and foul water drainage
    • 16km+ of mains water network
    • 6km district heating (3 networks)

RD&I and supply chain development

RD&I and supply chain development for low carbon and resource efficient technologies and materials will be achieved through the programme of collaborative research, development and innovation with businesses using the demonstrator. This includes a collaborative RD&I product development programme, delivered through a doctoral training centre. The programme will support 26 high technology businesses to carry out collaborative research, development and innovation with UK universities, to underpin the commercialisation of new products and services for global smart energy markets.

The SEND programme (ref. 32R16P00706) is part-funded through the European Regional Development Fund (ERDF) as part of the England 2014 to 2020 European Structural and Investment Funds (ESIF) Growth Programme, and is available to ERDF eligible companies. The programme is also receiving funds from the Department for Business, Energy and Industrial Strategy (BEIS).

Rethinking ancillary services – provides distributed generation reactive power for free?
Submission-ID 211
Karsten Burges 1, Michael Döring 2
1 RE-xpertise Baltzerstr. 13 15569 Woltersdorf, Germany
2 Ecofys - Navigant Albrechtstr. 10 c 10117 Berlin, Germany
For more than a decade, technical codes and planning standards for distributed generators in Europe have been defining minimum requirements with respect to reactive power capabilities being available for the network. The requirements have been justified by the increasing demand for voltage control and loss reduction in transmission and distribution networks. The increase in turn is driven by the ongoing implementation of distributed and renewable generation.

As a consequence of these technical minimum requirements, no economic value is allocated to the capability to provide reactive power. Network operators do not assess the long term demand based on a macro-economic cost benefit optimisation. Recent studies indicate that the current framework most likely will result in an overallocation of reactive power in distribution networks during the next decade [1].

More importantly, technical codes do not regulate the provision of reactive power in system operation neither do they address remuneration issues. In the past, reactive power was actively controlled at transmission level and in high voltage networks. Conventional power plants provided this ancillary service in the framework of bilateral agreements with the TSO. Depending on the regulatory framework this was backed by some remuneration.

At lower voltage levels, provision of reactive power by connected clients was uncommon. Hence, economic incentives or remuneration schemes never have been developed at this level.

Currently, distribution network operators more and more are deploying the existing capabilities of distributed generation to provide reactive power. For the plant operator this service, however, may be associated with substantial losses. For that reason, potential frameworks for a fair and non-discriminatory framework for non-frequency ancillary services are being discussed intensively in the industry, in Germany as well as in the European Union.

Due to the nature of reactive power the implementation of a dedicated regulative framework and related policy instruments is faced with some challenges:

  • Driven by technical codes, large capabilities have been implemented in the past. Capital costs represent a substantial portion of the service costs. New players may not face a level playing field.
  • Reactive power has to be provided (electrically) close to the source of the problem – the number of bidders at a certain node always will be limited.
  • Depending on the control objective (e.g. controlling voltage at PCC) the amount of reactive power provided cannot be measured easily.

The paper provides a structured review of cost components, operational processes, planning moments and allocation of monetary value for reactive power. It analyses the effectiveness of incentives discussed in the industry and, hence, supports an informed decision.

[1]: O. Brückl et al: „Zukünftige Bereitstellung von Blindleistung und anderen Maßnahmen für die Netzsicherheit“, Regensburg, 2016

How to get the power sector decarbonised – a lessons-learned tool for international knowledge exchange
Submission-ID 215
Karsten Burges 1, Frauke Röser 2, Markus Hagemann 2, Keno Riechers 2, Mia Moisio 2
1 RE-xpertise, Germany
2 NewClimate Institute Brunnenstr 195 10119 Berlin, Germany
As a consequence of the Paris Agreement, the participating countries have to undertake substantial efforts to decarbonise their power sectors within the next decades. Variable renewable energies (VRE) as wind and solar play an important role in that process. Because of the complexity of the transformation processes, extensive studies have been and are performed all over the world. They help to identify the most cost effective transitional paths and simultaneously must make sure that security of supply is not compromised.

Of course, there is a vital interest to learn from each other. Various publications exist comparing the status and achieved progress of selected countries, describing their experiences and drawing conclusions on success factors. However, generalisation of experiences and results is tricky. The specific conditions and key factors are differing substantially among countries. Some illustrative examples are the share of hydro or geothermal, the expected growth of energy consumption and characteristics of network topology (meshed versus sparse networks, transmission interconnections with neighbours etc.).

This variety makes it difficult to really learn from each other. Recently, the International Energy Agency presented a methodology supporting such a cross country comparison [1] defining some general key phases of VRE integration.

In the paper we will present a similar approach. It has been developed very much as a tool supporting system planners to identify country specific challenges, existing study approaches and potentially applicable results. We defined various metrics allowing classification of power systems. We will present application of the tool using some country experiences from Europe, Asia and Latin America.

[1]: “Getting Wind and Sun onto the Grid A Manual for Policy Makers”, International Energy Agency, Paris, 2017

Comparison of impedance characteristics of Multi-Megawatt grid simulator with LVRT-container during LVRT test
Submission-ID 216
Soroush Azarian, Torben Jersch, Shoaib Khan
Fraunhofer IWES, Germany
To reduce the time and cost of electrical certification of wind turbine, Fraunhofer IWES has engineered new test methodology and procedure based on electrical validation of wind turbine on test benches. In order to validate and verify the wind turbine performance under grid disturbances, wind turbine can be integrated to Power Electronic Grid Simulator (PEGS) of DyNaLab for conducting of grid compliance tests. Hence, to test the electrical properties of wind turbines on test benches in the wind industry, Fraunhofer IWES and manufacturers partners, Nordex, Senvion and Vestas cooperate in “Hardware in the Loop Grid Compliance Prüfstand (Hil-GridCop)” project. The aim of this project is to carry out validation and verification of the tests for the determination of the electrical properties of wind turbines (fast-running generator systems) with coupling to the grid simulation of the DyNaLab.

In order to validate and verify the test bench UVRT tests it is essential to show if grid simulator is capable to emulate same grid behavior and characteristics as state of the art test procedures such as UVRT-Container. This give use the motivation to purpose a scientific paper to make the first step toward validation of grid compliance tests on test bench, for the first step of validation process a comparison of the simulation results of developed LVRT-container test to grid simulator will be presented, where similarities and differences of these two LVRT test procedures will be pointed out.

By means of the test bench to be validated, the comparison of results of the field measurements test to bench tests with the aim of the certification of the electrical properties are to be carried out.

Gird simulator of Fraunhofer IWES DyNaLab consist of 4 voltage source inverter (VSI) units with 44 MVA short circuit power connected to a step-up converter transformer to emulate an artificial medium voltage grid up to 36 kV for simulation of various types of Fault Ride Through (FRT) tests. Because the grid simulator is considered as an ideal controllable voltage source including low step-up transformer impedance the voltage dips emulate by PEGS can have different properties than voltage dip by LVRT-Container.

One of the PEGS capabilities is to add artificial series impedance to the voltage source in favor of changing the virtual short circuit power at point of common coupling (PCC); hence according to the customer specification high or low short circuit ratio (SCR) can be realized, where the short circuit ratio is defined as ratio of grid short circuit power to MW power of integrated wind turbine. The lower the SCR, the weaker the grid will be. Therefore, in this paper the implementation and characteristics of virtual impedance on PEGS will be discussed in details followed by LVRT tests simulation results of LVRT-Container and PEGS including virtual impedance implementation in Simulink environment.

Balancing challenges for future North Sea Offshore Network
Submission-ID 217
Kaushik Das 1, Juan Gea Bermudez 2, Matti Juhani Koivisto 3, Poul Ejnar Sørensen 4
1 DTU Wind Energy, Denmark
2 DTU Management Engineering, Denmark
3 DTU Wind Energy, Denmark
4 DTU Wind Energy, Denmark
Future massive offshore wind power and the associated offshore grid development in North Sea pose many challenges towards power system balancing in North Sea neighboring countries. The locations of the offshore wind power plants are expected to be concentrated in relatively small areas where wind conditions are favorable. This spatial concentration of offshore wind power can cause high power fluctuations. The power variations in power systems are generally handled through prognoses of wind power at different time scales. Power is generally traded on the day-ahead spot-market based on the day-ahead prognoses, whereas the hour-ahead prognosis is the basis for the hour-ahead balancing performed by TSOs. The difference between the day-ahead and hour-ahead prognoses quantifies the need for balancing reserves to perform this hour-ahead balancing, while the difference between the real time wind power and the hour-ahead prognoses quantifies the subsequent need for real time balancing reserves to avoid deviations from the planned power exchange with neighboring countries. Real time imbalances are handled by automatic and manual frequency reserves. One of the major challenge involves estimation of adequacy of reserves to handle the imbalances caused by forecast error. There might be higher reserve requirements in those future operational scenarios. Availability of such high reserves all the time can be an expensive solution. There might be other market-based solutions to activate real-time markets closer to time of operation when forecast error is low. However, in order to perform these studies new simulation tools are required which encompasses different simulation capabilities starting from simulation of forecast errors, spot market-based unit commitment, inter-hour balancing, real-time balancing etc. This paper will investigate the challenges associated with power system balancing and as well as analyze the requirement of different components of simulation tool required for balancing studies.

Finding the limits to system flexibility
Submission-ID 219
Eamonn Lannoye 1, Aidan Tuohy 2, Erik Ela 2, Qin Wang 2
1 EPRI International, Ireland
2 EPRI, United States
This paper focuses on the assessment of system flexibility and the application of new analyses to determine whether a system has sufficient flexibility to meet its expected net load variability and uncertainty. By finding the technical limits of how much flexibility a system is capable of providing, system planners can determine the bookend when investment in new infrastructure is the only option available to ensure future system reliability. Coupled with existing analysis methods, these can provide estimations of the tradeoff between changes to operational policies such as operating reserve requirements and investment in new generation. This paper will review these methods and provide outcomes from realistic case studies where they have been applied.

Negative market prices and market premium support schemes – Impacts on wind integration in the German electricity market
Submission-ID 220
Marian Klobasa, Michael Haendel, Leopold Pfluger
Fraunhofer Institute for Systems and Innovation Research ISI, Germany

Today’s support schemes for wind energy are based on the market premium model in most EU countries. Market integration is incentivized by giving balancing responsibility to wind farm operators and by opening the opportunity to increase earnings by an optimized market value of the wind energy generation. This design has already led to RES curtailments at times of negative market prices when market premiums are fully compensated by negative market earnings.

