This paper look into synthetic inertia control of doubly fed induction generator (DFIG) based wind turbines to provide dynamic frequency support. In conventional synthetic inertia control if a large disturbance occurs in a power grid, two additional loops are implemented in each DFIG controller: droop
loop and rate of change of frequency (ROCOF) loop. Because of their fixed control gains, difficulties arise in determining them suitably, depending on the architecture of power system to which the wind turbines are connected and wind conditions. This paper proposes an optimal synthetic inertial control by means of using the particle swarm optimization algorithm (PSO) to calculate the optimum values of the control gains. The proposed approach aims to enhance the frequency nadir and ensure stable operation of the wind turbine.
The Kriegers Flak Combined Grid Solution (KF CGS) is the world-first hybrid-asset system, which serves as an interconnection between East Denmark and Germany via the offshore wind power infrastructure in the Baltic Sea. The hybrid-asset system includes offshore wind power plants of both countries interconnected via a meshed offshore HVAC grids and an HVDC Back-to-Back Converter Station at German shoreline. The KF CGS interconnection is controlled by the Master Controller for Interconnector Operation (MIO).
The MIO utilizes the optimal power flow (OPF) and performs the transmission capacity calculation applying wind power forecasts, equipment’s electrical data, availability and ratings including dynamic line rating (DLR), and onshore grid limitations. Due to wind power forecast errors or trip of transmission equipment, the MIO mitigates overloading of any KF CGS equipment, which may occur, according to the prioritized rules with minimum possible wind power curtailment.
This paper explains the method behind the transmission capacity calculations of the KF CGS hybrid-asset system, implementation of the simulation model for the grid-planning and grid-stability assessment, and presents the results of the MIO first onsite test. The lessons learned are useful for design and operation of future hybrid-asset systems combining both HVAC and/or HVDC interconnector offshore grids and wind power feeding into the same grid.
Hereby a modelling approach for wind power in large scale power system models is presented. Therefor a generic wind farm layout is implemented and an analytical wake model is applied to it. The generic layout is organized as a square grid in resolution of the rotor diameter matching the given area. The wind turbines are placed in a fixed pattern matching the total given power and aiming to match rules of ideal placement. From that an efficiency field in dependence of wind speed and wind direction is obtained. The efficiency is defined as the total power of the wind farm considering wake losses for a certain wind speed and direction divided by the power a single turbine would produce given that respective wind speed times the number of turbines. This efficiency field is then introduced into a power system model. The power system model is therefore able to model the wind power generation not only in dependence of wind speed, but also the wind direction with comparatively low computational effort. This allows maintaining a physically sensible interpretation of a wind power plant and keeping the performance factor to availability and uncertainty losses only.
For the generation of the efficiency field the CharL model is used. As an exemplary power system model the TransiEnt Library is used. For the validation of the approach the German wind power generation at a municipal level is modelled using the characteristic lines of a small and a large wind turbine. The efficiency fields are then generated for each turbine with a generic farm layout for three power densities (30, 60 and 90 MW/km²). The area of the wind farms is set to accompany 40 turbines at 60 MW/km². The resulting power profiles are scaled according to the installed power in each municipality, whereby the choice of turbine and efficiency field is made appropriately. For all farms the main wind direction is set to western. The wind power feed – as published by the TSOs – is used as reference for the validation.
With this approach the wind generation profile can be modelled closely and the impact of the wind direction can be shown as an additional parameter by successfully modelling power generation drops that relate to changes in wind direction.
The development of Variable Renewable Energy (VRE such as wind and solar power) at high share introduces a new source of uncertainty in electrical power systems. The resulting power imbalances should be integrated into the balancing studies, and in particular when sizing the automatic Frequency Restoration Reserve (aFRR). Traditionally, Transmission System Operators (TSO) use deterministic approach for aFRR sizing, even though some TSOs have already opted for probabilistic methods. Indeed, probabilistic method enables to tackle explicitly the uncertainty driven by VRE. Moreover, probabilistic method is undoubtedly going to be broadly adopted with the entry in force of European Network Codes (cf. article 157 of System Operation code).
“OPIUM” is a tool developed at EDF R&D. This tool is based on a probabilistic approach and aims to evaluate the needs in power margin for a given time horizon (e.g. 15 minutes for aFRR) while taking into account a predefined risk level, at the scale of a country and for each hour of the year considered.
More specifically OPIUM consists of a 3-steps process (performed for each of the 8760 hours of a year):
In the full paper, the methodology will be presented in detail. After being validated in the French case with 2015 data, OPIUM is then used considering prospective scenarios with higher VRE penetration levels (> 50%) developed in the European project EU-Sysflex. The results give an insight in the potential evolution in the aFRR sizing across Europe. Moreover, several conclusions can be drawn from the results:
In March 2017, the Australian Energy Market Operator (AEMO) issued a final report describing the results of the investigation of the 2016 South Australian blackout. While discussing the validation of dynamic models of SA wind power plants (WPPs), the Report compares against each other PSS/E-based RMS simulations, PSCAD-based EMT simulations and high-speed monitor (HSM) data measured during the Blackout.
This paper considers factors that can contribute to mismatches between WPP simulations and measurements and, also, the extent to which the accuracy of WPP models can affect the accuracy of a system-wide model with WPPs. The paper refers to comparison plots and observations in the AEMO report and, also, to information on South Australian WPPs available from the public domain. The analysis is based on experience with PSS/E stability studies for a variety of WPP projects in the USA.
For an individual WTG, the following factors are considered: representation of the mechanical drive train and power electronics converter; the role of the operating point on the WTG power curve; and reproduction of the output active/reactive power and terminal voltage. For a WPP as whole, considered are aggregation of the collector, the role of the central power plant controller and the impact of wind speed variation and meteorological conditions. The paper also discusses handling switching events and briefly comments on model validation and accuracy.
An emphasis is made on situations where assumptions and approaches widely used in model development for PSS/E-based stability studies can adversely affect the accuracy of fault simulations. While this paper outlines possible mechanisms and implications of a number of effects, it not necessarily intended to explain concrete phenomena observed during the Blackout. The AEMO report is used as an impetus for a discussion of a variety of WPP modeling aspects.
As per the analysis, the degree of detail assumed for WTG/WPP components may explain, at least in part, the effect of larger dips and slower recovery in the HSM data in comparison against PSS/E and PSCAD simulations. While measured data inherently reflect meteorological conditions during an event, it is difficult, if at all possible, to account for these factors when simulating the event. Presumably, for the case of three-phase faults close to WPP points of interconnection, one may well expect more pronounced oscillations with a greater effect on the overall WPPs’ dynamics and, consequently, more significant mismatches among simulations and measurements.
Based on the analysis, the paper identifies lessons to learn in order to enhance the modeling of WPP dynamics in PSS/E-based power system planning and operation studies. The paper concludes that despite numerous advances in WTG/WPP model development and the reliable WPP operation in the field, more insight is needed into many aspects of WPP modeling in planning and operation studies.
Modern power systems include a growing number of wind turbine generators (WTGs) and other new system components. Interconnection stability studies are usually based on commercial positive-sequence RMS dynamic programs (for example, PSS/E). A vendor-specific model (VSM) of a system component is developed by its manufacturer and aims to reflect essential characteristics of the component’s dynamic response. Often, a VSM intended for a specific dynamic program is implemented as a user-defined model (UDM), which means that it is not included in the library of standard models of this program. In addition, many VSMs are proprietary.
A VSM is supposed to be more accurate than standard/generic models applicable to the same component. Nevertheless, VSMs are gradually supplanted by standard and generic models. One of the reasons is that VSMs are often provided to the industry without sufficient verification and validation; as a result, they cause problems in interconnection studies and system-wide model development. Implementation of a VSM as a UDM complicates its use. It is no surprise that industry organizations are reluctant to apply UDMs. This has created a situation where UDMs of new power system components are discouraged or disallowed in actual studies. In the USA, some organizations restrict their use or no longer accept them at all, the corollary of which is that many VSMs may not be applied, either.
This paper provides recommendations on how to enhance the quality of VSMs: to make them more suited for interconnection studies, to help prevent modeling issues and to facilitate model investigations when issues occur. The proposed measures are based on experience with a variety of PSS/E VSMs/UDMs representing WTGs, solar PV inverters, HVDC transmissions, energy storage devices and other new power system components. These VSMs were supplied for interconnection studies for generation, transmission and energy storage facilities in North America during the past decade.
The paper considers the following aspects of the VSM development: model functionality (disabling modules; stand-alone modes; and model state indication); numerical solution (limitations on the integration time step and internal integration loops); model size and variables (reserved/redundant variables and confusion of variables); intermittency of energy sources (for wind power plants: deterministic modeling of wind variation; stochastic modeling of wind speed fluctuations; and need to account for wind variation); additional verification testing (errors in memory allocation and reproducibility of results). Additionally, the need for more specific information on eigenvalues pertaining to the component is discussed.
The recommendations are intended to help ensure that more VSMs remain in the modeling arsenal of a power system engineer and that these advantageous simulation tools continue to be used in interconnection studies, for which model accuracy is crucial.
Following from smaller-scale investigations of grid-forming converter control applied to wind turbines in 2017-8, this paper describes a much larger trial involving an entire wind park, operated by Scottish Power Renewables. To our knowledge this is the first UK converter-connected wind park to operate in grid-forming mode, and the largest in the world to date. The 23-turbine, 69 MW park ran in grid-forming mode for approximately 6 weeks, exploring inertia contributions of between H = 0.2 s and H = 8 s. A large amount of data was gathered at the turbine and farm level, recording responses both to deliberately-induced scenarios, and also to grid events. A number of unscheduled frequency disturbances occurred due to interconnector, CCGT and other trips, to which un-curtailed turbines were able to actively respond. While a significant amount of incremental improvement (software, hardware and energy storage) is still required to deal with the most extreme events which could occur, the turbines are able to provide stable and appropriate response at relatively high inertia levels to the frequency events commonly occurring today.
A requirement for power systems connected to larger systems through high-voltage, direct-current (HVDC) interconnectors is that frequency must be managed internally. In particular, connected generation needs to have sufficient inertia and emergency reserve to cope with loss of a major infeed or interconnector. Sufficient inertia is needed to limit rate of change of frequency (ROCOF) to a value that does not compromise loss-of-mains (LOM) protection on distributed generation, including wind farms. A ROCOF of 1.0 Hz/s has been adopted in Ireland. A ROCOF greater than 1.0 Hz/s is deemed to indicate loss of mains, triggering disconnection of the affected generation only.
The challenge for a system with significant wind and/or solar generation is that such generation does not generally provide inertia. Hence such non-synchronous generation, including HVDC import, is limited to a proportion of demand, including export. The proportion, known as system non-synchronous penetration (SNSP), is currently set at 65% in Ireland, with an ambition to increase it to 75%. When the SNSP limit is active, wind generation is curtailed. Such action is taken in preference to reducing HVDC import. SNSP-induced curtailment in Ireland for 2017 was 2.75% of potential wind generation.
Clearly, as wind penetration increases beyond the current level of 29%, so also will curtailment, even with SNSP increasing to 75%. Eventually the cost to wind farm developers and consumers will stifle investment in the sector. Hence it is important to study the factors affecting curtailment, and to assess possible remedies.
The paper will consider the effects of import reduction and export on potential wind penetration in the Iriosh power system for various curtailment toleration levels up to 10%. The scenarios are based on extrapolation of historic demand and wind power data. SMP data have also been used to explore the scope for extra penetration levels when curtailment is based on value rather than magnitude. It will be shown that import reduction can extend the practical wind penetration level, especially when based on curtailment value rather than magnitude. More significant improvement is possible if export capacity is fully exploited.
It will be argued that import reduction to avoid curtailment is justified by basic economics as well as by decreased emissions. However, the major benefit is that the system can accept a significantly greater wind penetration for a given curtailment tolerance.
This paper presents a detailed dynamic simulation study to demonstrate the potential capability of a 400 MW offshore wind farm to black start an onshore grid, using grid forming converter controls in the wind turbines (WTs) with P/f and Q/V droop control. The wind farm modelled comprises 50 WTs of 8 MW rating, and is connected via a 120 km long AC export cable to the onshore grid.
Simulations indicate that it is theoretically possible to black start an onshore grid by adding block loads of up to 50 MW, provided the grid forming controllers and electrical system are properly designed for black start, and there is adequate wind. Energization of the large offshore transformer, export cable and onshore transformer, result in significant transient voltage distortions, aggravated by transformer saturation, but do not cause a trip of the modelled WT protection. Practical challenges to allow an offshore wind farm to black start a grid are also discussed.
This project received financial and technical support from The Carbon Trust’s Offshore Wind Accelerator (OWA), a collaborative R&D program funded by nine leading offshore wind developers (E.ON, EnBW, Equinor, Innogy, Ørsted, SPR, Shell, SSE, Vattenfall) and the Scottish Government.
Wind energy has become an essential cost-competitive pillar for the energy transition, and the numerous possibilities of a wind power plant’s design motivate the use of optimization techniques. This work proposes a multi-objective function to size the cable diameter of an offshore wind power plant considering the corresponding power losses and costs of cables. With the help of Pareto optimality, the best set of cables for a given placement of wind turbines and the substation were chosen. The results were compared with a reference case, where the cables were sized to fulfill the current-carrying capacity. The result showed a more cost-effective design with the proposed optimization method.
Key European institutions influencing the development of power grids are of the opinion a well interconnected and integrated European grid is indispensable to deliver the ultimate goals of the Energy Union (competitiveness, security of supply and sustainability) to all Europeans. However, they also underline that these objectives will only be achieved with an adequate and smart infrastructure ensuring optimal connection and sectoral integration across Europe. In particular, they emphasise the necessity to avoid non-viable or inefficient investments via a better exploitation of existing interconnectors at transmission and distribution levels. This should enable rapid free capacity and improve public acceptance.
ACER decision on 27th February 2019 setting the common methodology for the calculation of cross-border capacities for the single electricity market coupling. It reinforces the obligation for the Core TSOs to gradually replace seasonal limits to calculate the maximum admissible current limit with a dynamic limit, which ensures that it represents the maximum current under expected ambient conditions for a given market time unit. As such, TSOs are mandated to compare the costs and benefits of installing the equipment needed to implement dynamic limit. This disposition shall be implemented by TSOs of the Core CCR no later than 1 December 2020.
