Keywords—Onshore wind power plant layout optimization, Non-dominated Genetic Algorithm, Mixed-discrete Particle Swarm, Bogdanci wind farm
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%.
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.
Keywords-HVDC; rectifier; wind power plant; FixReF; grid-forming control; FRT; asymmetrical fault; auxiliary power
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.
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.
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.
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:
 Selecting WFs that exceed their target capacities.
 Configuring TNs among WFs and access points (APs).
PVD and a GA are applied to address these challenges.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
* 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.
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).
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.
 International Energy Agency (IEA): “System Integration of Renewables”, IEA (2018)
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.
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.
Keywords-harmonic analysis, harmonic passive filter, connection assessment, offshore wind power, power grid
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.
Keywords / Indexing terms
Temporary overvoltages, TOV impact assessment, harmonic passive filter, C-type filter, connection assessment, offshore wind power, power grid
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
 TWENTIES project, “Demonstration project 6 – Improving flexibility of the grid (FLEXGRID)”, October 2013  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.  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.
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 .
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:
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.
The concept and first results of an experimental test-bed system, set up at the HTW Berlin has been presented in . 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.
 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
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.
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.
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.
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 .
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 . 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.
 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].
 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].
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) 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:
With a campus energy demand of: 39.2GWh pa - Gas • 23.8GWh pa - Electricity
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).
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 .
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:
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.
: O. Brückl et al: „Zukünftige Bereitstellung von Blindleistung und anderen Maßnahmen für die Netzsicherheit“, Regensburg, 2016
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  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.
: “Getting Wind and Sun onto the Grid A Manual for Policy Makers”, International Energy Agency, Paris, 2017
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Keywords- Wind power plants, synchronous condensers, voltage control, field tests, stability studies.
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.
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/