EU state aid guidelines introduced additional regulations for market premium support schemes by requesting that no incentive for generation should be given when market prices are negative. To fulfill this guideline no market premium is paid in the German market premium model when the market clearing price at the EPEX-Spot is negative for at least 6 consecutive hours. This current regulation raises the question on the im-pacts on market integration of wind energy today and in the future.

Based on historic data and future scenarios for wind energy development the paper analysis effects on wind power curtailment and earnings in Germany. Finally, it discuss the contribution of the current market design to the integration of large shares of wind energy and the efficiency of the overall system.


Historic market data for Germany is used to analyse market behaviour at times of low market prices and to monitor dispatch decisions of power plants. Next to historical data also the future development is assessed based on extrapolations of current market trends as well as energy policy goals on RES development. For this purpose, the future capacity development of wind energy and other RES technologies is derived. Furthermore the strike price development and the development of levelized cost of electricity. is estimated. As an indicator for negative spot market prices the residual load curve also considering export capacities and development of flexible generation and demand side is used.


The results show a substantial increase of hours with negative market prices up to more than 500 hours in 2025 on the German spot market. The affected capacities are mainly onshore wind energy plants. Impacts on wind curtailment can reach up to 4 TWh per year and are strongly influenced by future technology choice (e.g. low wind turbines) and by demand flexibility. Economic impacts increase until 2025 because mainly new build wind turbines will be affected. After 2025 losses of revenue due to negative prices decrease, because higher spot market prices are expected which in turn will reduce additional market premiums.


Historic and future market analysis indicate that with the current regulation dispatch decision of power plants can be distorted (e.g. increased conventional generation and reduced wind production). This has an impact on the efficient achievement of RES development goals and on the incentive for flexibility in the power market by reduced price spreads.

Voltage Support Provision as an Ancillary Service from Wind Turbines Installed in Distribution Networks
Submission-ID 221
Sanaz Namayantavana 1, Ali Bidadfar 2
1 School of Electrical Engineering and Computer Science, KTH, Sweden
2 Wind Energy Department, DTU, Denmark

Distributed energy resources (DERs), such as wind power in distribution networks can supply a part of network power demand. Additionally, the wind turbines can provide some ancillary services such as voltage support (reactive power management) and stability enhancement. Part of such services can be provided to the transmission networks. Interaction between transmission and distribution system operators, i.e. DSO and TSO can be possible by exchanging required data for the voltage support. In this paper, the reactive power that produced by wind turbines has been optimally controlled in order to support the voltage at the boundary between distribution and transmission networks. By controlling the voltage at this boundary some benefits, such as power loss reduction and reactive power flow control can be achieved.

This reactive-power-management is implemented based on a short-time ahead forecast of system loads and wind turbines production. In this paper it is assumed that the forecast values of loads and productions are available. To verify the interaction process as well as the effectiveness of the proposed control, simulation scenarios are conducted using the real data taken from the Scandinavian power systems data center, i.e., Nord Pool.

Analysis of HVDC and Wind Turbine Converters Response during Offshore Asymmetrical Faults
Submission-ID 229
Ömer Göksu, Nicolaos Cutululis, Poul Sørensen
DTU Wind Energy, Denmark
One of the main short-circuit analysis for the HVDC connected offshore wind power plants is the asymmetrical faults at the offshore network between the offshore HVDC converter and the wind turbines. There have been few requirements specified for these offshore asymmetrical faults, e.g. in the recent ENTSO-E HVDC network code, and few studies in the literature, focusing on the control algorithms for the HVDC converter and the wind turbines during asymmetrical faults. However, a fundamental theoretical analysis focusing on the underlying physics behind the offshore asymmetrical faults has been missing. In this paper, physical phenomenon occurring in the offshore network with the HVDC converter and the wind turbine converters during asymmetrical faults has been explored. It has been showed that the negative sequence fault current flow via the HVDC converter is strictly necessary and there is an interaction between wind turbine positive sequence fault current and the HVDC negative sequence current. The classical fault analysis approach has been utilized and complemented with time-domain simulations. The output provides understanding for the offshore asymmetrical faults and insights for related future grid code requirements and converter control design.

Variable renewable energy integration study and policy proposal in Japan by using demand-supply analysis and grid stability analysis
Submission-ID 230
Shota Ichimura
Kyushu University, Japan
Renewable Energy Institute, Japan
This study presents policy proposal to overcome the grid issues toward for expanding variable renewable energy (VRE) based on demand-supply analysis and grid stability analysis in Japan. In Japan, transmission system operators are facing mainly three grid issues toward for renewable energy expansion. First issue is to balance between demand and supply. In Kyushu electric power company, one of ten main transmission system operators, the hourly share of PV compared to electricity demand reached around 80% in April 2018. Second issue is delay of grid connection due to grid congestion. In Tohoku area, there is many requests of connection to the grid by new power plants such as offshore wind farm. Tohoku electric power company, as a transmission system operator in Tohoku area, started process to expand transmission line to accept new connection. Third issue is grid stability which is especially happened in Hokkaido area. Hokkaido area is connected to Tohoku area with 0.6GW of HVDC line but Hokkaido is not synchronized with Tohoku. Hokkaido electric power company concerned that grid stability caused by new variable renewable energy connection under plan and started process to accept new VRE connection on condition that new VRE pays for introducing storage system. Ministry of Economy, Trade and Industry (METI) started to reform grid operation rules and electricity market to overcome these issues. METI also started information disclosure regarding grid and power plants to encouraging VRE developers to predict future electricity business by using market and grid model based on information disclosed. For the purpose of discussion to overcome above grid issues authors jointly conducted variable renewable energy integration study by using demand-supply analysis and grid model built with public information in Japan. This presentation mainly reports on insight resulted from demand-supply analysis. The SWITCH model is used for the demand-supply analysis. The SWITCH model is a linear programming modeling platform used to examine least cost energy systems designed to meet specific reliability by modelling several demand areas and transmission line between them. By using SWITCH model hourly balance between demand and supply including VRE are evaluated in 2030 in each area in Japan. It is assumed that the cumulative installation of PV is 100GW and the cumulative installation of Wind is 36GW in 2030, which are quoted from PV and Wind developers association in Japan. The result of demand-supply analysis shows that the share of VRE will reach to 80% of total generation in 2030 in Japan. On the other hand, the result also shows that the share of VRE can drop to around 1% in some hours in 2030. In order to evaluate impact of expansion of VER, grid stability analysis is conducted by using grid model in these serious time durations. It will be presented in other presentation.

Mixed grid forming and grid following wind power plants for black start operation
Submission-ID 232
Jaime Martinez-Turegano 1, Salvador Añó-Villalba 1, Soledad Bernal-Perez 1, Ruben Peña 2, Ramon Blasco-Gimenez 1
1 Universitat Politecnica de Valencia, Spain
2 Universidad de Concepcion, Chile
Grid forming wind turbines allow for additional services to be provided by wind power plants. These additional services include self-start operation, islanding operation of off- shore ac-grids and contribution to system restoration after a black-out.

As the complete electric system will consist of both grid-following and grid-forming wind power plants, the aim of this paper is to study possible interactions between grid forming and grid following wind turbines during black-start operation.

The presented case study includes a 400 MW off-shore wind power plant connected by means of a 75km HVac cable to the on-shore transmission grid. The results show adequate islanded and black-start operation with less than 25% grid forming power.

It has been found that whereas stable steady state operation can be achieved for relatively low amount of grid-forming capability, large differences in speed of response between grid-forming and grid-following wind turbines imply the need of more grid-forming power to operate as a "slack bus" during transients, particularly during load rejection.

Do we need a network code on cyber security? How to address cyber security requirements in a power system with high penetration of distributed generation?
Submission-ID 237
Michael Doering, Edwin Haesen
Ecofys, a Navigant Company, Germany
The market liberalization and growth of renewable energy sources (RES) dominated the first phase of the transformation of the European electricity systems. The next phase of the transformation is characterized by the increasing penetration of information and communication technologies (ICT) in the energy system which requires an enhancement of system security and new answers how to ensure system security in the new environment. Therefore, the current draft of the Clean Energy Package proposes to develop a new European Network Code which covers the domain of “cybersecurity rules”. This builds on a “Security plan for critical infrastructure protection” requirement to all TSOs as already prescribed in the “System Operation Guideline” (SysOp).

Whereas in the past communication needs were limited to a few installations, in the future various actors need to communicate with and have access to millions of installations. The large number of communication technologies, systems and actors allows an inadequate assessment of who actually has security-relevant access to a wind farm or the power system. As a result, potential external attacks or system-inherent errors in the area of IT/OT systems can affect the generation of individual windfarms, supply of individual households or the system operation in the whole European synchronous network.

In this paper we will identify and assess cybersecurity risks and requirements for future system needs and discuss how these requirements should be addressed. The analysis is based on recent studies for the European Commission and German government. In this paper we focus on the communication infrastructure of network operators and third-party communication with distributed generation, like wind farms.The objective of this paper is to start the discussion about which aspects should be covered and not covered by a potential Network Code on cybersecurity.

In our paper we cover the following steps

  1. Overview and classificationofrelevant communication and controlling infrastructure(including mapping of actors and connection access) and related threat scenarios(considering recent attacks like Ukraine in 2015 and energy specific requirements like real-time operation)
  2. Proposal for a risk assessment methodology(based on ISO 31000)
  3. Overview of common mitigation measures in place(we will focus on common frameworks from NISTIR (US) and ENISA (EU) and map them to each other to show their coverage, but we also consider the information security management system standard ISO / IEC 27001)
  4. Assessment of cybersecurity risks(for the selected infrastructure and threat scenarios) and identification of additional mitigation measures(including enhanced compliance and audit requirements for specific actors in the energy sector) and first discussion of related costs
  5. Conclusiononfurther specifications and restrictions which should be implemented on European level (Network Codes) for specific actors

Novel Control Scheme to Enhance Frequency Response of Wind Farms Augmented With Energy Storage Systems
Submission-ID 238
S. Mohammad Mousavi Agah, Dave McNamara
Renewable Power Generation LTD, Ireland
This paper develops a novel control scheme to complement commercial controllers of wind farms augmented with battery energy storage (BES) systems. The novel scheme enhances smart grid flexibility by improving frequency response of the wind farms, i.e. their ability to change their active power output in response to a frequency change. The novelty of the paper lies in the coordinated control of frequency response from wind energy conversion (WEC) and BES systems. Distinct from existing methods, the developed scheme is applicable to small- and large-scale BES systems. Furthermore, dynamic models are developed for the WEC and BES systems to overcome technical issues of existing research, e.g. frequency oscillation. The developed control scheme is simulated using PSS/E software tool and the improved frequency response from an exemplar wind farm in Ireland is assessed against the grid code requirements. Ireland is facing insufficient frequency response problem before rest of the world and this research represents a significant opportunity to demonstrate solutions to a global challenge.