With this background, we assess the interconnector use for the past x years based on data from ENTSO-E transparency platform. The method proposed in Paper "An Objective Measure of Interconnection Usage for High Levels of Wind Integration"in 2014is applied to identify the interconnectors which show the highest potential for benefit gain by increasing the exchange capacity.
Focusing on these lines, we then performed detailed modelling of the interconnectors, to better understand the ambient conditions impacting the maximum admissible current. This involves identifying the line corridors, tower locations, topographical information like elevation, conductor type and physical properties, to construct a detailed model of the interconnectors. The CIGRE method is applied to calculate the hourly dynamic line rating of each span using wind speed, ambient temperature and solar irradiation data from the past x years. The paper presents the results of this detailed back-casting of the interconnectors' line rating.
The comparison to real-time physical flows obtained from the ENTSO-E Transparency Platform shows that the interconnector capacity gain made available by DLR is more than x% on all interconnectors evaluated. Furthermore, it shows how the Capacity Factor of the interconnector reduces by considering DLR.
The purpose of this study is to raise awareness about the level of potential gain there is to be made with regards to increasing interconnector use. Such a study will pave the way for assessment of Cost Benefit Analysis and assist TSOs who are mandated to implement dynamic limit.
This paper presents the system analysis 2019 of German TSOs on demand for reserve generation capacity to maintain system security.
System Security, Market Simulation, Load Flow Analysis, Load Flow Optimization, RES Integration, Congestion Management, Redispatch, Reserve Generation Capacity
In 2011 the German government decided to shut down existing nuclear generation units successively until 2022 and to accelerate electricity generation based on renewable energy sources (RES). Since then the RES target figures were repeatedly increased in accordance with the European energy policy objectives. As nuclear generation units were erected close to load centres in Southern Germany and wind power generation is mainly located in Northern Germany, a significant increase in North-South load flows during strong wind situations has been observed. This is amplified by changes in the power generation portfolio together with the political commitment to increase cross-border trading capacities beyond physical transmission capacities. Especially in high wind generation scenarios with additional transit flows through Germany, the maximum transmission grid capacities may be exceeded. The congestions in the transmission grid need to be mitigated by TSO instructed generation redispatch.
The so-called “Reserve Generation Capacity Ordinance”, revised 2016, was introduced 2013 and obliges the German TSOs to jointly assess/determine the measures necessary to maintain adequate system security. The annual TSO report is based on coordinated detailed analyses and is submitted to the national regulatory authority for confirmation.
The paper covers the periods 2019/20 and 2022/23 and considers annual trends, as well as severe scenarios. It takes the progressive reinforcement and reported delays to the transmission system into consideration, and the results show that the most challenging situations occur near peak load, combined with high wind energy generation or in case of high transit power flows through the German transmission network. The need for market-related measures to maintain system security increases continuously, as well as the corresponding demand for reserve generation capacity. The analysis determines for 2019/20 a total need of more than 15 GW redispatch capacity, of which 5 GW is derived from reserve generation capacity located in Southern Germany.
This paper provides insight into the legal, regulatory and analytical background of this system analysis. Beside refining the process for determining the robust reserve generation portfolio, in 2019 the flow-based market coupling methodology was implemented to adequately consider the future market framework as defined by the EU clean energy package. The methodologies are outlined and the results of the market and grid analyses are presented. Concluding, the paper illustrates the increasing challenges to maintain system security and the need for adequate transmission system reinforcement.
The grid-connection of the large offshore wind power plant (OWPP) at Kriegers Flak in the Baltic Sea to the East Danish transmission grid differs from the common radial-type connections of existing OWPP. The grid-connection is a meshed HVAC (220kV) cable system with a total 205 km cable length and including the two platforms, i.e. Kriegers Flak A (KFA, 200 MW) and Kriegers Flak B (KFB, 400 MW). The grid-connection goes to the onshore 220kV compensation substation in Bjæverskov, where an automatic voltage and reactive-power control (AVR/RPC) with a total 480 MVAr reactive capacity by the four switchable and variable reactors is established, and from which the grid-connection splits up towards the two existing 400/220/132kV substations in Bjæverskov and Ishøj. The Kriegers Flak grid-connection includes also a 400kV underground cable from Ishøj to the main 400/132kV substation in Hovegaard as the East Danish grid reinforcement.
From the 220kV KFB platform and via an offshore 220/150kV transformation on the KFB Extension platform (KFE), the connection continues to the German 150kV offshore infrastructure including the two OWPP Baltic 1 and Baltic 2, and ends in the German onshore substation in Bentwisch. Since the East Danish (Nordic) and the German (Continental European) systems are not synchronised, an HVDC-VSC Back-to-Back converter station (BtB, 400 MW) is established in Bentwisch.
Dynamic stability of the East Danish transmission grid has been conducted for several development scenarios which are prior, around and beyond the Kriegers Flak grid-connection establishment period. This paper will present the dynamic stability evolution of the East Danish transmission system for the four main stages: (i) prior to Kriegers Flak, (ii) after Kriegers Flak and no other development, (iii) prior to Kriegers Flak with retrofitting of the main wind power plants to converter-interfaced generation, and (iv) after Kriegers Flak with retrofitting of wind power plants.
A wind power plant composed of series-connected wind turbine generators is expected as a next-generation offshore wind power plant because substation-free plants can be realized. This type of wind power plant is composed of series-connected wind turbine generators, HVdc transmission line, a current-source thyristor inverter, and a synchronous compensator with duplex reactor, and it has many advantages such as huge cost savings, high reliability, and high quality of electric output with very simple configuration.
Since wind speeds fluctuate all the time and the situations of grid to be connected are sometimes changed, it is important to predict the dynamic performances of the wind power plant for the cases of changes in wind speeds and so on. In this paper, a dynamic model of the series-connected wind power plant is derived, and based on the model, dynamic performances of the plant for changes in wind speeds are investigated.
First, a set of equations valid for the transient state is derived to simulate the dynamic responses of the plant under assumptions such that leading angle of commutation of the inverter thyristors and overlapping angle of inverter currents, which are essentially time discrete quantities, are regarded as continuous quantities.
It is clarified that the system is expressed by 6th order nonlinear system, which is based on, as electrical system, 4th-order system that consists of dc inductance, d- and q-axis damper windings and field winding of the synchronous compensator, and as mechanical system, 2nd-order system in which the compensator torque is proportional to the second derivative of the phase difference angle between the internal emf of compensator and the terminal voltage of the grid with respect to time.
In order to confirm the validity of dynamic model and to discuss the basic dynamic performances, an experimental setup composed of two PMSGs (2kVA x 2) as wind turbine generators connected in series, current-source thyristor inverter, and synchronous compensator with duplex reactor was developed in our laboratory. The system output is 4kVA, 200V, 50Hz.
In the experiment, it is assumed that the wind speed flowing into one of two wind turbines changes suddenly and the dc output voltage of one of the wind turbine generators is changed while the plant is in steady operation.
For such a case, experimental results of dynamic responses of dc link current, system output current and power, leading angle of the inverter are compared to those of simulated results. From these results, we clarify the usefulness of the dynamic model of the current source wind power plant.
This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
Since power generated by wind power plants etc. is increasingly used in connection with existing power systems, grid codes are being established in many countries around the world. For example, in the Danish grid code, when supplying 1 pu (per unit) of active power, it is required to supply reactive power in the range of ± 0.33 pu (from the leading power factor of 0.95 to lagging power factor of 0.95).
At present, large-scale wind power plants installed on offshore etc. mainly use parallel connection systems that employ voltage-source type power converters. However, it has been recently noticed a series-connected wind power plant that uses a current-source type power converter.
This is because the series connection system has an advantage such as eliminating the need for the costly offshore substation in the parallel system.
In this paper, a simple method of controlling the power factor is proposed in which a variable reactance is connected to the output side of the current-source thyristor converter based wind power plant.
The series-connected wind power plant is composed of plural wind generators, of which the rectified outputs are connected in series in the dc link, a current-source type thyristor inverter, and a synchronous compensator with a duplex reactor. It should be noted that by using this system, the output current does not contain harmonic components in principle.
In this research, the steady-state characteristics of the system are investigated when the system reactances (subtransient reactance of the synchronous compensator and reactance of the duplex reactor) that affect the commutation of the thyristors of the inverter are changed for various short-circuit ratios of the synchronous compensator, and then we clarify how to select the output reactance for power factor adjustment.
Here, the steady-state characteristics are studied for the cases when the wind power plant generates DC power with DC link current of 1 pu and DC input voltage from 0.3pu to 1pu depending on the wind speeds of the wind turbines.
The reactance value to be connected to the output side of the wind power plant is clarified to realize system output power factors between 0.9 leading and 0.9 lagging while supplying 1 pu (per unit) of active power to the grid under the condition that the margin angle for commutation of the inverter is kept at about 20º or more.
The effectiveness of the basic performance of the whole system was confirmed through experimental study using a test system developed in our laboratory (PMSG x 2 sets, 4kVA, 200V, 50Hz).
This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
The paper provides an overview of the technical outcomes of three Expert Groups that have been established under the Grid Connection European Stakeholder Committee on: 1) Requirements for pump-storage hydro power generation modules, 2) Identification of storage devices, 3) Mixed customer sites with generation, demand and storage, and definition of system users. The Expert Groups were tasked to provide further interpretations, clarifications and potentially technical proposals for improvements of the current European Network Codes for Connection requirements.
This presentation provides an overview on key objectives of the new European energy policy legislative framework and on the essential role of Network Codes in implementing it. The revised competences of ENTSO-E, ACER, the EC and the new European DSO entity in drafting and amending network codes will be illustrated. Additionally, ENTSO-E’s view on the challenges for the power system implied by the updated energy policy targets and their impact on new and refined connection requirements is outlined.
Energy Policy Objectives, European Legislation, Connection Network Codes, RES Integration, System Challenges
The European energy policy legislative framework is currently changing. The existing framework will be replaced by a set of directives and regulations, known as the “Clean Energy for all Europeans Package (CEP)” with the main motivations of even more ambitious objectives to facilitate the European internal energy markets and to decarbonize the energy sector.
Network Codes and Guidelines are an essential vehicle for implementing the CEP. For new network codes, drafting committees will be established, which shall be chaired either by ENTSO-E, or jointly by ENTSO-E and a new EU DSO entity, as the case may be. Relevant stakeholders shall be actively involved already at an early stage as members of these committees. Topics for new network codes are still under discussion. Possibly more important than new network codes, existing codes and guidelines will be amended. The amendment process foresees a central role for ACER in collecting and assessing amendment proposals and proposing them to the EC.
Reconciling electricity system needs with energy policy objectives will remain a major challenge for Connection Network Codes. The massive RES integration entails an accelerated displacement of conventional, synchronously connected generators by mainly converter‐connected, non‐synchronous RES generation, which has a significant impact on system needs and characteristics, like the reduction of system inertia with its impact on frequency stability. Connection requirements need to anticipate such developments based on long-term scenarios and system analysis to conclude on the relevant capabilities of system users.
This presentation shall provide an overview of the key objectives and figures of the CEP. It will describe the process, roles and responsibilities for drafting and amending network codes and guidelines. An emphasis will be given on connection network codes, outlining how electricity system needs can be reconciled with the energy policy objectives.
As agreed with Thomas Ackermann, the presentation will not be accompanied by a paper, because the CEP key objectives and the processes for drafting and amending network codes are defined by the legislative package itself. It is however foreseen to provide the presentation in due time to publish it in the conference proceedings.
Large offshore wind farms (OWFs) connected to a modular multilevel converter (MMC) based high-voltage direct current (HVDC) transmission system may experience electrical resonance in a wide frequency range. The frequency-domain passivity theory offers an effective method for the stability assessment. This paper performs a systematic passivity-based stability analysis of an OWF connected to a MMC-HVDC, where the controller parametric effect on system passivity is thoroughly investigated, and guidelines for minimizing the non-passive region of OWFs and MMC-HVDC by optimizing controller parameters is provided. It is found out that much better stability robustness of the system can be achieved if the central frequency of the notch filter used in the OWF control is tuned lower than the network resonance frequency. Time-domain simulations are given to verify the theoretical analysis.
This paper analyzes the sequence current controllability of an offshore MMC-HVDC during asymmetrical faults. It is found that the separate limitation of the output positive- and negative-sequence current of the MMC-HVDC during asymmetrical faults may encounter physical limitations of the external faulted network, which leads to the sequence currents being out of control and the possible overcurrent tripping of the MMC-HVDC. To tackle this challenge, a fault ride through control method of the MMC-HVDC that limits its phase current and phase modulation signal is proposed in this paper, with which the magnitude of the fault current can be effectively limited. Time-domain simulations are given to confirm the effectiveness of the proposed method.
Type-III turbines based on doubly-fed induction generators (DFIG) are known to develop subsynchronous resonance (SSR) with series-compensated transmission lines. Last year we published a paper on practical experience and theoretical analysis of a similar subsynchronous resonance problem between type-III turbines and long overhead transmission lines that don’t use series transmission. In offshore wind farms with high-voltage dc transmission, the offshore HVDC converter serves as the “grid” for the turbines and may exhibit capacitive impedance below the fundamental frequency which can also lead to subsynchronous resonance with type-III turbines.
A common root cause for these resonance problems is the negative damping in the DFIG output impedance in the subsynchronous frequency range. In addition to the well-known induction generator effects, other factors such as the use of a phase locked loop and control of the dc bus voltage between the rotor and stator side converter also contribute to this negative damping phenomenon. The negative damping will lead to instability when the turbine forms a resonance with the grid impedance in the corresponding frequency range. Such negative damping is found to be inherent in DFIG and cannot be avoided completely.
A number of papers have been published on the mitigation of SSR between type-III turbines tand series-compensated grids. This paper presents design of supplementary active damping control for type-III turbines that reshapes the turbine impedance to solve SSR with weak inductive grids, series-compensated grids, and HVDC converters. In each case, the damping method aims at achieving overall positive damping at the system resonance frequency. The basis for the design is analytical small-signal models that accurately predict turbine output impedance over frequency, accounting for the induction generator effects as well as all control functions that affect turbine output impedance. The paper will also provide guidelines for practical applications of the proposed method.
As the importance of renewable generating resources has grown around the world, South Korea also trying to expand the proportion of renewable generating resources in power generation sector to 20% and to supply 28% of their new renewable energy capacity with wind generating resources. As wind generating resources are gradually replacing traditional generating resources, the system security and reliability are negatively affected. The increasing integration of wind generating resources in power systems leads to difficulties in power flow calculation due to the uncertainty and variability, which has a significant impact on the stability and reliability of power grids. Consequently, it is important to evaluate the security limit of power grids based on probabilistic approaches. In this paper, we propose the probabilistic security limit of power grids with large integration of wind generating resources. Power output forecasting of wind generating resources is considered using the enhanced ARIMAX model and then wind power forecasting outputs are fitted to probabilistic distribution through Monte-Carlo Simulation. The probabilistic distribution suitability is verified by the Anderson-Darling test and various combinations of power system operating conditions based on probabilistic distributions of wind forecasting outputs are produced. We perform the probabilistic power flow calculation based on the estimated distribution to evaluate the security limit of power grids. Probabilistic security limit analysis using the proposed method will play a key role to develop the hedging strategies of investment decision on power grid expansion planning with high wind power penetrations.
This presentation provides an overview on implementation of EU Regulations which establish the network codes on:
The presentation is completed by an outlook on the output of national implementations of EU Regulations on grid connection.
European Legislation, Connection Network Codes, Requirements for Generators, Demand connection, High voltage direct current system and facilities, Power Park Modules, RES integration, market facilitation
An ENTSO-E obligation of monitoring the implementation of the European Connection Network Codes has been established in each connection code (NC RfG, NC DCC and NC HVDC) in accordance with Article 8(8) of Regulation (EC) No 714/2009. According to the relevant articles of these Regulations (i.e. the connection codes) the monitoring shall cover in particular an identification of any divergences in the national implementation and assessment of whether the choice of values and ranges in the requirements applicable to the facilities under these Regulations continues to be valid.
For this purpose, ENTSO-E has been collecting the proposals for the national specifications of relevant non-exhaustive requirements since the beginning of the implementation process. After these national implementations were finished and approved by the designated entity of each Member State – the NRA by default – the ENTSO-E implementation monitoring report analyses the final approved and binding non-exhaustive requirements that are mandatory for system users to which the connection codes are applicable.
This presentation shall provide an overview on the national implementation of NC RfG, NC DCC and NC HVDC and provide identification of any divergences between the Member States and Synchronous Areas.
As agreed with Thomas Ackermann, the presentation will not be accompanied by a paper. The reason is, that the Implementation Monitoring report provides itself the relevant text for this presentation. It is however foreseen to provide the presentation in due time to publish the charts in the conference proceedings.
Power systems are facing various technical challenges with the ongoing energy transition, mainly due to the increasing use of intermittent renewable sources. Some of those challenges are being addressed and explored in depth recently. However, some others, such as the impact on the protection concepts, are still not receiving the necessary attention. To evaluate the impact on power system protection, mainly the distance protection for variable renewable sources connected to power grids, this study is aiming to determine how a large-scale wind farm, based on a variable speed drive, will affect the power system protection functions used today. The study is mainly focusing on the distant protection effectiveness, as the most common primary protection scheme used in transmission grids. There are some studies that are exploring these issues, however most of them are not considering detailed and realistic behavior during transient conditions, leading to oversimplified conclusions without a clear practical applicability. This study has used detailed PSCAD models of a wind farm connected to a high-voltage transmission grid. Typical distance protection functions have been analyzed from wind farm side, such as the impact of phase-to-ground faults and phase-to-phase faults within distance protection zone 1 reach. Several key issues regarding the protection functionality are demonstrated, such as the impact of fault resistances for the distance protection measurements, that might arise throughout the operation of wind farms during fault events. Possible solutions have been explored both with traditional phasor-based protection application schemes and novel solutions based on time-domain protection concepts. The new concepts have been analyzed and verified with positive results for the case study, mitigating some of the issues which are a challenge for the currently present phasor-based solutions. Finally, overall conclusions are provided.
Converter-interfaced generation has become a mainstream technology in the electric power supply. In Denmark, this development is seen as a significant increase of wind and solar power generation. Dynamic stability assessment has become a must for the long- and short-term grid planning and for the daily operation of the Danish transmission grid requiring accurate, reliable and validated simulation models of all equipment within the study case. Energinet receives and applies vendor-specific and standardised simulation models of large-scale generation units as the electromagnetic transient (EMT) and root-mean square (RMS) models. The EMT models are very detailed with characteristic time steps of tens microseconds and valid simulation periods of few seconds, and the EMT assessment is targeting the equipment specific study cases with grid equivalents including some passive networks and reduced or excluded other controllable assets such as power plants and control systems.
The RMS simulations are with time steps by few milliseconds and remain the main approach for the system-wide transmission grid assessment which includes hundreds of dynamic and static models in the Danish transmission grid and part of the neighbouring systems. The standardised approach of the RMS modelling is with reduction of fast dynamics of the converter-interfaced generation units. Specifically, fast dynamics of PLL modules, current control loops and DC-links have been reduced.
Undertaking the Danish transmission grid stability assessment, Energinet has begun observing diversity in the EMT and RMS simulation approaches. In simple study cases including a single converter-interfaced unit feeding in an infinite voltage source behind impedance, the discrepancies seem minor and inferior, but increasing at increasing impedance and reducing short-circuit capacity (SCC). By including the aforementioned modules in the RMS models of the major converter-interfaced units, Energinet has reduced discrepancies between the EMT and RMS models in the simple study cases for all relevant SCC levels for the Danish grid. For the system-wide transmission grid assessment of scenarios with more proposed converter-interfaced generation, such modified RMS models have shown adverse interaction both between one converter to another and also to some synchronous generators. Bringing in more synchronous generation has not resolved adverse interaction, though additional synchronous generation increases the SCC and is a classical, tried-proven measure of stability improvement. The conclusion is so far that the standardised RMS approach produces too positive, too stable results for the transmission grids with a significant share of the converter-interfaced generation. Energinet seeks for relevant experience and invites vendors, developers and system operators, to resolving the issues related to standardised RMS modelling of the converter-interfaced generation units for the system-wide stability assessment.
For the integration of renewable energies, system operators are moving toward a more active system operation in lower voltage levels. In order to fulfil the requirements, the system operators need to be able to enhance monitoring and controlling assets in their grids. In the project EU-SysFlex, a demonstrator is set up by a distribution system operator in Germany in order to improve congestion and reactive power management in distribution grid and to support transmission system operator, if flexibilities are needed. This demonstrator optimises the provision of active and reactive power flexibilities from power plants in the distribution grid. In the demonstration, mainly wind power plants are used for flexibility provision.
For processing this optimisation, processes and tools need to be developed. These processes enable coordinated flexibility use in grid operation for both distribution and transmission grid, without jeopardising with distribution grid operation. Different approaches for optimisation are investigated in a simulation tool and in a field test. The so far promising results show excellent applicability.
The simulation tool CorRES models wind speed in two parts. A deterministic part which uses meteorological reanalysis data and a stochastic part modelling the short-term variability not captured in the reanalysis data. The two parts of this model have been validated separately. The validation and parameterisation of their combination is the topic of this paper. The models are tested and parameterised using measured data from three different locations. The effect of the height and year of measurement, on the parameters of the model is also studied. The results of this paper indicate that the best-fitting parameters depend on the measurement location and height.
Today’s climate change debate reminds us to take action, while there is still control over our most important systems. One of them, the electric system is one of the a backbones of our society. Protecting the environment by increasing the percentage energy coming from of renewable energy sources (RES) while concurrently growing electricity demand are ambitious and challenging objectives. Under benign weather conditions, system operation with large amounts of RES on the grid has proved to be quite manageable with today’s technology. However, when planning the grid and its future secure operation, the effects of extreme weather events need to be examined and ultimately mitigated.
With grid operations becoming increasing automated, reliable weather information is essential. New tools are required to handle the growing need for weather information, not only for the day-ahead planning, but increasingly on the short-term horizon of hours and minutes, where local measurements can be assimilated to the forecasts in order to provide a more reliable picture of the current state of the system.
In this paper, we will describe a new probabilistic forecasting tool for ramping reserves to manage the current and future high penetration of RES from a case study in Ireland.
Ireland is the first country in Europe to experience Atlantic storms propagating from west to east. Information on the track and intensity of low pressure systems in the Atlantic is sparse. Therefore, Irish weather forecast uncertainty is higher than other European countries and the growth rate of the uncertainty is often even higher during storm events. For this reason, growing wind power capacity on the island of Ireland is a perfect show case for the future. EirGrid and SONI, the Irish transmission system operators are implementing solutions to improve their use of forecasts during balancing, market scheduling and real-time decision making in the control room.
During storm events, it takes less than one hour from when the wind speeds pick up at the west coast of Ireland until a major fraction of the wind farms experience a high speed shutdown (HSSD). This poses a significant risk of electricity supply demand imbalances during storm events. These risks are mitigated by limiting the wind generation in advance and ensuring sufficient reserve is available to cover any reduction in output due to HSSD. In some cases wind farms not impacted by the HSSD event need also to be limited due to the uncertainty of the event in time and space, which are then available to provide reserve when the events take place.
We will introduce a probabilistic ramping reserve tool required to handle such extreme situations with the planning analysts and control room staff in mind that need to take decisions that both optimize grid security and efficiency.
As the penetration of wind energy increases, a large wind farm must participate in the support of reactive power in accordance with the expansion of modern grid codes. Wind turbines with power electronic devices are able to generate reactive power promptly. Thereby, an operator could assign a certain reactive power reference to each WT to meet the requirements of the connected grid. It is considered a more economical way than configuring additional reactive power compensation equipment. This study proposes an optimal reactive power dispatch strategy to minimize an electrical loss, and is implemented more advanced than the conventional method considering the variation of the internal WF voltage. The reactive power references of each WT are chosen for the objective function, and a linearization method for ohmic loss is adopted to solve the problem. Electromagnetic transients using DC (EMTDC) software tools were used to perform an electrical loss estimation to verify the efficacy of this strategy.
Voltage source converter interfaced distributed generation units can be controlled either grid-feeding or grid-forming. Grid supporting functions such as voltage control and frequency support can be incorporated in both control concepts. Grid-forming control has so far mainly been used for distributed generation in microgrids, when operating in island mode. However, voltage control and inertia emulation capabilities of grid-forming control may also offer advantages in interconnected power systems. In this paper, the influence of grid-forming control in wind power plants on short-term voltage stability in distribution grids is analyzed. Symmetrical short circuits at the overlaying transmission grid are simulated using time domain simulations and the critical clearing times until stalling of dynamic load components are evaluated for several distributions of the wind power plants. A hierarchical voltage control strategy and a voltage-frequency droop control are considered. Both control strategies have features of grid- forming control. They are compared with grid feeding control with voltage support during fault ride-through. Results show, that grid-forming control has positive influence on voltage stability mainly for weak and resistive points of grid connection. Application to a test distribution grid shows, that, regarding voltage stability, usage of grid-forming control at the end of feeders improves stability margins, while it offers no advantage compared to grid-feeding control when applied to units near the substation.
Abstract--Northeast China is part of a very cold and wind-rich area. The coal-fired heating consumption is very large and air pollution is quite serious during the whole winter heating period. The energy vector coupling between wind power and heating has remarkable economic benefits and environmental value.
This paper proposed three measures aimed at solving the economic and technological problems for the coupling, considering the major forms of urban heat-sources, i.e. thermal power plants and coal-fired boiler centers. For coupling on the generation side, co-generation units combined with heat storage electric boilers (e-boiler) have been adopted, and a Northeast Power Peak Regulation Ancillary Service Market was established, which solved the problem of economic compensation of wind electric heating. For load side coupling, coal-fired boilers combined with e-boilers have been employed, and a direct trading model between wind power and thermal storage electric boilers (e-boilers) has been built. A third measure is establishing a coupling between the potential e-boilers and wind power curtailment by planned capacity of curtailment-based e-boiler and advanced on-line optimizing dispatch. The results show that wind power is achieving a significant level in Northeast China.
Index Terms-Energy vector coupling, cold and wind-rich area, wind power, coal-fired heating, thermal storage electric boiler(e-boiler), power peak regulation ancillary service, direct trading model, planning capacity of wind curtailment-based heating e-boiler.
Increasingly, the emphasis at the grid operators is moving towards an ever finer breakdown of the structures in the electrical grid in order to be able to better monitor and plan the effects of renewable energies and managed loads on the grid. In the project EU-SysFlex, a demonstrator for a distribution network in Germany is therefore being set up to support the transmission system operator for congestion and reactive power management. This demonstrator optimises the provision of active power and its flexibility from power plants in the distribution grid.
For processing this optimisation for the next hours and days it is needed to forecast the vertical loads, present on the transformers of the subgrid stations. As a requirement this forecast must distinguish electrical load and renewable energies types, as well as the switching state of the medium voltage grid. As there is only a combined measurement at the transition between medium voltage and the high voltage grid a purely statistical model approach is difficult because the splitting into producer/loads for the training of the forecast model is not easy to realize. Therefore, we favour a combined solution of a physical forecasting model with past processing together with a statistical model based on artificial intelligence.
We present the elaborated concept: How the single producer and loads are forecasted, how we administer the switching state of the sub grid and take it into account and how we combine all elements at the transfomer station. Furthermore, we show the realization in the IT-Systems, which has to be a demonstration system with real-time processing. Especially there we have to solve timing problems caused by the limited time to process a forecast.
This paper investigates the use of HVAC transmission systems with reactive power compensation as a potential cost effective solution compared to HVDC. A multi-objective optimisation technique is presented to identify the optimal reactor location and value. Two conflicting objective functions are considered: power losses and investment costs of installed compensation devices. Impacts of reactive power compensation on HVAC transmission system is analysed. The results show different optimal values, number and location of reactors depending on which objective function is prioratised. Also, a sensitivity analysis is presented to evaluate the impact of different voltage transmission levels and distances.
New system services, required for the efficient operation of systems with high shares of variable renewable generation, will change the make up of future generation portfolios. Including these new system services within an investment model demonstrates a shift towards more flexible technologies. The resulting portfolios lead to reduced operating costs and CO2 emissions, and highlight the importance of clear long-term signals for investors.
The ongoing energy system transition, empowered by technology developments, fueled by shifting investments, and motivated by decarbonization, is one of the 21st century’s most urgent tasks. However, our current suite of modelling tools has failed to adapt to emerging trends and deliver the holistic perspective required by decision-makers to navigate complex policy choices. Instead, energy system analyses are plagued by rigid model platforms and discretized researchers who focus on specific sectors (buildings, transport, power), spatial-temporal scales (municipal, provincial, federal), or energy vectors (electricity, fuels, thermal).