Much work has been conducted in the UK, Denmark, Germany and the US to provide ancillary services from wind farms. In particular, BES systems were proposed in research papers to improve the frequency response of wind farms. However, these studies have shortcomings, e.g. de-loading of the WEC systems and loss of efficiency, which make them less applicable to Ireland’s electricity grid, which is subject to greater and more rapid frequency deviations, due to the small system size and its synchronous isolation. Such a gap in the state of the art is addressed in this paper.

The research work of this paper is organised as follows. Section 1 provides a review of the literature dealing with the application of BES systems in wind farms in order to improve dynamic response of the WEC systems to frequency events. Section 2 develops dynamic models for the WEC and BES systems. In Section 3, a control strategy is proposed for coordinated control of frequency responses provided by the WEC and BES systems. The strategy is based on three integrated control loops: frequency control, active and reactive power control and electric charge control of the battery. In section 4, data from an exemplar wind farm in Ireland are used to assess the frequency response provided by the developed control scheme in the presence of a BES system. Then, optimisation strategies are developed for minimising size, cost and cycling of the BES system and tuning parameters of different control loops in order to achieve the best speed of response and time of sustained response. Section 5 verifies the effectiveness of the proposed control scheme using simulations in PSS/E software tool. Historical frequency events are simulated and the frequency responses from an exemplar wind farm fitted with the proposed control scheme are assessed against the grid code requirements, and finally, Section 6 concludes.

FRT Test System Compact for 27 MVA with less Grid Burdens is now in Operation
Submission-ID 240
Rainer Klosse
WindGuard Certification, Germany
The completely newly developed FRT test system of WindGuard Certification is now completed and in use since March 2018. The test system is designed for networks up to 30 kV and rated currents up to 630 A. This results in possible DUT up to 27 MVA rated apparent power. Due to a very small number of components, a single ISO container is sufficient to store everything together.

Technically, this test system switches from an series impedance to an autotransformer at the moment of fault simulation. By using the transformer effect, the network load is considerably lower for most numbers of test cases compared to conventional test equipment. For example, in the event of a 50% drop, the ½ short-circuit current at the same longitudinal impedance must be expected compared to a conventional voltage divider. In addition to UVRT, OVRT can also be simulated. Due to the not used capacitors the usual problem with resonance points is not given. The sin form of the voltage is much more consistent. This test system is also suitable for simulating not only pure amplitude changes but also vector jumps of the voltages.

Depending on the mains conditions, voltages of up to 150% of the input voltage can be achieved by the transformer. Initial measurements have already been carried out on the medium-voltage grid. These show that the basic assumptions are correct. However, the test system also uncovered properties that make correction factors necessary in addition to rough calculation. Due to the mixture of a simple coil and a transformer, there are no standardized models available. In recognition, the new system is compared with a conventional voltage divider. Due to the cost-effective design with the enormously increased application possibilities, a quick replacement of conventional voltage dividers is expected.

Wind integration costs - a useful concept that is complicated to estimate
Submission-ID 241
Hannele Holttinen 1, J Charles Smith 2, Simon Müller 3, Emanuele Taibi 4, Til Kristian Vrana 5, Daniel Fraile 6
1 VTT Technical Research Centre of Finland, Finland
2 ESIG, United States
3 IEA, France
4 IRENA, Germany
5 SINTEF, Norway
6 Wind Europe, Belgium
International collaboration under IEA TCP IEAWIND Task 25 was set out to study the different methodologies to estimate impacts and costs of wind integration in 2006. Integration studies at that time were often set out to estimate impact on reserve requirements and balancing costs - sometimes also capacity value and impacts to transmission reinforcement were analysed. During the years, recommended practices were formulated on how to take wind power properly into account when making simulations for power system operation, long term reliability, and short term operational reserve allocation. The main caveats were found out in the assessments of integration costs (Recommended Practices for wind integration studies, article http://consultkirby.com/files/EJ_Article_Cost_Causation_Nov_2011.pdf and WIW13 article Milligan et al Wind Integration Cost and Cost-Causation). The main problems in catching the integration costs are:

  1. how to choose the base case “no wind” simulation, in order to get the costs incurred by wind as a result of comparison with “wind” case.
  2. Quantifying the increase in balancing costs, as the major impact usually is a decrease in operational costs due to reduced fuel use that wind will replace.
  3. Allocating any difference in operational costs to wind power involving assumptions impacting the results.

Integration cost is very system dependent and driven by assumptions. An important finding is that flexibility in power systems make integration costs lower. There are also other technologies in the system causing or not contributing to providing system services - an ideal method should be technology neutral.

There is no longer interest in US to evaluate integration costs. Texas and Hawaii examples show that the costs are small, also in higher shares of variable generation. However, integration cost question still remains in many parts of the world. Both IEA and IRENA are confronted with questions of integration costs from new countries wanting to compare renewable scenarios with other alternatives.

It would be very useful to have a quantified “integration cost” number to add to the LCOE production cost of wind energy, when comparing with other alternatives. Also a system value approach could be taken where integration cost component would diminish the system value and again could be compared to the LCOE. However, capturing this “integration cost” component is the challenge. We know that total system costs can be calculated from future simulated systems, and compared for different scenarios. This approach does not try to withdraw an integration cost component, but instead compares the total costs, and benefits Also in this approach the impact of technology is system and assumption dependent. Flexibility is very important to keep system costs low even in high levels of variable generation. This paper outlines the methods for integration costs and discussing a method looking at total costs.

Kriegers Flak Combined Grid Solution – Principles of Voltage and Reactive Power Control for HVAC/HVDC Meshed Offshore Grids
Submission-ID 245
Vladislav Akhmatov 1, Anne-Katrin Marten 2, Reinhard Stornowski 2, Thomas Bentzon Soerensen 1
1 Energinet, Transmission System Operator of Denmark, Denmark
2 50Hertz-Transmission, German Transmission System Operator, Germany
Kriegers Flak Combined Grid Solution (KF CGS) will be an interconnector between East Denmark and Germany and utilize the already existing and under commissioning grid-connections of the large offshore wind farms in the Baltic Sea.

The already established 150 kV AC submarine and underground cable connections of the Baltic 1 (48.3 MW) and Baltic 2 (288 MW) wind farms with a total connection length of approx. 135 km to Bentwisch in Germany will be extended by the two 24 km long 150 kV AC cables to the Kriegers Flak B extension platform (KFE). On the Danish side, the two platforms Kriegers Flak A (KFA) and Kriegers Flak B (KFB) will collect 200 MW and 400 MW offshore wind power respectively. The KFA and KFB platforms will be linked through an approx. 9 km long 220 kV AC submarine cable and connected to the onshore, double-busbar compensation substation Bjæverskov via two approx. 80 km long 220 kV AC submarine and land cables. In Bjæverskov there will be the first 400/220 kV step-up transformation with connection to the Danish 400 kV transmission grid. From Bjæverskov to Ishøj there will be another 220 kV land cable with the second 400/220kV step-up transformation to the 400 kV Danish transmission grid. On the KFE platform there will be the 220/150 kV transformation.

Since East Denmark (the Nordic system) and Germany (the Continental European system) are not synchronised, there will be an HVDC Back-to-Back converter (BtB) in the substation Bentwisch connecting the 150 kV AC offshore system with the 380 kV German onshore system. The KF CGS will connect wind power infeed and permit as much as possible energy market trade between the countries utilizing the already existing equipment for the wind farm grid-connections and the additional equipment of the interconnector. The KF CGS holds the status of a “Project of Common Interest” (PCI), given by the European Commission, and is granted financial support from the European Energy Programme for Recovery (EEPR).

The KF CGS network resembles an HVAC/HVDC meshed offshore transmission system which requires careful, well-tuned and advanced voltage and reactive-power control. Because the KF CGS is a meshed offshore grid, the overall control shall be robust and working in different operational regimes such as switching of the power transport equipment, i.e. intended and unintended disconnection and reconnection of cables and transformers as well as connected and separated regimes of the interconnector.

This paper will present the major principles of the voltage and reactive-power control to be applied within the KF CGS, which have been designed and verified by simulations. Each control area includes several equipment and control functions, which will be presented and discussed.

Challenges with the design of cost effective series DC collection network for sea-based wind-farm
Submission-ID 246
Mohammad Kharezy 1, Torbjörn Thiringer 2
1 Rise Research Institutes of Sweden, Sweden
2 Chalmers University of Technology, Sweden
Today, when a sea-based wind park reaches a size of 100 MW or more, a platform, acting as the hub in the collection grid and a point of connection for the cable to land is needed. Among other equipment, the platform carries a transformer. Since ordinary AC cables becomes inefficient over nearly 100 km, it has become necessary to move over to DC transmission for some of the new wind parks. However, within the wind park, still a collection grid running at 50 Hz AC is used and 2 platforms of gigantic sizes are needed for 50 Hz AC/AC and AC/DC conversion increasing the cost substantially for the wind energy installation.

A possibility here is to use the DC technology also for the energy collection grid within the wind park. To do that, a key component is missing, the high-power DC/DC converter. Such a device can transform the voltage from the wind turbine to a high DC voltage using a much smaller converter unit compared to a 50 Hz transformer. An idea is to fit the converter system into a container on the outside of a wind turbine, thus utilizing the existing foundation out in the sea.

A highly interesting solution is then to connect the output of the wind turbines in series, and in this way making the voltage level to reach 100, 150 or even 200 kV. The idea would then be to continue the connection directly to shore without the need of a large transformer platform. A cable can transport up to 2 kA, and using a bipolar set-up, 800 MW can be reached without a platform, for the 200 kV case. This is a huge investment saving. However, here comes a highly important factor: The wind turbine that is located closest to the DC-transmission cables going away to shore must take up the full insulation on its high-voltage side. Today, the DC/DC converter technology is far away from such capabilities. This is where the proposed project comes in.