In this paper, we introduce a novel formulation to integrate distinct modules that span spatial-temporal scales – from individual buildings to system operation, system planning, and integrated assessment modelling – as well as infrastructure systems – the water, power, transport, and fuel distribution systems. The key novelty and functionality of this work centers around the development of so-called ‘bridging algorithms,’ which enable data flow between the distinct modules to generate insights across systems and scales. The bridging algorithms are composed of two key elements: (1) a unified platform for connecting individual modules via object-oriented Application Programming Interfaces (APIs); and (2) a policy optimization framework for consolidating insights across modules. To achieve part (1), we have leveraged existing, open-source API frameworks. To achieve part (2), we have explored various machine learning techniques, such as multi-objective active learning, which sequentially traverse the decision space to deduce the decisions that should be analyzed via expensive simulation in the next algorithm iteration. Active learning ensures that: (i) desirable policy trade-offs are identified and (ii) policy alternatives that have not been analyzed via the underlying expensive infrastructure systems model are accurately ranked. Insomuch, active learning efficiently evaluates multi-objective policies, by sequentially learning from previous policy evaluations to propose multiple new policy alternatives. The framework is extensible, to allow for the inclusion of alternative modules for collaborative research efforts, and adaptable, to explore new technologies and trends as they emerge.
The conventional focus on individual systems, which may adequately represent fossil dominated, centralized energy systems, ignores the opportunities found at the intersection of systems and scales and poses a significant barrier to decarbonization. Our framework moves beyond this discretized approach to an integrated approach, which could deliver timely and relevant insights to planners, policy-makers, and researchers as they seek design and operational solutions in a sector coupled future.
Developing an integrated pan-European energy system based on renewable energy sources (RES) has technical and economic benefits. In this way, harmonized rules for grid connection of RES are required at the international level. Wind energy is one of the most promising renewable energy worldwide. The integration of wind energy into the power system is overgrowing through onshore and offshore installations. The European network codes have been drafted and regulated for AC- and HVDC-connected power-generating modules (PGM) in two separate international network codes. This paper presents the main aspects of the regulated European network codes and compares them. Accordingly, it is recommended to define the European network codes based on RES connection type (AC and HVDC) rather than the onshore and offshore categorization. Also, the main requirements for HVDC-connected generations are being regulated all around Europe. Therefore, the integration of RES into European power systems via HVDC transmission would be easier.
This paper presents some of the results of a study of the very low frequency common mode oscillations that have emerged on the All-Island power system. This common mode oscillation is observed as system wide oscillation in frequency at approximately 0.05 Hz. If it does not continue to be well managed, this mode of oscillation could pose a threat to secure operation. This study incorporated detailed analysis of 5 months of 1 Hz frequency data and 15 minute resolution measures of system operation (e.g. wind, inertia, SNSP, demand) and high level analysis of 1 Hz frequency data for a full year of operation (2018). This analysis has demonstrated that these oscillations are not directly due to increased levels of wind generation and any correlation to wind/inertia appears to be through an indirect relationship that is driven by unit commitment for certain operating conditions. Indeed, this work indicates that wind can in fact be used as a tool to mitigate the threat of common mode oscillations on the system
To derive an accurate image of the current and future network state of the transmission and distribution grid, exact information about active and reactive power (for consumption and generation) as well as voltage amount and angle at every point of the grid is needed. Power flow calculations for the different voltage levels based on the network state are essential for optimally utilizing the grid and minimizing curtailments due to system security reasons.
Rising shares of renewables require steadily improving forecast methods in order to identify and heal overloaded grid elements in an efficient way. The weather situation, and therefore also the generation of renewables, changes on a regional scale of a few kilometers. To reflect this, the regional resolution on which renewable generation is predicted has to be high. Grid operators additionally require these regional forecasts to be assigned to the actual grid topology. This assignment depends on the distance to the transformer stations as well as on the current state of the grid. In this work, we use an extreme learning machine in order to predict the vertical power flow at transformer stations connecting the transmission and the distribution grid directly. The vertical powerflow can be understood as the sum of generation and consumption per transformer station. As training target for the extreme learning machine, historical vertical power flow measurements of single transformers are used. The input features include common information used to predict wind and solar generation as well as consumption:
- Numerical weather prediction parameters (wind speed, wind direction, temperature, air pressure, dew point, solar radiation)
- Sun position
- Time information (time of the day, day of the week, day/night)
- Consumption forecast for Germany
Results show that the approach is able to predict vertical power flows, which are mainly characterized by wind generation, solar generation, or consumption, but also combinations of all of them, with high accuracy. This is astonishing as (i) the model does not take spatio-temporal input information into account and (ii) there is no feature selection applied to distinguish different transformers regarding their main generation or consumption characteristics. Common pitfalls like changing network topologies due to new transformers or power lines or different switch states are discussed and appropriate solutions are presented.
In this paper, a virtual synchronous machine (VSM) controller of grid-connected power converters for rms studies in DIgSILENT PowerFactory is presented. This control structure emulates the dynamic behavior of a synchronous machine (SM) in order to enable a larger penetration of converter-connected generation in the grid. However, compared to the SM, the converter should be able to limit the current flowing through the converter to the maximum allowed current during transients. Therefore, a current limitation control is also implemented. Simulation results are presented which verify the performance of the VSM by comparison to the dynamic response of an equally sized SM during disturbances. %The current limitation control is verified by application of a three-phase to ground fault in the grid close to the converter. The functionality of the current limitation controller is illustrated by simulation of a three-phase to ground fault in the grid, not far from the converter.
Recent system level studies have shown that the use of Grid Forming control schemes such as the Virtual Synchronous Machine (VSM) for grid connected power electronic convertors may offer significant benefits to increasing the penetration of convertor based distributed generation when compared to Grid Following control schemes. However, many of these studies have employed a simplified model for the power converter and its control system to enable large-scale power systems to be efficiently simulated. For example “RMS” models for convertors can mask some of the behaviour of the power convertor during abnormal and fault conditions - the converter is a highly non-linear device, and can be subjected to high current stresses when there is a small, short duration mismatch between inverter and grid voltage.
This paper will discuss the challenges of designing a VSM control algorithm for a real power converter, which must operate under normal and fault conditions. The paper will present the development of an experimental VSM controller for a 15kVA battery energy storage system, and will then discuss the development of appropriate power and current limiting modes of operation so that the system can respond to abnormal grid conditions such as a short circuit, a phase step due to load change etc.
System level studies have shown that the use of Grid Forming control schemes such as the Virtual Synchronous Machine (VSM) for grid connected power electronic convertors may offer significant benefits to increasing the penetration of convertor based distributed generation. However, the VSM control scheme inherently makes the convertor system operate in a voltage controlled, rather than a current controlled mode and this makes it susceptible to disturbances on the grid. This paper will investigate this phenomenon when harmonics and imbalance are present on the grid supply.
A VSM algorithm has been used to control an experimental 15kVA battery energy storage system (BESS). The BESS can be interfaced to the local distribution grid (supplied to the lab from a 1MVA transformer), or to a 90kVA programmable three phase, four wire source. When operating from the grid the VSM system draws a significant 5th and 7th harmonic current due to the harmonics contained in the supply voltage, and the three phase currents are unbalanced due to imbalance between the three supply phases. The same VSM system draws only sinusoidal currents when operating from the programmable supply with an (almost) ideal supply waveform.
This paper will present experimental results demonstrating the VSM behaviour in the presence of imbalance and harmonics and then describe how these characteristics can be removed from the VSM currents with small modifications to the VSM algorithm. The paper will then discuss the challenges for convertor design, rating and operation caused by the harmonic and imbalance effects. For example there will be an increase in the ripple on the DC capacitor of the VSM convertor and this will need to be factored into the convertor design. There will also be an increase in the rms inverter ac current: for a given power device rating, how should the inverter current be apportioned to active power, reactive power and harmonic current for a particular operating condition?
To document compliance with grid code requirements an extensive test program is needed in many countries. Irish Transmission System Operator, EirGrid have a very thorough test procedure for Grid Code testing on every Wind Farm Power Station, where everything in the Grid Code is tested on every WFPS. This is a long and time-consuming process that is highly dependent on wind and grid conditions.
But can we do this in a more efficient way by using type testing combined with field testing to demonstrate compliance to the Grid Code?
This paper will be based on EirGrid‘s test procedures and present a split between performance testing and functional testing according to the working draft of the IEC 61400-21-2. The idea in the upcoming standard is to conduct performance testing at every WFPS, however functionality testing is instead to be done in a “hardware in the loop” simulation set-up, that is valid for all WFPS using the same power plant controller and software.
The paper will present results from Grid Code testing on a WFPS in Ireland and compare this to functionality tests completed on the power plant controller in at simulation set-up in the laboratory.
This will show a consistency between the results from the WFPS and the simulation and indicate that large part of the testing can done covered by a type test.
The following sub-set of the full suite of Grid code tests will be proposed as the (on-site) performance tests:
• Active power control, DMOL test from frequency control and start up APC on and APC Off
These tests are proposed as the function (or type) tests:
• Active power control setpoint handling
• Frequency control
This split between simulator and site tests will reduce the number of tests on site and the dependency of wind and grid conditions, without compromising the quality of grid code compliance achieved.
The sustainable expansion of the German transmission grid is crucial for the successful conversion of the energy system in Germany. In addition to the construction of new 380-kV-AC transmission lines, high-performance HVDC links based on the VSC-MMC technology are required. This is the prerequisite to transport energy generated by wind farms in the north of Germany to the load centres in the south. The first of these HVDC links will be the ‘Ultranet’ and its extension to the north (‘A-Nord’) which together will also form the first multi-terminal system within Germany. In the future, more and more power-electronic converters (HVDC, STATCOM etc.) will be integrated into the grid. In some regions, they will be electrically close to each other which includes the possibility of unwanted (controller) interactions via or with the AC system. Such AC-side grid-interaction phenomena involving power-electronic converters are not yet fully understood and simulation models are not as standardized as for classical power-system-stability phenomena. This paper describes a unique test-bench that enables a realistic investigation of AC-side grid-interaction phenomena, the derivation of mitigation measures and the validation of simulation models. For the most part, the hardware components and the overall arrangement of the test bench are described. In addition, an exemplary simulation study is presented which demonstrates the importance of converter control principles on the system stability.
In power systems all over the world, renewable power generation based on power-electronic converters is increasing and is replacing synchronous machines. In large synchronous areas – like the ENTSO‑E Continental Europe Synchronous Area – this development is critical in case of system-split scenarios. Within this context, especially the control of power-electronic converters plays a major role. Therefore, this paper analyzes the impact of converter control on system stability in such events. The relevant influencing factors and different converter-control principles are taken into account. A major outcome of the analysis is that conventional control methods with current-injecting-control schemes are only feasible up to a certain ratio between synchronous machines and power-electronic converters. In contrast, control methods with voltage-injecting control schemes (grid-forming control) allow for stable system operation regardless of this ratio.
Short circuit level is an important identifier of strength for the power system. An increase in percentage of inverter based generation in large interconnected power systems causes a reduction in the available fault current, and as a consequence, results in a reduction of traditional system strength. The reduction in fault current is due to the displacement of synchronous generation and introduction of current limited inverter based resources. In addition to this, both planned and unplanned outages would result in a change in network topology and possibly online generation which may further reduce system strength. This new reduced value of system strength could be at a value for which the controls of inverter based generation have not been designed to operate and thus, it may cause inverter controller instabilities. Knowledge of the short circuit strength of the local grid is hence essential to achieving a safe and stable system with increased percentage of inverter based resources. It is even more imperative that observability of the reduction in short circuit strength be obtained in both long term and operational planning system studies
A tool developed by EPRI called the Grid Strength Assessment Tool (GSAT) calculates various system strength metrics from only a steady state analysis of the network model. However, this computation can also provide insight into transient stability of inverter-based resources. In addition to computing conventional steady state generic short circuit ratios, weighted and composite SCR, a new advanced short circuit strength (SCS) metric is proposed for evaluating the potential for inverter controller instability. This proposed SCS metric uses dynamic data (controller gains, time constants) of inverter-based resources to identify potential instability. GSAT can also determine metrics during outage scenarios thereby providing a comprehensive overview of system strength.
This paper will outline the challenges associated with low system strength in inverter-based systems, the difficulty determining potential instabilities adn provide details of the GSAT tool and the metrics available including the advanced SCS metric for as a screening method for inverter instability.
As part of the effort to minimize the C02 emission, offshore wind farms are one key solution. Regardless of whether the offshore wind farms are HVAC or HVDC connected, the realization is associated with high investments and technical risks. Within this paper, measures to minimize technical risks in the scope of e.g. electrical compatibility of wind-energy plants, HVDC and the grid system and the influence of electrical and structural mechanics are given, utilizing a high-performance calculation cluster for realtime system analysis.
Concerning the electrical system, amongst others, inter-compatibility risks in the sub harmonic, the harmonic and the resonant frequency range have to be taken into account. For this, realistic modelling of the wind-energy plants and the HVDC system and the related real control algorithms as well as the offshore grid is a must. Concerning the mechanical system of each wind-energy plant, the holistic system including coupled structural mechanics and aero-elastic as well as the turbine control a have to be assessed.
In this scope, the enormous pressure on prices in the wind business in the last years have increased the technical risks. Therefore, from the authors point of view, a combined assessment of electrical and holistic mechanic system is the optimal solution for risk mitigation. Due to the size of wind farms, the simulation efforts for the sole electrical systems rise enormously if details like converter control generation of each wind-energy plant and HVDC should be taken into account (due to the converter switching, the time step of the solver has to be chosen in the range of 1 microsecond). Furthermore, the simulation of the holistic mechanical system of each wind-energy plant is associated with relevant computation effort.
As solution, a combined electrical and structural-mechanic realtime analysis is proposed. The authors have jointly implemented a demonstration system with a high-performance calculation cluster enabling the combined electrical and structural-mechanic realtime analysis of an offshore-wind farm with MMC-based HVDC-connection systems, offshore AC-grid and the 100 wind-energy plants including structural-dynamic models.
One scope of interest is the perfect process integration of such powerful analysis systems like e.g. combination with high-power power-hardware-in-the-loop systems, realization of high-resolution real-time twins for modelling, certification, predictive maintenance and model-based aging-optimized wind-energy plant control.
The rapid development of renewable energy sources drives the European power systems toward a green transition. Growing shares of wind power create the need to develop accurate models to describe the variability in wind generation. The reanalysis approach is arguably the most popular approach for modelling the variability.