The proposed design and optimization approach was introduced earlier in a PhD work where it was applied on two down-scaled 50 kW, 1/3 kV, 5 kHz prototype medium-frequency transformers. These optimized designs have later been manufactured, and successfully measured, fulfilling the efficiency, power density and leakage inductance requirements that the prototypes were designed for. To move further up in insulation level, more insulation material must be added reducing the power density of the unit, increasing the length of the winding, thus decreasing the efficiency.

In this research project, financed by Swedish Energy Agency, the aim is to study the size of such a DC/DC converter accounting for the increased voltage strength and thus insulation level. Both obtainable voltage withstand levels as well as life-time of the insulation are issues that must be investigated carefully. The success of this project puts academia, research institutes and industry in a very good position to take a lead into the development of this key enabler for the cost-effective harvesting of wind energy from offshore wind turbine installations.

Wind turbine future
Submission-ID 251
Eslam Khalife
University of kassel, Germany
Fluting Wind turbine in space and mobile wind turbine for electrical vehicles. Concept of combining wind turbine systems solutions within electrical vehicles for charging purpose and the innovative design for wind turbine.

Impact of technical parameters and data quality on wind energy modeling in Germany
Submission-ID 255
Daniel Beulertz, Armin Schnettler
Institute for High Voltage Technologies at RWTH Aachen Universtity, Germany
This paper presents a model framework that is capable of determining the generation from wind power plants on a large scale while taking technical parameters into account. Turbine characteristics and hub heights as well as data transformation methods are included in the approach. Most relevant input data, especially wind speed data and power plant data are analyzed regarding their comprehensiveness and level of detail. To account for the impact of different wind speed data sources the model is applied to a set of single wind power plants and the results are compared to historical daily generation data. Afterwards the model is extended to german scope, focusing on the impact of power plant data and model parameters on the modelled wind generation compared to historical data for Germany.

A correction method to improve the quality of the wind forecast-A case study for Wangjiangping station
Submission-ID 256
Yan Shen
National Meteorological Information Center, China Meteorological Administration, China

As a substitute to burning fossil fuels, hydropower and wind power belong to clean, renewable, abundant power and produce no hothouse gas radiations during operation compared with the non-renewable power sources. So more and more countries and organizations pay attention to the development of hydropower and wind power with no exception of China. Hydropower (wind power) is mostly dependent upon precipitation and elevation changes (wind speed and wind direction); high precipitation (wind speed) levels and large elevation changes are necessary to generate significant quantities of electricity. In it, meteorological data including hi-resolution, long-term precipitation and wind observations and forecast can do more contribution on the power site selection, power monitoring, prediction and early warning.

Based on the WRF (Weather Research and Forecasting) model, the wind speed at the 80 meters above the ground surface is forecasted ahead of 3 days with the 15-min temporal interval at the Wangjiangping station. However, the forecasted wind speed contains large errors owing to the problems in the complex physical process, improper boundary and first-guess values and high stochastic effect of the wind speed at the 15-min temporal resolution. Therefore, an efficient way to improve the quality of the forecasted wind speed is to correct the errors based on the in-situ wind speed values. In this research, a Probability Density Function (PDF) method is employed to correct the errors in the forecasted wind speed for the Wangjiangping station located in the Sichuan Province of China. The key to conduct the PDF is to obtain the sufficient samples to co-pair the observed and forecasted wind speed. In this study, the different number of co-pair samples is selected and conducted the correction. We used the 3-month data from August to October, 2017 to collect the samples and 1-month independent forecasted data in March 2018 to validate the PDF correction results. The independent validation result is indicated that the bias of the forecasted wind speed of one-day ahead has been improved from 1.113 m/s to 0.084 m/s after the PDF correction.

Sector coupling: Renewable gas from offshore wind and offshore electrolysers to decarbonise heat and transport
Submission-ID 257
Malte Jansen, Iain Staffell, Samuel Hill
Imperial College London, Centre for Environmental Policy, United Kingdom
Heat and transport are much harder to decarbonise than the electricity sector with little progress made in the past decade. Hydrogen can offer the energy system a low-carbon fuel that has direct exchangeability with natural gas, a higher specific energy density and more flexibility than electrification. However, this ‘sustainable’ fuel of the future is currently produced using a high-carbon process. This study proposes a more sustainable alternative. A techno-economic model of an offshore wind farm, offshore water electrolysers (both alkaline and proton exchange membrane) and salt-cavern storage is built. The model output is the Levelized Cost of Gas (LCOG), or the lifetime costs portioned across the lifetime hydrogen production. The LCOG has been calculated using deterministic and stochastic approaches. The deterministic model is based on discounted cashflow analysis whereas the stochastic model uses Monte Carlo analysis to calculate the expected LCOG by varying input parameters. Three scenarios were modelled, and alkaline electrolysis cost the least at 8.38 EUR/kgH2. This is, at minimum, four times the cost of the most common conventional hydrogen generation method and twelve times that of natural gas. States are in a unique position to make renewable hydrogen more competitive by reducing the uncertainty around private investment with a supportive policy environment. Reducing the risk of investment alone could see the LCOG of alkaline electrolysis fall to 5.32 EUR/kgH2, near competitive with conventional generation methods.

Concepts for the Connection of Electrical Boilers and Electrolysers to HVDC links
Submission-ID 258
Mario Ndreko 1, Mart van der Meijden 2
1 TenneT TSO GmbH, Germany
2 TenneT TSO B.V., Netherlands
In Germany and the Netherlands, the exploitation of large scale offshore wind power far from shore in the North Sea has gained a large political and environmental importance. Due to the large distance of the planned offshore wind sites from the onshore grid connection points, high voltage direct current (HVDC) transmission technology is regarded as the grid connection alternative. The massive grid connection of large offshore wind power to the transmission system brings congestion challenges and demands for additional flexibility in the system level. Due to grid development constraints, caused by lack of societal understanding and acceptance, it is a challenge to build new transmission lines in order to relief serious grid congestion. In that frame, the provision of flexibility by offshore wind power is a need and it can be achieved by means of transformation of the energy flows using various energy carriers, including both heat and gas. The planned onshore grid connection points are located in the close vicinity of oil and gas industries. Part of the industrial processes use heat which is mainly generated onside by means of boilers which are running on natural gas. Hence, power-to-heat and power-to-gas conversion integrated with HVDC technology becomes a relevant topic for the provision of flexibility.

Concerning the power-to-hear conversion, the state-of-the-art electrical boilers which are already used today in large industrial processes apply high voltage electrode boiler technology connected usually to the medium voltage level (15-35kV). Their rating varies from a couple of MW up to 60 MW. Complementary to the power-to-heat option, in the case of power-to-gas, the generated offshore wind power can be stored as hydrogen by means of applying electrolysers. The stored hydrogen could be physically transported by means of pipe-lines or logistics and be used to provide power infeed by large fuel cells at different grid connection points in the power system.

This paper explores various electrical connection concepts for the integration of power-to-heat and power-to-gas facilities intro the HVDC infrastructure used for offshore wind power. Different grid connection alternatives are assessed starting from the connection of the boilers and electrolysers in AC and in DC connection using AC or DC electrical boilers. For the case of DC connection, the need for a DC-DC converter is stressed especially for the case of power-to-gas. A popular high voltage DC-DC converter topology which fits the above requirements is analysed for its feasibility to be integrated within HVDC links. Furthermore, grid ancillary services are discussed which become relevant in future systems with high penetration of RES. As a conclusion, the integration of power-to-gas and power-to-heat facilities to HVDC links could provide flexibility and accommodate storage as well as virtual transmission capacity in a cost effective manner.

DLR use for optimization of network design with very large wind (and VRE) penetration
Submission-ID 263
Ana Estanqueiro 1, Jake P. Gentle 3, Lennart Söder 2, Patrik Hilber 2, Kateryna Morozovska 2, Thomas Kanefendt 4, Joaquim Duque 1
1 LNEG, Portugal
2 KTH, Sweden
3 INL, United States
4 Fraunhofer IEE, Germany
Due to the stochastic nature of wind, the integration of wind generation in the power system poses serious challenges to the long-term planning of transmission systems. It is worthy to note that the grid reinforcements involve relevant direct costs and the average load factor of the wind dedicated transmission lines is usually low. In very windy sites, the same high wind resource that produces large amounts of wind generation and may congest transmission lines transporting it to distant consumption centres also has a beneficial effect in increasing the transmission capacity of those lines. In fact, the occurrence of wind not only contributes to the loading of the connecting line, but also increases the line capacity, via the convective cooling of the cables - one of the main heat transfer mechanisms in conductor heat balance.

Dynamic line rating (DLR) enables using “hidden” capacity of existing transmission lines to accommodate additional wind power generation. The purpose of DLR is to enable power system operations with higher thermal ratings on existing transmission and distribution networks without compromising the physical operating limits of overhead lines. These operational limits hinge on two main criteria: maximum conductor temperature, and minimum distance above ground – or clearance. Using a DLR approach one may compute a realistic set of values for the line capacity, thus it can be used as a cost-effective solution to alleviate line congestion problems and achieve both an optimal loading of the grid for different climate conditions, and also minimize the cost of new connections to that grid.

A few operational DLR analysis systems were recently developed (e.g. LNEG, KTH, INL) mostly based on CIGRE and IEEE methods for thermal rating calculation of overhead lines. Some tools also associate to DLR analysis the calculation of an optimized power flow, others address reliability aspects of the electrical network.

This paper introduces multiple transmission line rating methodologies, including a comparison between Static Line Ratings (SLR) and Dynamic Line Ratings (DLR). Moreover, case studies with a focus on thermally constrained and critically congested overhead lines will be presented. Case studies have been selected from representative areas throughout North and South Europe, and North America. Each case study will incorporate seasonal rating scenarios, as well as looks across systems with and without wind generation as the primary load on the electrical lines.

The paper aims to demonstrate the value of using DLR during the planning phase of new transmission lines in the areas with high wind probability and to implement DLR operational in-time tools with wind forecast systems thus allowing to assess the added ampacity of the lines (or not) with respect to the meteorological conditions and to estimate the value of that (a) line capacity proving the value of having a DLR approach when operating transmission lines in windy regions.

Optimal Provision of Frequency Containment Reserve with Hybrid Power Plants
Submission-ID 265
George Alin Raducu, Jonas Funkquist, Claudiu Ionita, Nikolaos Styliaras
Vattenfall R&D, Denmark
This paper presents the development of a controller, used to steer renewable hybrid power plants (RHPP), consisting of wind power plants (WPP), solar power plants (SPP) and battery energy storage systems (BESS) that are connected in the same point of common connection (PCC). This controller, referred to as Renewable Park Controller (RPC), will communicate directly with controllers of the individual components of the hybrid plant via set-point commands.

The development of such a controller is expected to facilitate the integration of new units (generation and/or storage) to already existing sites. Some scientific publications regarding optimization-based control can be found in the literature, but basic questions regarding the integration of new units to existing sites have not been addressed often. The exact functionality of this controller can vary depending on the requirements for the specific site, e.g. primary frequency control, power ramp limitation, power limitation, increased renewable energy utilization, etc. Batteries and solar panels are being installed in existing wind farms at an increased rate. It is crucial that the new units will be synchronized with the operation of the wind farm and that the power quality at the PCC will satisfy the requirements imposed by the grid codes.

The design of the controller and a basic dispatch functionality have been presented in the article “Design and Implementation of a Hybrid Power Plant Controller” - Hybrid Power Systems Workshop, Tenerife, 2018. Three functionalities can be mentioned: following a setpoint, power limitation and curtailment strategies. The power limitation functionality ensures that the RHPP never produces more power than the allowed power in the PCC. By defining a curtailment strategy the RPC will select an appropriate priority for power reduction in order to follow the PCC reference set-point. As an example, the wind turbines can be set with low curtailment priority, in order to reduce the mechanical loads.

With the proposed RPC architecture, different market-related operations can easily be performed, such as primary frequency control (PFC), arbitrage and imbalance market. Moreover, a proper combination and coordination of these functionalities has a great potential to increase revenue. The aim of this work is to develop and integrate the already mentioned market related functionalities in the overall RHPP structure for an optimized operation. It is desired to validate the control and optimization algorithm through a Hardware-In-The-Loop (HIL) test and to deploy it in Vattenfall’s future hybrid power plants.

The RPC development adds significant value to the operation of hybrid power plants, proving that such a controller can solve integration issues for newly added components to existing installations. It will also enable further enhanced optimization functionalities based on energy market spot prices, weather forecast and grid demands.

Operational Planning Strategies of Wind-Powered Electric Vehicle Charging Stations for Charging Demand Dispersion
Submission-ID 269
Yerim Lee 1, Yunsung Cho 2, Jin Hur 1
1 Sangmyung University, Korea, Republic of (South)
2 Daegu Catholic University, Korea, Republic of (South)
Jeju island in South Korea plans to deploy wind generating resources into power grids with electrical vehicles (EVs) according to the Carbon-free island Jeju by 2030. Due to the dynamic characteristics of the electric vehicle charging demands, the charging demands can be concentrated in local areas. If electric vehicle charging time is converged, the charging demand connected to power grids will increase and system instability may be induced. Wind Generating Sources (WGRs) can provide as much of the charging energy as possible. In this paper, we propose the operational planning strategies of wind-powered electric vehicle charging stations in order to distribute the charging demand of the electric vehicle with the wind generating resources. To validate the proposed approach for electric vehicle charging dispersion, we use the empirical data from the Jeju Island’s wind farms in South Korea.

Wind Farm Fault Ride Through – An Irish Context
Submission-ID 275
Aaron McDonnell, John Kelleher, Stephen Hunt, John Whelan
Renewable Energy Institute, Japan
In 2017 Ireland installed 426 MW of new wind turbines and had an average power consumption of 3 GW. This made Ireland the European state with the highest level of installed wind capacity relative to its power consumption. Ireland had an annual penetration rate of 26 % from wind in 2017 and has a target of 40 % by 2020. Furthermore Ireland is trialling a System Non-Synchronous Penetration (SNSP) limit of 65 %. This has resulted in an increased focus on the ability of wind farms to contribute to system stability during system disturbances such as faults or other step changes in voltage. In this paper the key Fault Ride Through (FRT) requirements specified in the Irish Grid Code are outlined. The modelled performance of four wind farms connected to the Irish distribution system are compared against the clauses relevant to the FRT requirements. The wind farms in question comprise of turbines with different technologies and from different manufacturers, allowing for a comprehensive overview of the common clauses where compliance may not be fully achieved.

A generic control approach to enable the participation of wind farms in frequency control services
Submission-ID 278
Julien Callec 1, Ye Wang 1, Gauthier Delille 1, Quentin Morel 1, Dorian Sarvary 3, Chuan Shu 2
1 EDF R&D, France
2 EDF EN, France
3 EDF ENS, France
Operating power systems with high penetration of variable renewables and converter-connected resources will require to overcome significant technical challenges, such as reduced system inertia and frequency stability concerns. In this context, higher reserves, new reserve products and new sources of flexibility will be needed. In particular, frequency control services, which are still today mainly supplied by synchronous units, could be procured from wind generation in the future. Previous literature, including from the authors, have demonstrated ways to implement such features at the turbine level. These solutions based on specific control algorithms are, however, not always applicable to plants already in operation; moreover, they are presently not standardized and thus may be difficult to manage on a large scale for companies operating several turbine technologies. That is why this paper introduces a generic approach allowing the participation of wind farms in frequency control which only requires that they can respond to curtailment orders with a suitable response time. The proposed arrangement is described and its performance is then studied though an experimentation on a real 8-MW plant. The generic approach is based on several control blocs that aim at computing, at the wind farm level, an active power setpoint taking into account reserve constitution and the expected response to frequency variations and/or to requests from grid operators. One of the main difficulties of this process is to be able to estimate the Maximum Power Point (MPP) of the plant at any time, no matter the actual operating point is, if possible without requiring additional hardware. In the proposed approach, this task is performed by a “MPP estimator” that uses pieces of data very commonly available on wind farm SCADA such as wind speeds, pitch angles and turbine availabilities; its training process and accuracy level will be presented. This control approach have been tested on a wind farm composed of four 2-MW DFIG turbines located in the south of France. To do so, the proposed algorithms have been implemented within an advanced communication infrastructure based on IEC 61850 standard ensuring interoperability and a high level of cyber-security. The paper will summarize the learnings of this test, including key conclusions on the response time of the whole process. No matter its technology or brand, the presented generic approach allows a wind farm to participate in frequency control services without modification of its internal power and speed controllers provided by the manufacturer. Furthermore, the proposed approach ensures, from the operator’s perspective, a high level of replicability among existing and future wind farms as well as a high level of evolutivity to adapt to changes in grid codes’ requirement. Further work includes the characterization of techniques to assess the quality of frequency control services provided by resources of variable nature.

A Study on Future Power System Database Construction According to the Renewable Energy Expansion of Korean Electric Power System
Submission-ID 280
Seongjun Park 1, Junghoon Han 1, Hyukil Kwon 1, Yunsung Cho 1, Jin Hur 2, Hyunjin Kim 3, Heasu Sin 2
1 Daegu Catholic University, Korea, Republic of (South)
2 Sangmyung University, Korea, Republic of (South)
3 Korea Electric Power Corporation, Korea, Republic of (South)
In Korea, reduction of nuclear power and coal power generation equipment and expansion of renewable energy were planned according to the renewable energy 3020 plan. It is planned to expansion of the renewable generation that amount to 20% of the total power generation by 2030. So it is necessary to build the future power system Database(DB). This paper describes the construction of future power system DB according to expansion of renewable. DB construction, system stabilization and stability analysis are performed automatically by using python-based simulator. The simulator is linked to the PSS/E, the power system analysis tool, through the application programming interface. Algorithm modeling is carried out about system DB construction and stability analysis for generating simulator. And about the PSS/E initial DB construction method of the power system raw data through the generated simulator will be described. First, construction a topology composed of existing generators without renewable sources and stabilize the power flow. And then describe the addition of a renewable generator and a method for power system analysis and stabilization of the constructed initial power system. This paper describes the modeling of renewable energy source and the control of shunt and the addition of FACTS devices for stabilize the power flow. And Transformer Tap ratio adjustment method for stabilizing grid bus voltage are described. Fault analysis is performed to check whether the fault current exceeds the allowable range according to being added of the renewable energy source. In addition, we describe a steady-state and contingency accident state analysis simulator. and describe method to overcome line overload. In this paper, we describe the line linkage method by calculating WSCR and SCR index. A renewable center point is selected in the area with low WSCR index, and SCR index calculation is used to describe the HVDC linkage with the heavy load area. Once the powerflow convergence, line overload and fault analysis are completed, dynamic analysis stimulation is described. It will be checked the stability of the system by checking the initialization of the dynamic stimulation and the divergence of the result graph.

Is cheap electricity storage essential to build an energy system based on wind and solar power?
Submission-ID 282
Emil Nyholm, Lina Reichenberg
Chalmers University of Technology, Sweden
This paper provides a counter-argument to the claim that the cost for storage is crucial to the economic viability of a power system based on renewables. We test the effect of cost reductions for two different generic storage types on the system cost for a European power system based on renewables. The generic storage types are (i) those where we vay the energy investment cost €/kWh], represented by Li-ion batteries, and (ii) those where we vary the capacity investment cost [€/kW], represented by flow batteries. The term greenification cost, the difference between the system cost with X% renewable energy and a business as usual case with very little renewable energy, is introduced in order to compare effects of different cost scenarios. It is found that a cost for Li-Ion batteries of 150 €/kWh, i.e. approximately half of the current cost, will lead to a 10% lower greenification cost, compared to a baseline scenario with a cost of 200 €/kWh. For flow batteries, a 5% lower greenification cost is reached with a cost of 300 €/kW, which amounts about one fourth of the current investment cost. In the most extreme low-cost scenario, where Li-ion batteries are assumed to cost 75 €/kWh and flow batteries 100 €/kW, the reduction in greenification cost compared to baseline is 25%.

Reactive Power Management of a Large Scale Wind Power Cluster in Northern Sweden
Submission-ID 284
Ingmar Leisse
E.ON Energy Networks, Sweden
In the Nysäter cluster in northern Sweden it is planned to establish eight wind farms with a total capacity of 1167 MW. The cluster will be connected to the transmission network at the 400 kV level by a dedicated 400/130 kV substation. The Swedish transmission network operator, Svenska Kraftnät, did put the requirement to control the reactive power exchange at the connection point continuously. In normal operation the reactive power exchange should be controlled to 0 Mvar. However, the wind farm cluster should also be able to absorb or feed-in reactive power corresponding to 10 percent of the actual active power production.