For modelling using the reanalysis approach, meteorological data along with the technical parameters of wind power plants (WPPs) are needed. This paper focuses on the technical parameters, as previous work shows the importance of accurate technical WPP information in wind time series modelling. Specifically, missing hub height and turbine type data, which are essential inputs for the modelling, are estimated.
Machine learning algorithms have been used widely, e.g., in power forecasting, fault detection and condition monitoring of turbines. This paper applies the random forest (RF) algorithm for estimating the missing technical WPP parameters. Applying the RF algorithm, missing hub height and turbine type data were estimated for WPPs covering the whole of Europe. As a result, a complete dataset of approximately 16000 WPPs is available for pan-European wind generation simulations.
The resulting parameters from the RF model area presented and compared to a baseline model. The baseline model includes simple imputation methods; mean value for the missing hub heights and most frequent type for the missing turbines types.
To compare the RF and baseline model results in large-scale wind generation modelling, time series simulations are compared for European countries. Wind generation simulations are carried out using the same underlying meteorological data with both the RF and the baseline WPP datasets. The results indicate that especially for countries with a lot of missing technical data, RF shows significant improvements compared to the baseline model.
Finally, the applicability of the presented methodology to simulate future scenarios with changing WPP installations is demonstrated. Using the modelling validated on measurements, both hub heights and turbine types are changed to assess wind generation with high hub height and low specific power installations. The studied scenarios show significantly higher CFs compared to the installations effective in 2015.
Ambitious goals have been established in the EU to decarbonize the energy system towards 2050. This paper investigates the optimal development of the heat and power system of the North Sea region. The optimization of investment, decommissioning and transmission expansion has been performed with the open source energy system model Balmorel. The results show a large deployment of Variable Renewable Energy (VRE), power transmission, Power-To-Heat (P2H), heat storage, and flexible bio fueled Combined Heat and Power (CHP) plants towards 2050. Most of the electricity generation is expected to be provided by wind, solar PV, hydro and nuclear sources (96% by 2045, 63% just wind and solar). On the heating sector, most of the generation is expected to come from bio fuel CHP plants and P2H (68% and 24% by 2045 respectively), especially to satisfy the high temperature demand of industry. P2H increases the power load 10% by 2045. This development, largely influenced by the assumed increase of the CO2 tax and decrease in VRE costs, leads to a massive reduction of the CO2 emissions of the system, although not to a complete elimination of them.
Wind power generation is one of the promising renewable energy sources. However, the generated power fluctuates with change in the wind speed, and suitable locations for power generation such as mountains, coasts, and oceans are often far from power grids. The transmission line from the wind generator to the main power grid tends to have a long distance. It is concerned that the generator power and voltage may be unstable due to an interaction between the wind gene-rator and the long transmission line with high impedance. The authors have started research on the interaction problem between the wind generator and the high impedance transmis-sion line, through experiments in the laboratory using a small-capacity generator for demonstrating the wind generator. This paper describes results of the research including calculation of the maximum theoretical power of the wind turbine simulated by the experimental setup, measurement of generator output characteristics, power controller design, confirming experi-ments, and experiments connected to a high impedance line.
With the rise of wind power generation in the power grid, plant operators and transmission system operators identify the need to procure different ancillary services from wind power plants. Self-energization of a wind turbine and black start capability of a wind power plant are functions that offer multiple benefits to reduce costs of development and operation of a wind power plant. This paper presents two test benches that make use of co-simulation and hardware in the loop methodologies to develop self-energization and black start capability of a 1 MVA Type 4 wind turbine. The obtained results for the validation of black start capability are presented in this paper.
Curtailments due to grid congestions (often called EinsMan or Eisman) are of high (economical) interest especially for direct marketers but also for grid operators and providers of wind power forecasts. To establish a curtailment forecast these stakeholders can benefit from, a method is presented to analyse the importance of several commonly available parameters / features and the interaction between them. The analyses shown are carried out for a wind park located in Schleswig-Holstein, the state with most of the curtailments in Northern Germany and generally concentrated on. Reasons to define the forecast as a binary classification problem are given and the usage of Matthews Correlation Coefficient (MCC) is proposed as the cost function for machine learning algorithms to select the best performing model and be able to compare the forecasts of parks with a different curtailment rate. With a model based on an extreme learning machine (ELM) with logistic regression the performance of a day-ahead forecast of the probability for an occurring curtailment is demonstrated (MCC: 0.73).
As power systems evolve and the resource mix changes to accommodate very high levels of renewable generation with high levels of non-synchronous instantaneous penetrations, systems are being operated closer to the boundary of stability. While the transmission system is being operated prudently and securely always, the risk of a major system disturbance especially at very high penetrations of wind generation is ever present. There have been documented cases on smaller or islanded transmission systems around the world in recent years of transmission system collapse or partial system blackout during major weather events, with and without high levels of variable, renewable generation.
Preparing and training for the blackstart and restoration of the transmission system, in the event of full or partial system collapse is an essential part of the remit of transmission system operators. As the resource mix has changed in recent years and variable renewable generation has replaced thermal generation, system operators are investigating the possibility of utilizing wind generation for blackstart or to improve restoration time frame in the event of a full or partial system blackout. TSOs with large penetrations of renewables around the world, such as Ireland, UK, Australia, Hawaii, Japan have tended not to consider variable renewable generation in blackstart and restoration due to resource variability, grid stability and protection concerns.
Should a blackout occur on a day with high renewable penetration many thermal plants will be offline and may have long cold start up times. Wind and solar may have high availability during such events and incorporating this resource as early as possible will speed up the process of restoring the system. In addition, type 4 wind farms can provide reactive power capability with no active power output, providing a dispatchable resource for voltage control. In this paper, the inclusion of a wind power plant early in the restoration process will be evaluated, providing a methodology and considerations for those developing restoration plans and for operators during restoration addressing issues around forecasting, grid stability and protection.
Machine learning models are increasingly used in the field of wind power forecasting. In most cases a nonlinear regression model is trained, whereby the power feed into the grid of a wind farm is forecasted based on local weather forecasts. Such models learn to reproduce the interaction between weather variables and wind farm power based on historical observations. The model implicitly corrects systematic local errors in weather forecasts and considers orographic properties, turbine characteristics or the layout of the wind farm. However, in contrast to a physical model, these properties are not attributed to certain explicitly described causes. Instead, the machine learning model must rely on the possible weather conditions being comprehensively covered in the historical data set.
While the quality of the forecast is mostly reliable, large and implausible errors may occur from time to time. A common cause is that no comparable weather condition was included in the training data set. The output of the model in such a situation is pure extrapolation and is rather a guess than a well-founded estimate.
In order to cover more of all possible weather conditions, it makes sense to combine the data sets of several locations and thus to increase the amount of training data. However, a simple model trained with the data of several locations loses the ability to map individual wind farm behavior.
Multi-Task Learning describes machine learning methods which are suitable to solve several tasks with one model. The forecast of an individual wind farm can hereby be interpreted as a single task. In this article, we use a hard parameter sharing approach to train a deep artificial neural net. This approach is able to model each wind farm individually, while most of the parameters are trained with all available data of all wind farms. Only a small part of the parameters is tailored to individual wind farm behavior.
Our results show that the multi-task approach is promising. Although the improvement compared to a single-task (local) model is not significant with a sufficiently long training period, the presented procedure for shorter training periods represents a considerable improvement. With just one month of training data, comparable good results can be achieved as with a single-task model and one year of training data. This capability makes the multi-task model particularly attractive to add new wind farms to an existing portfolio or to react dynamically to changes in the behavior of the wind farm (e.g. repowering, degradation, etc.).
Keywords: Wind Power Forecasting, Multi-Task Learning, Artificial Neural Networks
Modular Multilevel Converter-High-Voltage Direct Current (MMC-HVDC) transmission systems can interact with the grid impedance, leading to harmonic instability phenomena. To screen the risk of harmonic instability in multi-vendor MMC-HVDC-based transmission systems, an automated impedance measurement toolbox is developed for Transmission System Operator (TSO), which allows the TSO to extract the harmonic impedance model directly from the vendor-specified black-box electromagnetic transient models of MMC-HVDC systems. The automated toolbox can be seamlessly incorporated with the PSCAD software environment, and it is benchmarked in two steps in this work: first for a generic MMC system against its analytically derived impedance, and then for a vendor-specific MMC installation against the model used by the vendor. Finally, the application of the toolbox for a generic HVDC system tested in a transmission power network is presented.
The IEA Wind Task 36 on Wind Power Forecasting connects several hundreds of meteorologists, wind power forecasters and end users towards the aim of improving the forecasts, and increasing the their value. As there emerge more and more high-penetration areas, forecasting requirements change and technoogy advances are on the agenda in order to facilitate the coming challenges. More than ever, we need the exchange of experts experiences and opinions, ideas and visions in how to best assist emerging markets with steep learning curves and changing markets with tools to meet the challenges they face.
The IEA Wind Task 36 management team therefore wants to apply for an entire session to create an open space workshop, where we can discuss the results from phase I (2016-2019) and open up for ideas on how to achieve the objectives for the new phase 2 (2019-2021).
The session should start with a brief 15min overview of the structure of the IEA Wind Task 36 and the major results of the recently concluded first phase. The major results include a Recommended Practices Guideline on forecast solution selection, which has 3 parts, an information portal (ieawindforecasting.dk), a profound review of the use of probabilistic forecasts in the power industry, list and lonks to recent research projects, a common workshop with Task 32 on minute-scale forecasting and a list of open access measurements and measurement campaigns. In the next phase, it is planned to also look into uncertainty propagation, data and meteorological measurement handling and standardisation for benchmarks and system operation, both from the meteorological side and the power forecast side. Propagation of energy related output parameters and metrics in the NWP models of the national met centers is another topic on the agenda.
The details on how to achieve these objective is what we want to discuss with the session participants in order to create new material for the industry's end-user. The open space technology offers a perfect framework for people to create disruptive ideas, as they ca
n walk from group to group, bring in their experience and discuss ideas and visions.
The planned Open Space topics for the session are:
(1) Standards and industry guidelines for data exchange and IT solutions in power industry: where do we need them ?
(2) Meteorological Measurements and instrumentation tandardization for integration into grid codes: what can we learn from the WMO ?
(3) Application of probabilistic forecasts in grid operation and marketing: what should the guideline contain ?
(4) IEA Wind Recommended Practices on Forecast Solution Selection: which areas are not covered sufficiently
(5) Uncovering uncertainty origins through the entire modelling chain: which applications can benefit from that knowlege ?
As renewable power sources are increasing worldwide, those of South Korea is also expanding. South Korea is carrying out various researches on renewable energy policies and technologies in order to install the renewable energy of about 58GW until 2031. In particular, South Korea is establishing a new grid code criteria based on the expansion of renewable energy. Currently, its grid code criteria consist of the frequency range, voltage range, and LVRT(Low Voltage Ride Through), etc. This paper explains the new grid code for active power, reactive power, and FRT(Fault Ride Through) that are carried out in South Korea. Firstly, we create databases that include renewable power sources to simulate the grid code criteria. Secondly, we introduce the frequency range about the active power control for over and under frequency. thirdly, we present the reactive power control method, which is the reactive power requirement and operating range according to the voltage range. Fourthly, we cover the FRT requirements: LVRT. When the renewable power source is expanded and the existing LVRT standard is applied, if the disturbance occurs, the renewable power source is dropped due to the violation of the LVRT standard. Therefore, we applied a new LVRT characteristic curve. In this paper, we performed various grid analysis on the newly found grid code for the power system that includes renewable power sources. The active power, reactive power, and FRT criteria were examined based on PSS/E Python.
With the establishment of the Paris Climate Treaty in 2015, South Korea has set its ambitious goal of 'Reduce 37% compared to 2030 Greenhouse Gas Emissions Outlook (BAU)' in June 2015. As a measure to reduce carbon, countries are increasing the ratio of renewable energy sources in each country. South Korea is also target to increase the proportion of renewable energy source to 53GW by 2030. In this paper, we will study the methodology for the power system development plan of the future power system which consists of various scenarios such as the amount of renewable energy and the installation of new transmission line. First, we describe the construction process of database w.r.t the ratio of renewable energy and load level. In order to increase the stability of future power system in South Korea, we make the various scenarios for the mix of transmission line, VSC-HVDC and FACTS. Then, the comprehensive tests based on static and dynamic analysis are processed using the proposed simulator based on PSS/E Python. Through the above process, the optimum future system reinforcement plan is calculated. The automatic simulator with various function of DB creation, simulation and decision based on PSS/E Python are developed to validate the proposed methodology and tested using the large-scale future power system.
The share of wind power in the electrical power system was significantly increased and is both concern and challenge for system operators. They demand certain performance from wind power plants, through various grid codes, to ensure stable operation of their electrical power systems. In general, grid codes require wind power plants and wind turbines to ride through short-circuit faults - validated simulation models are utilized to perform assessment studies. Wind turbine manufacturers perform fault-ride-through tests and validation of electrical simulation models as part of the development process of wind turbines in accordance to international and national standards and guidelines.
Due to inherent problems of field testing (weather condition, long test preparation time, special requirements for installation and operation of test equipment, etc.), the duration of those measurement campaigns with approximately 6 to 12 months and an higher focus of manufacturers to time to market for new wind turbine platform developments, manufacturers are forced to investigate alternative test methods as for example test bench tests to shorten the measurement time and covering more extensive grid code requirements. Due to the system limitation (for example special equipment necessary for UVRT and OVRT) and grid restriction (SCR, different system characteristics) it is not possible to measure, verify and validate UVRT and OVRT characteristics in all operation areas of wind turbines in detail.
To validate and verify the UVRT tests performed at the DyNaLab test bench it is necessary to show that these tests have similar behavior and characteristic as field tests. The development of measurement and test procedures with test benches in standards is an ongoing process. For example, the measurement and test procedure for test benches are included in Annex D of the FGW TR3, Rev. 25 and the IEC 61400-21-4 will focus on sub-systems and components. Thus, it is essential to show if test benches – DyNaLab grid simulator – are capable to emulate same grid behaviors and characteristics as state-of-the-art test procedures.