For the Nysäter cluster new power lines need to be raised to transport the power from the wind farms to the 400/130 kV substation. As the power lines are dimensioned according to the maximum power transfer from the wind farms, they will be heavily loaded during periods with maximum power production and a lot of reactive power will be consumed. Calculations have shown that in total 500 Mvar of reactive power are consumed by the 130 kV overhead lines and the adjacent transformers. Moreover, considerable voltage variations will arise in the 130 kV grid as a large amount of reactive power is transferred.

Providing a total of approximately 620 Mvar reactive power at its maximum (500 Mvar network losses and 120 Mvar to the TSO) and maintaining the voltage at the same time is a quite challenging control task. The requirement of continuous control of reactive power increases the complexity further as the need for reactive power is changing with the variation of the active power production.

In previous projects reactive power compensation was mainly achieved by shunt capacitors. However, due to the voltage variation caused by their switching the size needs to be limited as well.

One possible solution would be to install a large number of small shunt capacitors which would be an expensive solution as a large number of bays would be needed, a lot of switching operations are expected and the control is not really continuously.

Another solution would be the installation of an SVC, Static Var Compensor. This type of equipment is based on power electronics which are able to control the reactive power continuously but their cost per Mvar is quite high.

However, modern wind turbines grid connected by electronic power converters normally have the possibility to provide reactive power with both inductive and capacitive power factor. As the wind turbines are located far out in the grid their possible reactive power contribution is limited due to voltage variations.

To fulfil the requirements for the voltage but also the requirements from the TSO and keep the connection costs for the wind farm developers low, in this project it has been chosen to combine traditional shunt capacitors with the ability of continuous reactive power control from modern wind turbines.

Economic and Technical Analysis of Reactive Power Supply with Renewable Energy Power Plants
Submission-ID 285
Hartmudt Koeppe 1, Robin Grab 2, Bernd Engel 1
1 Technische Universität Braunschweig - Institut für Hochspannungstechnik und Elektrische Energieanlagen - elenia, Germany
2 Fraunhofer-Institut für Solare Energiesysteme - ISE, Germany
Variable renewable energy (VRE) power plants have an increasing share of the electricity production. Recently, their influence on power grids is rising, which means that they are asked to contribute to grid quality and stability. One of the key factors is the secure provision of reactive power, not just for local voltage stability issues, but also for grid balancing, and potentially grid optimization.

Presently, a VRE plant’s reactive power capability is usually limited to requirements such as the German TAB HV or the BDEW MV guideline. In these guidelines it is defined that during times with low or no active power generation, VRE are not obligated to provide reactive power or only a small amount. Due to the intermittent nature of their energy sources, this is quite frequently the case for wind and PV power plants. This limits the availability of reactive power from these sources. An improvement can be achieved if the reactive power limitations are extended up to phase shifting operation. With this expansion, the system operator can use VRE for grid optimizing.

Although, the increased provision of reactive power entails an additional burden to the VRE. It leads to higher currents and losses within the power plants, higher stress for the internal components and potentially to a curtailment of active power when the reactive power feed-in is compulsory for grid stability. It is clear that power plant operator need incentives to feed-in reactive power for grid balancing.

To determine the value of these incentives, the operational costs of VRE are analyzed in this work for an actual wind and pv park. The park has circa 35 MW installed power and is modelled in PowerFactory and the park is integrated into a centralized reactive power management of an actual high-voltage grid. The losses as well as the capability of the park are analyzed.

Pen y Cymoedd - Delivery of Enhanced Frequency Response in UK by Batteries
Submission-ID 287
Jonas Persson 1, Sebastian Gerhard 2
1 Section Manager, Power Technology, Vattenfall R&D, Sweden
2 Head of Batteries, Business Area Wind, Vattenfall, Germany
Vattenfall has developed an energy hub inside the existing wind farm Pen y Cymoedd in Wales, UK. By using the existing grid connection, electrical infrastructure, and available land it is a success to combine batteries inside an existing wind farm.

Enhanced Frequency Response (EFR) assists National Grid’s (the TSO in UK) obligation to maintain system frequency within +/- 1% of the target value of 50 Hz, by offering a sub second response for a max duration of 15 mins from the batteries.

Operating independently from the wind farm, the storage system will provide EFR and Capacity Mechanism services for the first four years and thereafter Firm Frequency Response (FFR) services to the network

The storage system has been commisioned in March 2018.

Sneak Preview: PowerDynamics.jl – An Open-Source library for analyzing dynamic stability in power grids with high shares of renewable energy
Submission-ID 290
Tim Kittel, Sabine Auer, Christina Horn
Potsdam Institute for Climate Impact Research, Germany
PowerDynamics.jl is an Open-Source library for dynamic power grid modeling built in the latest scientific programming language, Julia. It provides all the tools necessary to analyze the dynamical stability of power grids with high share of renewable energy. In contrast to conventional tools, it makes full use of the simplicity and generality that Julia combines with highly-optimized just-in-time compiled Code. Additionally, its ecosystem provides DifferentialEquations.jl, a high-performance library for solving differential equations with built-in solvers and interfaces to industrial grade solvers like Sundials. PowerDynamics.jl provides a multitude of dynamics for different node/bus-types, e.g. rotating masses, droop-control in inverters, and is able to explicitly model time delays of inverters. Furthermore, it includes realistic models of fluctuations from renewable energy sources. In this paper, we demonstrate how to use PowerDynamics.jl for the IEEE 14-bus distribution grid feeder.

Machine Learning Approach for Probabilistic Wind Power Forecasts with Discrete Probability Density Function
Submission-ID 292
Anton Kaifel, Martin Felder, Frank Sehnke, Kay Ohnmeiß, Leon Schröder
Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Germany
Neural networks trained on historical feed-in time series of wind turbines or parks are able to deterministically predict power over the next hours to days, by minimizing a scalar cost function on a training data set. Yet similar to the analog ensemble method, the training algorithm can also be adapted to analyze the uncertainty of the power output from the spread of possible targets found in the historical data for a certain meteorological situation. For this new approach of probabilistic forecasts, the uncertainty estimate is achieved by discretizing the continuous time series of power targets into probability density functions (PDF). For each forecast horizon, a deep neural network then predicts the PDF of power output. The resulting empirical probability distribution can then be analyzed to determine percentiles or statistical moments. The advantage of the proposed method is that it avoids the use of costly numerical weather prediction (NWP) ensemble runs. Although a selection of several deterministic NWP forecasts as input can be used. We demonstrate the application of this new method in case studies for a German wind power feed-in as well as a wind farm in Germany and Chile using state-of-the-art deep learning technology.

The North American Renewable Integration Study and the Interconnections Seam Study
Submission-ID 298
Greg Brinkman, Aaron Bloom, Jonathon Ho, Josh Novacheck
National Renewable Energy Laboratory, United States
The National Renewable Energy Laboratory (NREL) recently completed the most detailed North American grid integration study, the Interconnections Seam Study. NREL is also well on the way to completing an even larger and more detailed study, the North American Renewable Integration Study (NARIS). Both projects incorporate co-optimized generation and transmission planning models and unit commitment and economic dispatch models. Both projects use these tools to analyze future North American power systems operating under high penetrations of wind and solar power. At the 2017 workshop, we presented the goals and preliminary results of both studies. At this year’s workshop, we will present the final results from the capacity expansion production cost modeling for the Interconnections Seam Study. We will also present final capacity expansion modeling results and preliminary results from the production cost modeling for NARIS.

Interconnections Seam Study

The Interconnections Seam Study examines the potential economic value of increasing connection between the Eastern and Western Interconnections in North America using high-voltage direct current (HVDC) transmission and leveraging capability across the continent. The study conducted a holistic multi-model analysis which used co-optimized generation and transmission expansion planning, production cost modeling, and AC power flow. Four future designs were developed and studied to quantify and observe potential benefits. The results show increasing cross seam HVDC transmission may have a benefit-to-cost ratios that reach as high as 3.3 and annual operational savings exceeding $2 billion US. These results indicate significant value to increasing the transmission capacity and sharing of resources between interconnections.

North American Renewable Integration Study (NARIS)

NARIS investigates pathways to modernize the North American power system through the efficient planning and operation of transmission, generation, and demand. Power from wind, solar, hydro, and natural gas continues to expand throughout Canada, Mexico, and the United States. We analyze this transformation to the entire North American power system using planning and operational models, collectively spanning time scales from decades to 5-minutes.

For NARIS, we use a diverse set of tools, including a distributed generation adoption model, a capacity expansion model, and a production cost model, among others. We use these tools to investigate four scenarios: Business As Usual, Low-Cost Variable Generation, Carbon Constrained, and Electrification with Carbon Constraints. Within each scenario, we also model a suite of sensitivities to understand the robustness of our results. Capacity expansion results suggest future North American power systems with wind and solar meeting 40-75% of all load, depending on the scenario.

Advanced Inertial Response Control based on Disturbance Observer in Microgrid with Wind Power
Submission-ID 300
Yokohama National University, Japan
Recently, penetration of wind power is going on worldwide. It is an important issue to keep system frequency within allowable range in power systems with a large amount of wind power because its output is uncertain depending on weather condition. Here, it is possible that wind turbines contribute to mitigate the frequency fluctuation by utilizing the controllability of their output. For example, active power curtailment is available when frequency increases due to oversupply. Moreover, it is well known that wind power output can be temporarily increased by using kinetic energy stored in rotating blade even when the output reaches the rated output, namely, inertial response control. Based on this approach, wind turbines contribute to both upward and downward balancing control.

On the other hand, the frequency control is important also in small scale isolated power system, namely, island microgrid. Because the moment of inertia of the entire system is smaller compared to bulk power systems, the frequency fluctuates easily and it is expected that time cycle of the frequency fluctuation is also shorter. Therefore, there is a possibility that the inertial response control works more effectively even though the duration time of the temporal power surge is limited. The authors have developed the wind turbine model with inertial response control so far, and its effectiveness in the microgrid has been tested. However, since the proposed control strategy in those papers was based on droop control methodology, the control effect might be improved by introducing more advanced control technology. To this end, the authors have worked on application of disturbance observer to inertial response control. The disturbance observer can estimate imbalance in the entire system at high speed based on inverse function of inertia model, system frequency, and output change of all the relevant generators. Recently, concept of rate of change of frequency (ROCOF) is often used to provide the required output to imitate synchronous generator, and this concept gives the estimation of disturbance by using only frequency change and inverse function of the inertia model. In the case of disturbance observer, the accuracy of the compensation control of disturbance is improved by using the information of generation output of the other generators.

Hence, the advanced synthetic inertia control by using disturbance observer was developed in this paper. Here, output of a part of generators are estimated supposing the controller models for those generators are known. The proposed method is verified through numerical simulation based on the microgrid model with wind turbines and photovoltaics. The effectiveness of the proposed method is discussed compared to asymmetric synthetic inertia control in which the synthetic inertia control works only when the system frequency moves away from the normal value.

Stability Analysis of Offshore Wind Farms with Fixed Frequency and Diode Rectifier HVDC Connection
Submission-ID 301
Cord Prignitz 1, Hans-Günter Eckel 1, Sven Achenbach 2
1 University of Rostock, Germany
2 Siemens AG, Germany
The German energy policy objectives require continuously increasing renewable energy percentage related to conventional power supply. To reach their targets, utility companies try to locate wind farms offshore with long-distance HVDC connections to the onshore grid. State of the art HVDC topologies are characterized by voltage source converters (VSC) applied as modular multilevel converter (MMC) installed on huge offshore platforms. A promising solution to reduce the size of these platforms and the cost of offshore wind energy seem to replace the offshore VSC by uncontrolled Diode Rectifier Units (DRUs).

Since diodes are passive components, a DRU grid connection requires essential control of voltage and frequency by the wind turbine (WT) converters within the AC offshore distribution grid. Furthermore, WT converters have to compensate the total reactive power demand of the offshore island grid.

In the recent years mainly two types of control concepts for WT converters operating at diode rectifier grid connection have been published. The first type is based on a variable frequency in the offshore grid, balancing the reactive loads of the WTs by droops using the offshore frequency. The second type is characterized by a fixed frequency in the offshore grid with all WT converters operating in a common reference system. Thereby droop functions are used to share reactive power in an advantageous way between the WT converters. Because the DRU grid connection is a nonlinear system, a stability analysis for DRU grid connections is characterized by higher complexity and hasn’t been done this detailed before. Hence a stable operation of these droop control strategies for DRU grid connection as well as their interactions within the offshore island grid has to be verified.

This paper will show different methods of the stability analysis of a DRU connected offshore wind farm. It will explain various approaches to design control models of the DRU grid with varying complexity and point out constraints for the linearization of the nonlinear DRU system. Based on these models, the droop coefficient for reactive power sharing will be designed and validated in an industrial concept of a wind farm topology with PSCADTM/EMTDCTM simulation results. Furthermore, it will be implemented and tested on a laboratory test bench model of a wind farm consisting of downsized WT converters in a 25 V offshore grid.

Bounded Probabilistic Wind Power Forecasting using Mixture Density Recurrent Neural Network
Submission-ID 302
Sebastian Haglund El Gaidi, Mihai Chiru
Greenlytics, Sweden
Uncertainty of wind power forecasts is typically only assessed as the resulting forecast spread from using ensembles of numerical weather predictions as input to the forecast model. Machine learning methods have proven efficient in quantifying forecast uncertainty even when ensembles are not available. Mixture density recurrent neural networks learn time-dependent uncertainty patterns from historical input-output mappings and subsequently estimate the uncertainty of forecasts when confronted with unseen input data. However, mixtures of unconstrained distributions are not suited for expressing bounded variables such as wind power production. Furthermore, the on-off nature of wind power production suggests it is a stochastic variable that could appropriately be described using a mixed discrete-continuous type distribution. To address these issues, we propose a mixture of bounded truncated Normal distributions and point masses at the interval end-points to model the uncertainty of wind power production in a principled manner. We apply the developed model on the GEFCom2014 dataset consisting of wind power production data from 10 wind farms in Australia. The model is compared with baseline forecast models and gives high forecast skill, especially after the first few forecast hours.

Future flexibility valuation in power systems with high penetration of variable generation
Submission-ID 304
Luis Hurtado, Mats F.J. de Ronde, Martijn R. Duvoort
DNV GL, Netherlands

Setting aside the benefits of renewable energy sources (RES) in energy de-carbonization, their stochastic, uncontrollable, and unpredictable nature of renewable sources inevitably increases the uncertainty in the operation of the electrical power systems, either at the system level such as supply-demand mismatch, or network level like transport congestions. At the system level, the increased penetration of RES, inevitably increases the system's need for power reserves and flexibility sources.

The combination of energy market modelling and power system analysis tools is a very powerful method to provide decision makers and responsible agencies with the tools to ensure an appropriate system planning. This area of interest falls in between classical power system studies on the one hand (covering frequency stability, system controls and system balance dynamics) and energy market models on the other hand (that simulate expected economic dispatch, reserves and capacity expansion under future scenarios of fuel prices, etc.). Currently, combining these two domains has proven difficult. It is not only highly data- but also computationally intensive. Thus, power system stakeholders and system planners require an approach that enables to capture the real time dynamics of the power system, i.e., to assess the technical impacts of uncertainty, in the long term planning time frame.

DNV GL Energy, has developed an approach that allows power system planners and decision makers to assess the impact of RES not only from an economic point of view, but also from a technical perspective.

Future flexibility valuation methodology

Given the rapid development and future plans of adding RES capacity, the addition of sufficient ancillary services will likely be a key component in order to maintain a balance in the system, its reliability, and cost efficiency, (minimal curtailment leads to lower costs). Based upon experience with wind and renewable power integration across Europe and the United States, DNV GL proposes an approach that integrates a detailed analysis of expected variability in power with an assessment of the technical and regulatory possibilities for increasing operational flexibility. The methodology consist of an iterative process consisting of two main steps:

Long-term system planning

The objective here is to provide the guidelines and tools for the optimal scheduling of generation and demand in an effort to integrate wind and solar power into the power system with attention to preserving the security of supply, transmission constraints, reliability and commercial aspects of the power system operations.

Dynamic performance of the power system

The assessment of the dynamic performance of the power system is meant to reduce the impact of the remaining imbalance, i.e., output from the long-term system planning. It consists of determining a required amount of spinning reserve to capture the last imbalances within a certain confidence interval.

Submission-ID 306
Tsuji Takao
Yokohama National University, Japan

Although the forecast systems are developed to estimate the renewable energy sources generation, the predicted error is high, causing many problems. Therefore, Probabilistic Power Flow techniques are applied to calculate the risk of introduced Renewables. In this paper probabilistic power flow is created by Monte Carlo Simulations and developed further by Quasi-Monte Carlo Simulations. Monte Carlo method has the most accurate results but has enormous computation burden, while this is exclusively important when dealing with large systems and real-time applications. Thus, in this paper improvement on computation time is shown by changing the pseudo-random numbers to low-discrepancy sequence. Here, computation time is decreased to the desired value, but the accuracy became reduced than Monte Carlo method within an acceptable range. In addition, in the traditional PPF calculations, renewable energy sources and their forecast errors are taken separately as random variables by their probabilistic density functions, but in this paper relationship between them are taken into consideration. Multivariate functions are used to combine them.

Keywords – Probabilistic Power Flow (PPF), Monte Carlo Simulations (MCS), Quasi-Monte Carlo Simulations (QMCS), probabilistic density function (PDF), Sobol sequence, low-discrepancy sequences.

Grid investment needs for Renewable Capacity Integration - A different approach
Submission-ID 307
Hélder Milheiras
1- ERSE - Entidade Reguladora dos Serviços Energéticos, Portugal
In order to meet European green targets set on renewable penetration and reduction of greenhouse gases, During the last decade, Portuguese National Electric System faced major changes on generation, with more than 6 GW of new renewable generation units commissioned, of which of 3 GW of wind farms. Nevertheless, until 2030, Portugal is expected to commission additional 15 GW, reaching 80% of the total installed capacity.

In order to integrate such capacity strong networks are needed to deliver electricity from generation areas located far away from consumption areas. Over this decade, transmission network grew 25% in circuit length and doubled its substations’ transformation capacity.

Nevertheless, this evolution won’t allow the integration of new generation expected for the coming years. In fact, several areas offer no available connection for new capacity for the near future. This lack of connection capacity is explained by transmission operator due to network simulation results of several generation scenarios for hydro and wind regimes cross checked with demand scenarios for peak and off-peak hours. Shall any simulation result in violation of the approved network security standards, then no available capacity will be published, even if this scenario has very low probability.

According to the National Regulator, this practice should be reviewed as it is preventing more renewable generation to be connected to the grid, forcing promotors to hold their investments for years, until more investments are commissioned. However, from an economic point of view the Regulator is not willing to accept a continuous growth in transmission grid without an equivalent growth on connection capacity. Thus, Regulator asks for a deep review existing regulation and national codes, as a result of recent approved “European Request for Generators Network Code” which offers the transmission operator new grid management tools allowing remote dispatch of RES if necessary, which didn’t happen before.

National Regulator also recommends more dynamic network studies taking into consideration the probability associated with scenarios, resulting in hourly connection capacity to be made available to new generations, allowing the connection of units that best remaining available grid capacity, even if generation will be restricted during off-peak hours.

This approach would result in much more renewable generation capacity penetration helping achieving National and European political targets. And, of course, it would postponed many millions of euros on grid investments and extra charges for consumers.

The paper will discuss all topics related to new renewable capacity; the current practice of connection capacity calculation and the resulting lack of available capacity; the need to updating existing practice; the need for dynamic and probabilistic analysis and a view on economic impact of postponements big investments.

Frequency Regulation of Power System in Japan with Large-Scale Integration of Renewables by using Electrolyzers
Submission-ID 309
Takao Tsuji, Jingting Qi
Yokohama National University, Japan
It is an important issue to properly manage the system frequency of power systems with a large amount of renewable energy sources due to their uncertain output fluctuation. Since the power system in Japan does not have international tie-lines, the frequency easily changes and frequency regulation technique becomes very important. There are various approaches to mitigate the frequency fluctuation in addition to the conventional methods such as load frequency control (LFC) and economic dispatching control (EDC). For example, charging and discharging control of rechargeable batteries equipped with electric vehicles can contribute to the stabilization. Also, the output control of renewable energy sources with active power curtailment has been used for frequency regulation in some countries.

Supposing an extraordinarily large amount of renewable energy sources are introduced to power systems in Japan, there is a possibility that excess power often occurs and it is effective to install the electrolyzers to convert power to hydrogen in order to efficiently use the excess power. It is well know that control speed of the electrolyzer is so high that it is possible stabilize the frequency as a part of primary or secondary control reserves. Here, it should be noted that control speed of the electrolyzers might be oftern limited to keep the stability of the process to store the generated hydrogen depending on the type of the storage devices.