The paper focuses on the following aspects: (1) fault-ride-through measurement setups (field tests and DyNaLab tests) and tests performed on the SG-8.0 MW wind turbine. (2) comparison of test results based on field tests, DyNaLab test bench and their validation using PSCAD model and discussion of the results and boundary conditions (e.g. impedance behavior during UVRT and OVRT events). (3) presentation of additional tests performed utilizing the extensive grid simulator capabilities (e.g. emulate various grid conditions, fault scenarios). (4) assessment of advantages and limitations of the two test methods applied on the SG-8.0 MW wind turbine and outline of recommendations for further development of standards and guidelines.
This paper presents an open-loop voltage control approach for the offshore voltage source converter (VSC) of a HVDC transmission system, which forms the AC offshore grid with a defined voltage vector. The common multi-loop approach including current controls is avoided, which generates challenges for the dynamic performance of the voltage control regarding overcurrent limitation and disturbance rejection capabilities.
Furthermore, the offshore voltage control is used within the context of PSCADTM/EMTDCTM simulation studies on AC meshed offshore grids, where two offshore wind power plants (OWPPs) operating at different HVDC transmission systems are interconnected. The simulations demonstrate how the active and reactive power flow between both interconnected offshore grids for can be controlled for different control objectives.
With the continuing expansion of renewable energies, transmission grids have to adapt to the increased demands in transmission capacity. Especially transmission grids that connect regions with a high energy production to consumers in remote regions experience higher loads. Instead of building new transmission lines, most existing lines are able to carry much higher loads depending on the cooling of the conductors. The temperature and thus the ampacity of overhead conductors is strongly depended on the meteorological parameters like wind, ambient temperature and solar radiation. In weather situations with high wind speeds the ampacity of conductors can be up to 2 times higher than the nominal maximum ampacity . The overall ampacity of a circuit is determined by sections with the smallest maximum ampacity called „hotspots“. Consequently, in order to determine the ampacity of a larger transmission line the weather situation has to be monitored only on a few hotspots.
The main goal of this study is to establish a process that determines such hotspots in circuits from historic meteorological situations.
Using meteorological parameters for windspeed, -direction, ambient temperature and solar radiation for a longer historic period (several years) and the electrical properties of the conductors, ampacity values can be evaluated using the Cigre Model . Aside from conductor properties, windspeed and -direction have the greatest influence on the resulting ampacity. Thus, to increase the accuracy of the results, additional methods are discussed that model local wind effects.
After the evaluation of the ampacity its hotspots can be determined for a single circuit. This is done by approximating the global ampacity of a circuit with the ampacity of only a few points on the circuit. The points that represent the global ampacity optimally are marked as hotspots.
The second goal of this study is the optimal positioning of meteorological measurement stations that cover identified hotspots using a fixed number of available stations. If hotspots are found that are geographically close, it is possible that the meteorological parameters are highly correlated. So putting up a station at one of the hotspots makes it possible to monitor multiple hotspots. Therefore, a clustering algorithm is used that pools hotspots based on how accurately the ampacities can be evaluated if a station is set up at a certain point. The stations then can be used by grid operators to determine in real-time how much current a circuit can carry.
Studie zur Abschätzung der Netzkapazität in Mitteldeutschland in Wetterlagen mit hoher Windeinspeisung, Lange, Focken, 2008
The thermal behaviour of overhead conductors – Section 1 and 2, Electra No. 144, 1992
Large amounts of wind and solar generation are installed in power grids all over the world. One of the main challenges with the usage of power electronic converter based generation units is the stable control of the power system frequency. This is especially severe in smaller synchronous areas as in Great Britain and Ireland. Higher and faster frequency swings can occur due to the reduced mass in the power system with higher renewable penetration. In future power grids, converter based generation units are expected to fully participate in the frequency control to allow a reliable system operation. Converter based units have a different structure than conventional units and can therefore control their output faster. The effect of some of the possible frequency controls in wind turbines to the frequency reliability of wind integrated power systems is determined.
Therefore, a wind integrated test system is set up to analyze the system reliability with additional frequency controls. Until now, the reliability is only analyzed with the established grid codes. For the reliability assessment is it important to analyze the probability of wind blowing, the active load in the power system and the impact and probability of different events occurring. The load defines the needed number of generation units being in operation to satisfy the demand of the grid. The total system inertia varies therefore severely durin gsystem operation. The determined system reliability indexes are the expected number of abnormal frequencies (ENAF), the expected abnormal frequency duration (EAFD), the expected energy not supplied (EENS) and the expected wind energy wasted (EWEW).
Additional frequency control strategies for converter based power units and their implementation have been highly researched before. One control of interest is short term droop based frequency control. This control is significantly reducing number and duration of abnormal frequencies. In contrast, the wasted wind energy rises. Another analyzed frequency control is designed to limit the rate of change of frequency. This also rhas the potential to reduce the maximum frequency deviation, but it is only active for a short amount of time. This inertia emulating control is tested with variations in the controller gain and the allowed power change magnitude. This control also increases the system frequency secuity depending on the implementation.
This article summarises challenges and solution to frequency stability challenge in the Nordic synchronous area. Various options to address the challenges have been considered and assessed. Clearly, increasing inertia is not the only measure to secure frequency stability. In the Nordic synchronous area securing frequency stability will in the future be ensured by introducing a new fast reserve, Fast frequency reserve (FFR), as a compliment to the primary reserve for disturbances (FCR-D). FFR takes over as the first mitigation measure at situations with low inertia and large reference incident. Already in place is the mitigation measure of reducing the online reference incident.
Subtitle: CIGRE C4.49 Multi-frequency stability of converter-based modern power systems
Body: The electrical infrastructure is becoming more complex due to the introduction of long HVAC cables, HVDC connections, widespread penetration of renewable energy sources (e.g. PV plants, wind power plants) and offshore electrical network development. The number of power electronic converters (PV-systems, wind turbines, STATCOMs, HVDC transmission systems, etc.) in modern power systems is rapidly increasing. In the past, devices such as wind turbines or PV-systems were directly coupled to the low voltage grids and connected to the medium and high voltage via the dedicated step-up transformers. However, with the greater availability of modular multi-level VSCs, power electronic devices are increasingly directly coupled to the HV and EHV grids. This trend creates challenges such as operational coordination of grid-connected converters and small-signal stability both in the sub-synchronous and harmonic (super-synchronous) frequency region. This is mainly due to such systems being characterized by relatively low damping and hence the observation of resonance interactions.
No commonly agreed methods are available for the analysis of potential sub-synchronous and harmonic (super-synchronous) stability problems. Hence, there is a need to provide a general overview of the topic highlighting the root-cause of the sub-synchronous and harmonic stability issues of grid-connected power electronic devices supported with a state-of-the-art literature survey as well as industrial experience.
This paper presents the overview, outline and current status of the new CIGRE working group entitled “Multi-frequency stability of converter-based modern power systems”. It provides concise definitions, a literature review covering state-of-the-art contributions, and aspects of modelling and analysis methods are discussed and aided by examples. The working group’s objective is to describe the phenomena, harmonize definitions and explain available methods for analyses with their advantages and disadvantages as well as providing a common understanding on modelling, analysis, evaluation and mitigation techniques. Furthermore, guidelines on the general approach to such studies and the availability as well as choice of tools will be provided by the working group.
The Vertical Grid Load (VGL) is defined as the power transferred between different layers of the power grid, e.g. from the distribution grid to the transmission grid. Power transformers operate as grid junctions between these layers and are potential bottlenecks in grid operations. Thus, monitoring and predicting the VGL at power transformers is a major issue in grid management. VGL forecasts can be used to avoid critical congestions, to plan transformer maintenances at off-peak times and as input data for day ahead congestion forecasts (DACF-process) or more generally for predictive load flow calculations.
The VGL is measured at these power transformers and can be mathematically described by the balance equation: VGL = production - consumption. With a high share of wind and pv power in the grid, the power production is subject to significant weather dependent variability. In order to separate the weather-dependent from the non-weather-dependent components of the VGL, the balance equation can be rephrased as VGL = Wind + PV + Residual. In this equation the Residual represents the sum of both, the other unknown producers and the total consumption. The Residual-component can be further split into other known components, if more data is available, e.g. schedules for biomass plants, coal plants or load profiles.
The wind and pv real-time estimates and forecasts are provided by our forecast systems previento and suncast, which have proven to be highly accurate. In order to achieve accurate VGL predictions, we forecast single wind and pv power plants and model the power flow to each transformer. This leads to precise effective wind and pv predictions at the power transformers. With the combination of the decomposition and the forecasts for wind and pv power production, the weather induced uncertainties in the VGL are minimized. Additionally, the residual-component can be predicted statistically, e.g. by using the "reference day method", which requires historic measurements of the VGL at the power transformers.
Based on our real-time estimates for the known technologies wind and pv, we provide short-term correction for the forecasts of these components. In addition, with real-time VGL measurement data available, we can also provide a short-term correction for the residual prediction, hence for the VGL itself. We provide predictions of the actual VGL and of the potential VGL:
The actual VGL at power transformers considers curtailments of wind and pv plants such as EinsMan or other scheduled maintenances. The potential VGL shows the expected load level at power transformers, if no curtailment measures would be present. This enables the grid operators to safely schedule power feed-in curtailments in order to prevent grid congestions.
This paper compares operating experiences across seven power systems around the world that have successfully integrated high penetrations of variable renewable energy (VRE). These systems range from the small stand-alone island grid of Kauai Island Utility Cooperative (KIUC), to the synchronous grids of EirGrid and the Electric Reliability Council of Texas (ERCOT) that have HVDC interconnections with neighbors, to balancing areas within larger synchronous grids such the Southwest Power Pool, Xcel/Colorado and Energinet, to the South Australia state which is loosely interconnected to the rest of the National Electricity Market (NEM). All have very high penetrations of wind with the exception of KIUC which has high penetrations of solar. South Australia has both. Some of these systems are stand-alone (or part of larger) competitive, wholesale markets. Xcel/Colorado and KIUC are vertically-integrated utilities.
All of these systems have high and growing levels of VRE and face challenges in system balancing. The inverter-based nature of wind, PV and batteries leads to its own set of challenges: displacement of synchronous generators with inverter-based resources (IBR) results in lower inertia systems and weaker grids. This paper examines leading operational practices to manage both the challenges from variability and high instantaneous penetrations of IBR. Each system makes the most of what is practical for their situation: Energinet has interconnector capacity with neighbors that is roughly equivalent to its peak demand and is planning additional interconnector capacity, while KIUC has significant energy storage and KIUC’s future storage capacity is planned to roughly equal its peak demand. Some systems employ unique solutions, such as Xcel/Colorado’s use of wind to provide both upward and downward regulation reserves or KIUC’s conversion of a gas turbine to be able to run as a synchronous condenser during high IBR penetrations.
All systems share some commonalities, such as reduction of minimum generation levels to allow for a wider operating range on their synchronous generators. But while planners may know that such a mitigation option will enable VRE integration, getting plant owners to undertake the change is a completely different challenge, because they need cost recovery for the change and the change may mean that they now run at a lower output and earn less in the energy market. Unique market solutions, such as EirGrid’s new Synchronous Inertial Response provide a path forward for competitive wholesale markets.
The goal of this paper is to showcase leading practices, and to give a better understanding of what is essential for high VRE/IBR integration and how that might be achieved across different grid types and market situations.
Introduction: Nowadays, wind power has significant contribution to the global electricity production. Wind power capacity has reached 11% growth over the last year, i.e. 52 GW added globally in 2017. Doubly-Fed Asynchronous Generator (DFAG) Wind Turbines (WTs) – called type III WTs – are equipped with reduced-capacity power converters. Such switching converters produce harmonics and interharmonics which often are stochastic in nature, i.e. can vary depending on WT operating point, converter control scheme and switching pattern.
Problem definition: The main power quality issues in Wind Power Plants (WPPs) are related to harmonic emission from WTs and harmonic resonances within the electrical network. Harmonic current flow to the Point of Common Coupling (PCC) from WTs is defined as Primary Emission (PE) while harmonic background contribution as Secondary Emission (SE) and the total harmonic current at the PCC is the combination of both. It is known that the application of ideal current source modelling based on the IEC 61400-21 standard as well as harmonic summation formula from IEC 61000-3-6 can provide unreliable results in WPP’s harmonic studies. Thus, the Transfer Function (TF)-based method to decompose the PE from SE is further explored in this paper. It can provide accurate harmonic distortion estimation at the PCC, if the system modelling is done carefully, i.e. including WT converter control and passive components as well as WPP passive component and external power grid.
Methodology: The contribution of each WT to the total harmonic current at the PCC can be calculated using the PE TF. Moreover, the additional contribution of grid harmonic voltage distortion to the WT harmonic current distortion at the PCC can be calculated using the SE Transfer Admittance (TA). The TF-based approach shown in the literature does not consider the WT output impedance. Hence, in this article, the DFAG rotor part (including Rotor Side Converter (RSC) and Asynchronous Generator (AG)) and the grid part (including Grid Side Converter (GSC) and LCL filter) are included based on the average modelling.
Contribution: This article proposes updated TF-based modelling of WPPs which can improve the results accuracy while maintaining the fast execution time. Furthermore, the detailed studies are done in time and frequency domain to provide adjusted α-exponent to the IEC 61000-3-6 summation rule.
Validation: To validate the outcome, three different analysis methods are used and compared: (i) frequency domain model to perform TF-based harmonic propagation studies including the detailed modelling of type III WT; (ii) time domain model to perform harmonic propagation studies based on detailed dynamic simulations where different WT’s operating points are considered; (iii) harmonic current source model to perform harmonic propagation studies according to the IEC 61000-3-6 summation rule with standard and adjusted α-exponents.
This paper investigates the stability of the grid-supporting converter of a wind energy plant, acting as a current source with superordinate droop control. The small-signal model of a grid-side converter was extended and the movement of eigenvalues investigated. Different approaches to droop-control were compared.
This article addresses the problem of state estimation. In general, not all process states of control systems are measured, so state estimation can provide additional information during the run-time. Typically, observers are used to fulfil this task. The paper focuses on a wind energy system with LCL filter and an observer is applied to estimate the grid current of the filter. The observer model is characterized by several features to suit the application best. First, it treats the grid current as a harmonic signal, which corresponds to the ideal case. An extension of the observer takes higher-order harmonics into account as well. Lastly, a change of frequency of the grid current is considered, which leads to a non-linear observer. Therefore, the stability properties have to be analysed with a non-linear theory, and for this Lyapunov’s theory is applied. Numerical simulations of the proposed observers are carried out to prove the effectiveness and for comparison.