It is expected that various balancing power are available for frequency regulation in the future as above. Hence, in this paper, the frequency control is simulated supposing large-scale integration of renewable energy sources is realized in the power system in Japan. Due to the reduction of system inertia, frequency drop after a disturbance will be more significant problem. Therefore, based on various operational conditions in which the ratio of the conventional generators in operation is very low, the frequency response in both normal and contingency conditions were simulated. The simulation model is based on IEEJ AGC30 system model and rechargeable battery, droop and inertial response control of renewables, and electrolyzers models were newly added to the system model.

Maximum likelihood wind field state estimator with LIDAR measurements for wind farm control
Submission-ID 311
Bahri Uzunoglu
Uppsala University, Division of Electricity, Sweden
In this study, LIDAR measurements with medium fidelity models to be used in real time wind farm control applications to increase the wind farm efficiency, life span, operation and maintenance. The challenge is to make use of very limited observations from point measurements such as LIDAR positions and employ this information for the wind state estimations for whole wind farm area. The model in question here will be a two dimensional medium fidelity model that can run in real time. The model in question is not a high fidelity model since these models are computationally very demanding as a result no recorded implementation of them for real time actual controls exist. Once the medium fidelity model is chosen, the question is how to assimilate limited real time LIDAR observations with full farm state which will be addressed in this study. To achieve this assimilation, a novel Maximum likelihood wind field state estimator with LIDAR measurements for wind farm control will be introduced.

Operational Experience with a Type 3 WPP in a Weak Power System Region with Type 2 WPPs and Synchronous Condensers
Submission-ID 316
Amr Abdellaoui, Mohamed Asmine
Hydro-Québec (TransÉnergie), Canada
With the increasing integration level of wind generation over time, different Wind power plants (WPPs) types were integrated in a weak transmission system region. This raises challenges regarding voltage control schemes that have to be implemented in order to maintain the reliability of the system. Thus, thorough dynamic studies have to be performed to optimize the voltage control schemes of the WPPs which should be followed by field testing to ensure a reliable operation. This paper presents the Hydro-Québec operational experience with the implementation of a voltage control strategy scheme to coordinate the voltage regulation between a type-III WPP and two synchronous condensers at a common point of coupling in a region with two type-II WPPs.

Keywords- Wind power plants, synchronous condensers, voltage control, field tests, stability studies.

Improved Methodology for Determining Responsive Reserve Requirements in ERCOT
Submission-ID 321
Julia Matevosyan
ERCOT, United States
Responsive Reserve (RRS) is a critical service ERCOT employs for arresting and recovering grid frequency during large generation trip events. ERCOT determines RRS amounts for each 4-hour block of an upcoming year based on historic synchronous inertia conditions. The RRS quantities are published before the start of the year and procured in the Day Ahead market every day. ERCOT is also evaluating sufficiency of RRS in real time based on actual inertia conditions before the start of and during an operating day. In previous years, a gap has occurred during real time operations between the year ahead calculation of RRS quantities and real time RRS needs. This project seeks to address this sufficiency gap.

The project developed an improved methodology for determining RRS quantities before the start of the year that significantly improves balance between economic efficiency (i.e. reducing over procurement) and reliability (i.e. reducing under procurement).

The new methodology analyzes inertia distributions separately for each time period under evaluation (i.e. every 4-hour block within each month) then, based on regression analysis, finds optimized parameters for each time period. As ERCOT is determining RRS quantities for the upcoming year, the optimized parameters will allow to minimize over- or under- procurement of RRS in real time.

The proposed methodology is implemented in Python. The code contains tunable parameters to allow for different scenarios to be analyzed, as well as a risk analysis to identify times of year that are particularly at-risk for under procurement and give additional sensitivity to these risky hours.

Results from the proposed methodology show a 16.81% increase in RRS sufficiency against benchmark validation data. It will help the year ahead Ancillary Service study run more accurately and faster, leading to lower risk of reliability issues due to RRS insufficiency in the future.

Usability of Flexible Demand and Generation in the BDEW Smart Grid Traffic Light Concept
Submission-ID 327
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Usability of Flexible Demand and Generation in the BDEW Smart Grid Traffic Light Concept
Submission-ID 328
Leonard Hülsmann 1, E. Tröster 1, U. Ohl 2, M. Koch 2
1 Energynautics GmbH x, Germany
2 EWR Netze, Germany
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MIGRATE project and Future Power Systems
Submission-ID 329
Jako Kilter
Elering/Tallinn University of Technology, Estonia
MIGRATE stands for Massive InteGRATion of power Electronic devices and is an EU-funded project under the framework of Horizon 2020. The aim of MIGRATE is to find solutions for the technological challenges the grid is currently and especially in future faced with. By 2020, several areas of the HVAC pan-European transmission system will be operated with extremely high penetrations of Power Electronics(PE)- interfaced generators, thus becoming the only generating units for some periods of the day or of the year – due to renewable (wind, solar) electricity. This will result in growing dynamic stability issues for the power system (possibly a new major barrier against future renewable penetration), the necessity to upgrade existing protection schemes, and measures to mitigate the resulting degradation of power quality due to harmonics propagation.

European TSOs from Estonia, Finland, France, Germany, Iceland, Ireland, Italy, Netherlands, Slovenia, Spain and UK have joined to address such challenges with manufacturers (GE, Schneider Electric) and universities/research centres. They propose innovative solutions to progressively adjust the HVAC system operations.

Firstly, a replicable methodology is developed for appraising the distance of any EU 28 control zone to instability due to PE proliferation and for monitoring it in real time, along with a portfolio of incremental improvements of existing technologies (the tuning of controllers, a pilot test of wide-area control techniques and the upgrading of protection devices with impacts on the present grid codes). Next, innovative power system control laws are designed to cope with the lack of synchronous machines. Numerical simulations and laboratory tests deliver promising control solutions together with recommendations for new PE grid connection rules and the development of a novel protection technology and mitigation of the foreseen power quality disturbances. Technology and economic impacts of such innovations are quantified together with barriers to be overcome in order to recommend future deployment scenarios. Dissemination activities support the deployment schemes of the project outputs based on knowledge sharing among targeted stakeholders at EC level. Project website for more information: https://www.h2020-migrate.eu/

Large Disturbance Rotor Angle Stability Analysis in Power Systems with High Penetration Levels of Wind Power
Submission-ID 330
Da Wang
TU Delft, Netherlands
Unlike synchronous generators used in conventional power plants, wind turbines cannot directly respond to large disturbances, due to their power electronic converter based interface and maximum power control strategy. Therefore, the decommissioning of synchronous generators and the integration of wind turbines will influence the large disturbances rotor angle stability of existing power systems. How to solve this challenge? Three questions should be answered: what is the influence of wind turbine? How to assess/monitor this influence? and finally how to mitigate this influence? This presentation introduces the work to answer the above 3 questions, carried out in the framework of MIGRATE.

Operation of Wide-Area-Controls in Iceland
Submission-ID 331
Birkir Heimisson
Landsnet, Iceland
The Icelandic power system experiences large and rapid frequency deviations because of its low inertia and large loads relative to the system size. Using fast synchrophasor based signals to trigger new flexible demand- and supply-side resources, fast response services can reduce frequency deviations and the probability and impact of islanding. This presentation reports on the experience of the first live operational fast response services addressing angle and frequency stability. The approach improves reliability and access to the grid in constrained locations.

Experimentation results : Grid Forming Control Interoperability Tests and Current Limitation
Submission-ID 332
Thibault Prevost
RTE, France
Different grid forming controls have been developed/improved within WP3 of MIGRATE project and simulations are being performed to validate their behavior on small networks. Interoperability issue will also be addressed between the different controls on such small network. Moreover, current limitation strategies, which is of critical importance for such inverters will presented and first tests on reduced scale hardware will be presented.Different grid forming controls have been developed/improved within WP3 of MIGRATE project and simulations are being performed to validate their behavior on small networks. Interoperability issue will also be addressed between the different controls on such small network. Moreover, current limitation strategies, which is of critical importance for such inverters will presented and first tests on reduced scale hardware will be presented.

Relay and System Protection Challenges in Future Power Systems
Submission-ID 333
Ruben Andrino Gallego
REE, Spain

Power Quality and Grid Codes in View of Massive Integration of Power Electronic Devices
Submission-ID 334
Marta Val Escudero
Eirgrid, Ireland
This presentation will present some of the results from work package dealing with power quality in MIGRATE. The main aspects of power quality will be discussed considering the changes taking place in current and future power systems. Results from various power quality related questionniares and studies made in MIGRATE will be presented considering technical and legislative framework. Results and their influence to grid connection rules and codes will be discussed.

Review of European Grid Codes for Wind Farms and Their Implications for Wind Power Curtailments
Submission-ID 335
Elis Nycander, Lennart Söder
KTH Royal Institute of Technology, Sweden
In order to accommodate the increasing wind power penetration in power systems grid codes for wind power plants are being continuously updated by TSOs. In this paper we review several European grid codes for wind power released from 2015-2018. Specifically, we focus on the comparison of the new ENTSO-E grid code Requirements for Generators, released in 2016, with national grid codes to see to what extent these are in agreement and how this contributes towards harmonization of grid codes within Europe. Also, we discuss the implications of the grid codes for performing curtailments of wind power plants.

Spine Toolbox and Spine Model for Open Source Energy System Analysis
Submission-ID 336
Juha Kiviluoma 1, Mikael Amelin 2, Erik Delarue 3, Jody Dillon 5, Maren Ihlemann 3, Steffen Kaminski 3, Toni Lastusilta 1, Manuel Marin 2, Jon Olauson 2, Fabiano Pallonetto 4, Kris Poncelet 3, Erkka Rinne 1, Pekka T Savolainen 1, Lennart Söder 2, Per Vennström 1
1 VTT Technical Research Centre of Finland, Finland
2 KTH - Royal Institute of Technology, Sweden
3 KU Leuven, Belgium
4 University College Dublin, Ireland
5 Energy Reform Ltd, Ireland
EU project Spine is developing an open source toolbox and model for energy system modelling. The purpose is to offer a platform for data acquisition tools, data processing tools, and models of different temporal and/or geographic scope. The toolbox will help users to manage data, scenarios and modelling projects. It will allow connecting models of different scope through the toolbox interface. The poster presents the Spine Toolbox user interface, data structure and the interface for connecting external tools and models. It will also introduce the Spine Model, which is an adaptable energy systems optimization model integrated with the Spine Toolbox.