In 2017, Denmark had a share of 62% RES of electricity demand, with 45% provided by variable RES. Aiming for 100% electricity share in 2030, the country enters the next phase towards a future, when not only electricity, but energy has to be fossil free in 2050.
The Danish TSO being responsible for the electricity and gas infrastructure investigated possible pathways towards that future. So far, a strong coupling between the electricity and heat sector has harvested synergies for several decades already, strong transmission lines have benefited exchange possibilities with neighbours, power plants are maximal flexible and controllable and proper means of electricity market design as well as up-to-date forecasting means have provided best conditions for variable RES integration.
For the countries’ further decarbonizing it is important to determine economic viable pathways with all sectors coupled, i.e. the electricity, heat, gas and transport sectors.
To investigate this sector coupling, the Danish TSO has modeled the countries’ energy systems and has spread so called “energy plants” of several sizes across the country. These energy plants link various sectors and act dependent on power prices. Determination of their size is part of the results showing sector coupling potential depending on their location and related individual conditions.
To capture the development of the whole region, the ENTSO-E’s future scenarios for 2030 and 2040 have been applied. They reflect a consistent regional development, as not only Denmark, but all Europe is expected to go green. Big electrification activities are foreseen in all countries with an impact on hourly energy prices, import- and export potential of electricity - as variable RES often is correlated across country borders. New applications are expected to enter the markets on the generation and consumption side as well. To name a few, there are electric vehicles (EV), big data centers, power to gas (P2G) units, power to X (P2X) units linking especially gas and transport sector closer to the classical electricity system.
The modeling results of a sector-coupled Denmark including potentials for P2G and PtX will be presented, embedded in the regional development of Northern Europe between 2030 and 2040.
The “Load Flow” project is aimed at creating a system that can model a stable predictive power flow of the electrical networks (EN). Whereas stability is achieved through bottleneck analysis module and contingency analysis (n-1 criteria). The model contains a flexible system, that can be adjusted to different electrical networks types. Currently, the program uses the CGMES data format that is used as a data exchange standard between the transmission system operators (TSOs) and the distribution system operators (DSOs). The model is based on a modified Newton-Raphson method, that is well-known and widely used in power flow studies.
The ultimate goal of EU electricity market is a liberalization of the electricity sector and creation of a truly internal electricity market. However, the TSOs have not previously designed their networks and interconnections between them with the primary objective of facilitating internal power trade. Consequently, the integration of the national electricity markets is impeded by the limited amount of cross border transmission capacity.
Bottleneck analysis: The key feature of the “Load Flow” project is the possibility to forecast the bottlenecks in the EN, what is achieved through the deep integration with other emsys products. Based on forecasted energy production and loads the system can perform predictive power flow simulation and point the critical elements of EN.
Contingency analysis: Another important feature of the project is a resilience check of the EN (n-1 criteria checking). The idea is to recognize and prevent appearances of dangerous regimes of the EN. The program changes topology of the considered network for each EN element. After each topology modification, a full load flow calculation takes place in order to monitor load flows of the elements. If the system finds an overload on some monitored elements, this is to be recorded. After a completed calculation, a load flow module report is created. Based on the results of the n-1 criteria-test, a grid operator may perform actions to increase stability under critical circumstances and a fault tolerance of the system.
Based on the data and models generated in the “Load flow” project a grid operator can perform different redispatch measures to achieve a stability of an EN. The awareness of the network bottlenecks may help grid operators to restructure its own transmission network to minimize the electricity redispatch and therefore achieve better utilization of the generated energy.
Increasing renewable energy penetration comes with a reduction in the amount of online synchronous resources. A reduction in synchronous system inertia has already been observed in some systems and is expected to increasingly provide a challenge for transmission system operators (TSOs). As system inertia decreases, TSOs face new challenges in planning, operating, and protecting transmission systems and operating electricity markets. Smaller or islanded electricity systems with large renewable energy capacities around the world have already identified concerns related to inertia-related issues, and they have been developing innovative solutions for real-time operation and markets.
In recent project work, summarized here, EPRI worked with European and Asian members to survey the issue from several viewpoints, including transient stability, system protection and the market and regulatory aspects related to reduced inertia levels. Technical, operational and market challenges and solutions were identified and described in detail, including the theoretical background as well as the various technical solutions proposed. A qualitative assessment of the different solutions was provided and various potential market compensation schemes described, including discussion of the trade-offs between markets that aim to spur investment in new resources compared to changes operational changes for existing resources. In the future, changes can be expected not only in how thermal plants are operated to ensure minimum levels of inertia, but also how generators are compensated for providing services, including inertia.
New technologies such as emulated inertia or the use of ‘virtual synchronous generators’ are promising, but their capabilities need to be understood and demonstrated at scale. At the same time, new market products need to be studied for their potential to supplement or fulfill the need for synchronous inertia. The experience of several system operators conducting early tests of some measures adds perspective and underscores economic and regulatory factors in planning for low-inertia systems.
EPRI continues to work with members in the US, Europe and Asia on several aspects related to low inertia operations, and will describe these in the presentation. This includes online inertia and frequency response measurement methods, studies to calculate minimum levels of inertia and frequency response and investigation of the use of power electronics to compensate for reduced synchronous generation levels. The presentation will conclude with a summary of the most critical technical and economic choices that power systems face in anticipation of low-inertia operation.
This paper introduces a virtual synchronous machine (VSM) control scheme which is based on a minimized emulation of the model of a synchronous generator (SG). Based on this VSM control scheme, state-space representation and block diagram of the small-signal model for the VSM-controlled grid-forming inverter connected to an infinite bus are derived, which are used for small-signal stability analysis of the power angle in the VSM control scheme. This power angle small-signal stability (PAS) is proposed as a simpler way for analyzing the influence on the rotor angle stability of power system from large amounts of VSM-controlled grid-forming inverters. Influence of parameters in the power loop of the control scheme on the PAS is investigated by analyzing the eigenvalue trajectories of the state matrix.
The ongoing energy transition is transforming the fossil-fuel based energy system of today into a more sustainable energy system of the future. This transition is accelerated by the Paris Agreement, which aims to significantly reduce the global CO2 emission levels. The grid-connected Voltage-Sourced Converter (VSC) is the underpinning technology for the success of this energy transition.
Within PROMOTioN WP 16, the risk of a harmonic resonance within a power system of high penetration level of VSCs was identified. The theoretical framework of the harmonic resonance analysis (i.e. impedance-based analysis) was summarized for two-level and Modular Multi-Level VSC respectively. This paper summarizes the MW-scale demosntrator realized in DNV GL Flex Power Grid Lab, Arnhem, the Netherlands, with the objectives to:
1. Propose a power test circuit for the measurement of the input-impedance of a vendor specific wind turbine converter power unit (i.e. a 1 MW power converter unit, henceforth referred as Device-Under-Test)
2. Establish a Controller Hardware In the Loop (CHIL) setup to measure the input-impedance of the wind turbine controller replica of the same vendor
3. Compare the input-impedance measurement results from the power test circuit and the CHIL setup and provide recommended practice for the future industrial application.
Wind power forecasts have been used operatively for over 20 years. Despite this fact, there are still several possibilities to improve the forecasts, both from the weather prediction side and from the usage of the forecasts. The International Energy Agency (IEA) Task on Wind Power Forecasting organises international collaboration, among national weather centres, forecast vendors and forecast users. The Task looks back on the first 3 years, and just started the second three-year period. Collaboration is open to IEA Wind member states, 13 countries are already therein.
The Task is divided in three work packages: Firstly, a collaboration on the improvement of the scientific basis for the wind predictions themselves. This includes numerical weather prediction model physics, but also widely distributed information on accessible datasets and a benchmark. Secondly, we try to improve the derived power forecasts and deal with forecast vendor related matters to bring the entire industry forward. Thirdly, we will be engaging end users aiming at dissemination of the best practice in the usage of wind power predictions.
The main result of the first phase is the IEA Recommended Practice for Selecting Renewable Power Forecasting Solutions. This document in three parts (Forecast solution selection process, Designing and executing forecasting benchmarks and trials and Benchmark metrics) takes its outset from the recurrent problem at forecast user companies of how to choose a forecast vendor. The first report describes how to tackle the general situation, while the second report specifically describes how to set up a forecasting trial so that the result is what the client intended. Many of the pitfalls we have seen over the years, are avoided. Other results include a comprehensive review paper on the use of uncertainty forecasts in the power industry and an information portal related to forecasting.
In the second phase of the Task, we will take up additional topics like a full uncertainty propagation through the modelling chain, the use of distributed measurements for forecasting, and some initial standardisation of data flows and formats.
In short, the poster presents the IEA Task 36 on Wind Power Forecasting, opening a forum for international collaboration in this important field for meteorologists, wind power forecasters and end users. For collaboration, please contact the author (email@example.com) and see the website at www.ieawindforecasting.dk.
There has been considerable interest in convertor solutions which to a greater or lesser extent mimic the behaviour of synchronous machines, thus overcoming many of the disadvantages of the existing technology which are potentially destabilising at high penetration. These solutions are frequently referred to as Grid Forming or VSM Convertors.
For offshore installations, where some equipment is on shore, locating equipment offshore is usually more expensive and carries greater commercial risks, requiring extensive testing. This is particularly significant for GB and where there are significant quantities of offshore generation. Onshore solutions to stability are therefore desirable for OFTOs (Off-Shore Transmission Owner) networks. Such solutions could also be applied by retrofitting to existing conventional converter plant.
Consequently, National Grid ESO and University of Strathclyde embarked on a project to investigate hybrid solutions for offshore networks where the STATCOM onshore is replaced by alternative options such as synchronous compensator and VSM converter of similar or appropriate rating with the aim to achieve levels of Grid-Forming capability. Findings from the first stage of the research will be presented and discussed in this paper.
In performing an assessment of potential solutions, it was necessary to establish:
a. The proposed solution is general purpose and can be applied to a number of Wind Turbine Generators types
b. Was appropriate and outperforms or performs at least as well as conventional technologies such as synchronous generators and synchronous compensators
c. Was benchmarked against a variety of test conditions and provides an appropriate response under all conditions
d. Tests that are particularly important in identifying Grid Forming behaviour and performance
e. Characteristics of the convertor, OFTO network and Wind Turbine Generator that are important
In this paper a typical OFTO network is first constructed and tested using wind turbine generator (WEC and IEC) and plant controller models. A variety of test conditions are applied to the network and WTG, and these are bench marked against a conventional synchronous generator and standalone VSM solution. The onshore STATCOM is then replaced with a VSM convertor or Synchronous compensator, retested and the results presented. Finally, parameters and ratings are adjusted to improve and provide comparable performance with the Synchronous Generator and standalone VSM solution.
There has been considerable interest and discussion regarding Virtual Synchronous Machines (VSM) and Grid Forming Converters (GFC) in GB through National Grid GC0100 Working Group proposal and the subsequent workshops, in Europe (through the ENTSO-E Implementation Guidance Documents), in US (through ESIG) as well as in many other parts of the world. The key concept of the VSM/GFC is to operate the converter/power park module as a voltage source rather than a current source, which has found to be beneficial to system dynamic stability (even achieving 100% penetration) and other aspects such as faster fault current infeed, and mitigating unbalance and harmonics. In particular, it has been found that hybrid GFC options have a great potential in facilitating the recovery of angle swings after disturbances.
In this paper, an enhanced control algorithm based on the aforementioned VSM controller is proposed which employs modifications to further improve performance on angle swing events without incurring significant increase in converter rating. With the proposed enhanced VSM control algorithm applied in the offshore windfarm model, its performance is tested through case studies such as network angle shift and compared with alternative solutions such as STATCOM, synchronous compensator and the unmodified VSM controller.
The algorithms and performance are compared/discussed mainly in RMS simulation but real-time physical implementation in a VSM convertor has also been attempted and selected tests performed in hardware.
There has been considerable interest and discussion regarding Virtual Synchronous Machines (VSM) and Grid Forming Converters (GFC), with various commercial models being proposed. Furthermore, as VSM/GFC controls are mainly implemented in software, various parameters such as H (the inertia constant) can be adjusted which is not the case for conventional generation. In addition, there are further design options in relation to derating the output or incorporating energy storage, or the use of a combination of both.
The question of the fitting of energy storage and/or derating are likely to be influenced by various factors including cost, curtailment at high output, and how frequently VSM modes of operation are required. With so many parameters and the complexity of commercial considerations, the modelling and dispatching of VSM/GFC for system operators and owners is by its nature both more complex but also more flexible, than conventional rotating machines where most of the parameters are fixed.
The dispatch of VSM/GFC modes of operation are more likely to occur at high output for some renewable sources and less likely for others. It is therefore probable that different sources would benefit from different amounts of storage or perhaps in some cases no storage, where reducing output may be more economical, potentially further complicating their dispatch.
Whilst the above, are arguably of greater importance to generators and manufacturers, system operators must be confident that the plant has sufficient reserve to ensure Low Frequency Demand Disconnection (LFDD) will operate correctly.
This paper discusses various parameters, general dispatch principles and proposes a possible algorithm to demonstrate a method of dispatch which aims to overcome many of the issues discussed above. The technical solution provided is discussed and not tied to commercial parameters allowing generators and operators to select these independently.
It also provides an overview and introduction to the variety of aspects associated with the VSM algorithm described in the other papers presented by Strathclyde and Nottingham University, and discusses additional features of the VSM dispatch algorithm as proposed in National Grid’s GC0100 option 1 proposal such as variable fault contribution for stressed Distribution Networks.
Additionally, the paper summarises the findings of the various projects with possible implications for definition of Grid Code, in particular focussing on changing the emphasis to the performance requirements from a system perspective as opposed to the implementation.
Dynamic Line Rating (DLR) systems are effective solutions for managing grid congestion. Grid congestions may be caused by increasing power flows due to growth in peak demand or distributed generation, or power flows that are rerouted due to maintenance outage. Wind speed has the strongest impact on conductor cooling and therefore the DLR. An increase in wind speed from 0.5 m/s to 2 m/s results in a 50% gain above the static line rating. This is why, the business case for wind integration appears to be the most attractive use case of DLR systems. In Belgium, DLR systems are installed on numerous 70 kV, 150 kV and 380 kV lines. Cases from each voltage level are reported in this paper. The DLR systems collect wind speed, load current, ambient temperature, and seasonal static ratings, and calculate the dynamic rating every 5 minutes. Where the line loading increased with higher wind infeed, the correlation of this data with DLR was analyzed. The results are noteworthy because they are based on actual measurements rather than theoretical modeling. The results show that, at all voltage levels around 10-15% latent capacity above the seasonal static ratings are available 90% of the time. The extra line capacity, made accessible by the DLR system, reduces the instances of high line loading by 5-10%. Furthermore, this congestion management effect applied to a radial connection of a wind farm could be used to reduce the need for curtailment by up to 10%, increase infeed by up to 50%, and increase hosting capacity by up to 40% without the need for any curtailment.
Standards and industry guidelines in general are necessary in order to judge work objecitvely against a reference and to ensure that different working processes or interactions can be interlinked.
Wind power forecasting or more general renewables forecasting has matured in recent years. In this process it becomes evident that efficiency and interaction between supplier and end-users need to become more streamlined and pre-defined processes. The most important processes to achieve more ease and efficiency in implementation of forecasting processes require a common understanding of terminology, data structures and formats and communication layers.
By establishing a "business layer" through communication with pre-defined terminology between forecast users and forecast providers the "technical or logical layer" of data transfer and product definitions are easier and reduce misunderstandings, mismatches and decision making.
Other benefits are the comparability of products for the end-user and clear guidelines for newcomers as to at which level, or "standard" one's quality of service needs to be.
There are many different ways of developing a standard or a industry guideline. Our suggestion follows a 2 tier pathway: Level (1) defines the logical layer terminology and grouping information into logical entities and relevant parameters. Level (2) defines the data transfer protocols with exact description of parameter names, detailed meta data description, supported formats etc.
In this part of the workshop we want to discuss organisation and development of these levels and see
k for input from the participants. Forecast providers as well as forecast users are invited to join the work and dicuss how and what to:
▪ Review in relation to existing and related standards
▪ Structure the process for developing, reviewing and releasing a guideline
▪ form a key working group
State of the art wind power forecasting methodologies utilise, besides wind speeds from weather forecasts, onsite real-time power measurements from SCADA systems and meteorological measurements from met masts or alternatives thereof to compute wind power.
The combined use of the trend of the forecast and measured meteorological variables is the state-of-the-art method to be able to predict wind power in the next few hours, as well as high speed shut-down and critical ramping events. This explains the need for high quality measurements, even though similar considerations are applicable in the management of dispatch, i.e. ranging down to cover also lower wind speeds.
Today, there are no standards or guidelines on the quality requirements for instrumentation or on the type of instrumentation itself that would help system operators to develop their grid codes. The IEC 61400-12 standard, guidelines from MEASNET and some recommended practices from IEA Wind Tasks are applicable only in resource assessments. The US environmental protection agency (EPA) provides a “Meteorological Monitoring Guidance for Regulatory modelling Applications”, which is a guideline on the collection of meteorological data for use in regulatory modelling applications such as air quality. All these guidelines and standards provide recommendations for setup and maintenance of instruments, the measurement and reporting for all main meteorological variables. However, only the EPA guidelines deal with real-time usage in relation to meteorological modelling applications. All these guidelines and practices need to be studied and adjusted for the real-time usage in the power industry in order to develop specific guidelines or recommended practices for the real-time environment of the grid management and operation.
We will briefly introduce and discuss known standards from other areas, their limitations and benefits and develop a list of key elements that are important to take into account when developing requirements in the grid codes of transmission system operation.
In this open space session we are seeking for input from the participants. Forecast users with a need of standarising met measurements as well as forecast provides, academics, consultants, engineers with experience of met measurement campaigns of any type are invited to join the discussion and help us with the planning of further work in developing guidelines for meteorological measurements and instrumentation standardization to be integrated into grid codes.
In this open space session we are seeking for input from the participants. Forecast users with a need of standarizing met measurements as well as forecast provides, academics, consultants, engineers with experience of met measurement campaigns of any type are invited to join the dicsussion and help us with the planning of further work in developing guidelines for meteorological measurements and instrumentation standardization to be integrated into grid codes.
Today, there is a basic lack of understanding about probabilistic forecasts, how to best make use of them and which methodologies are available for the specific problem cases encountered in the power industry with increasing amounts of variable weather driven generation. This was the result of an industry survey conducted in 2017 in the IEA Task 36. The lack of understanding very often leads to mistrust in uncertainty forecasts, which echoes the mistrust in deterministic wind power forecasts in the industry 5–15 years ago.
In order to mitigate these gaps, the IEA Task workgroup has started to produce publications that shed light into these gaps and explain methodologies in a practical engineering way to increase the awareness on the added value of forecast uncertainty information, and to provide public, objective information for end-users as reference when they reach out to incorporate uncertainty forecasts into their operation. Only if there is confidence in and how to use this information it will lead to business practices.
Forecast providers also need to improve the communication of forecasting quality to the end-user and use a common language. The typical industry criteria to choose a forecast service vendor is currently mostly oriented around point forecast quality (e.g., normalized mean absolute or squared error). Concepts like sharpness, calibration or forecasting skill scores are more difficult to interpret by non-statisticians and are today barriers at the end-users to pursue this road. One way to overcome this situation is the design of games that help end-users to understand, e.g. the impact of a poorly calibrated forecast or too wide forecast intervals.
Moreover, the forecast evaluation might also focus on critical events identified by the end-user (e.g., ramps, phase errors) and, in this case, event-oriented evaluation metrics might provide a more intuitive interpretation and evaluation possibility of uncertainty forecast skill for the end-user.
In the same vein, forecasting providers must work to improve their visual representations of uncertainty so that the information becomes more intuitive to the end-user.
The current practices in system operators control rooms show that simplicity in the visualization tools and flexibility to adjust the forecast visualization for different situations are key requirements for operators. Warnings about large ramps and respective probability of occurrence are already operational in some control rooms and will be a pre-requisite to manage large amounts of RES in the future.
In this topic we want to discuss with the participants our recommendations and present our idea on how to develop business practice guidelinesfor better utilization of forecast uncertainty in practice and invite the general participants to come with own ideas and feedback and end-users with their challenges in order to have a broad fundament for the development work ahead.
In phase 1 of the IEA Wind Task 36 a group of experts prepared an IEA Recommended Practice on Forecast Solution Selection (RP-FSS), which provides guidance on the process of selecting a new or additional forecast solution, the execution of a forecasting trial or benchmark, and the evaluation metrics and methods used to assess forecast quality.
The RP-FSS is composed of three documents. This set of documents provides guidance on almost all aspects of the selection of a renewable power forecast solution. The first part, “Forecast Solution Selection Process”, deals with the selection and background information necessary to collect and evaluate when developing or renewing a forecasting solution. The second part, “Benchmarks and Trials”, of the series offers recommendation on how to best conduct benchmarks and trials in order to evaluate the relative performance and the "fit-for-purpose" of alternative forecasting solutions. The third part, “Forecast Evaluation”, provides information and guidelines for the effective evaluation of the performance of forecasts and forecast solutions.
The effectiveness of forecasts in reducing the cost of managing the variability of wind and solar power generation is dependent upon both the accuracy of the forecasts and the ability of users to effectively use the forecast information in application-based decision-making processes. Therefore, there is considerable motivation for stakeholders to try to obtain high quality forecasts that have a format and content that can be effectively used as input to operational processes or market-based transactions. The RP-FSS documents are intended to provide guidance to stakeholders who are seeking to initiate or optimize a forecasting solution that will maximize the benefit for their specific applications.
A key objective of the second phase of the IEA Wind Task 36 is to update the initial version of the RP-FSS documents to maximize their relevance and usefulness to the members of the stakeholder community. The first step towards this objective will be to collect feedback on the first version from a broad sample of the stakeholder community and especially users of operational forecast information. It is envisioned that the knowledge gained from this feedback will guide the update process and result in a second version of the RP-FSS documents that will more effectively address the needs of the stakeholder community.
This paper is designed to provide a summary and background information of the RP-FSS for further discussions in the workshop.
The evaluation of forecast uncertainties is a topic often discussed in the international community of wind power forecasters but seldom with much attention in the industry. In the IEA Wind task 36 we want to relate protocols for evaluating NWP uncertainties to those derived for full wind power forecast models. This can include a decomposition approach to understand how known uncertainties propagate through the modelling efforts into the power forecast, and their ultimate effects. Validation targets should include ramp events, storms, and weather windows for installation, operation and maintenance, mainly offshore.
In the task, we aim to investigate and present the best practices for quantification of the input uncertainty and corresponding methodologies for uncertainty propagation further in the forecast modelling chain. Both aleatory and epistemic input uncertainties are to be listed per application, as well as the statistical (e.g. Monte Carlo approaches) and non-statistical (e.g. Polynomial Chaos Expansion PCE, Grid Collocation) methods to propagate the uncertainty along the deterministic or stochastic modelling chain. We plan to include as many forecast applications as possible (i.e. short-term, long-term; micro-scale weather observations, meso-scale weather predictions; high frequency power output, low frequency power output, etc.). Final validation targets should extend to ramp events, storms, and weather windows for installation; as well as availability and reliability including operation and maintenance, mainly offshore.
In this topic we want to present our idea on how to work through the uncertainty origins and invite the participants to come with own ideas and feedback in order to have a broad fundament for the development work ahead.
Key words: renewable generation; wind power plant; harmonic impedance; resonance; harmonic cancellation.
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 analysis in power system with large wind power plants.
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.
For analyzing the harmonic behavior of large wind power plant, both detailed and aggregated wind turbine models will be applied. Moreover, the harmonic cancellation effects will be discussed in detail.
Concerning the harmonic impedances of large wind farms 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.
The session will give an overview and status of the latest standard developments from the Technical committee 88 - Wind power generation systems – in relation to grid connection requirements, focusing on measurements, test, verification and simulation of components, wind turbines and wind power plants (WPP). The session presents the actual status of the standards IEC 61400-21- series for the measurement, test and assessment of wind turbines and wind power plants, as well as the latest development of the IEC 61400-27 – series in relation to the generic simulation models and model validation procedures for the wind turbines and wind power plants. The overview, usage and challenges of these standards as well as other standard activates, will be presented from different perspectives; Standard organizations, manufactures, developer and system operators – which are presenting their experience of these standards, best practices, as well as future needs for standardization in relation to test, simulations, certifications, assessment procedures etc.
The session will be followed by an audience discussion focusing on the use of the standards for grid code compliance evaluation e.g. the use and scalability of component level / subsystem tests and use of generic simulation models. Additional discussion points will be the future needs and harmonization of standards to support the latest grid connection requirements, e.g. AC / DC connection, new control capabilities, converter systems.
Dipl.-Wirt.-Ing. Frederik Kalverkamp, M.Sc. Josef Rauber, M.Sc. Syed Mansoor Ali, M.Sc. Erfaan Makki, Dipl.-Phys. Bernhard Schowe-von der Brelie; FGH GmbH and FGH Zertifizierungsgesellschaft, Germany
Ireland and Northern Ireland are not only in focus due to Brexit but also because they strive towards ambitious goals on the development of wind energy. Both countries are committed to generating 70% of all electricity from renewable sources by 2030. This leads to an installed capacity of up to 10 GW of wind power in 2030. However, with these developments the Green Isle is also facing serious challenges concerning a stable and sustainable energy supply. Driven by the national implementations of the European Directive (EU) 2016/631 for establishing a network code on requirements for grid connection of generators strict prerequisites are in discussion on the one hand concerning electrical characteristics and on the other hand concerning compliance verification schemes for newly installed wind power plants. Nevertheless, this is not totally new, as due to its geographical location within the European transmission system Ireland as well as Northern Ireland have been facing special conditions in the electrical energy system for decades. In particular, frequency stability has always been a topic on the island. Therefore, technical requirements and a system of compliance testing for wind farms have been established early in 2005. The technical requirements are defined in EirGrid Grid Code and SONI GRID CODE for Ireland and Northern Ireland respectively. FGH is providing essential services in this field for several globally acting wind turbine manufacturers in the evaluation of grid code compliance testing results. Specialized algorithms had to be implemented in order to evaluate compliance performances and to deduce precise measurement results out of the complex data pool. Falling back on our expertise in software development, certification of grid code compliance as well as in technical and system solutions for grid integration, FGH is analyzing and evaluating the test results listed below describing the individual wind farms’ compliance, i.e. on
Hence, this paper gives an insight on the Irish and Northern Irish compliance testing process with proper preparation, a technical base as well as lessons learned for the expected pan European compliance testing schemes resulting from the ENC RfG implementation.
Since several years there has been a constant effort at international level for the standardization and the validation of different types of wind turbine (WT) dynamic models. The paper focuses on the IEC 61400-27-1 type 4 model, proposing and fully describing two benchmark systems for the assessment of dynamic performance. The first system is a simple setup and it is designed to specifically assess the transient response of the WT controls. The second system is based on the IEEE 9-bus test system, and it is a multi-machine configuration allowing for the inclusion of the typical power system dynamics of synchronous machines and corresponding primary regulators. System configurations, models and parameters are designated with the purpose of interoperability and replicability, aiming to models comparison and validation.
The increase of converter-interfaced generation (CIG) typically related to renewable energy sources is progressively causing a significant change of power systems operation. The curtailment of synchronous generation with the consequent reduction of the overall kinetic energy is one of the related issues, currently under investigation in academia and industry. The work considers the problem of overall inertia reduction from a large-scale point of view, analyzing the impact of different CIG integration levels to the frequency response of the power system of Continental Europe. The kinetic energy is evaluated for each country of the synchronous area, and the system is modified substituting conventional synchronous generation with converter-interfaced generation. The models of power converters and corresponding controls follow typical representation for stability studies: they are modeled as controlled current sources with active and reactive power control loops, implemented as user-written equations within the overall mathematical model of Continental Europe power system. Comprehensive time-domain simulations are performed for each scenario, assuming the system subjected to a power plant outage: the obtained frequencies for Western, Central and Eastern Europe are reported, and typical frequency metrics are used to evaluate the response of the system across the different scenarios. Results show how the penetration of CIG affects the frequency response of the system, in terms of instantaneous frequency deviation and maximum frequency rate. A significant change in the inter-area oscillations is also observed, with a progressive increase of the oscillation frequency of the East-West mode, as confirmed by the modal analysis of the system.
The existing power network of a Mediterranean island is used as test bed for the study of grid-forming converter dynamics in weak grid conditions. The control scheme adopted for the converter falls in the category of the so called virtual synchronous machines (VSM). Particular attention is committed to the interaction between the converter controls and the synchronous machines dynamics. Time delays typically associated to measurements and filters are considered in the study, and included in the system model with a specific mathematical approach. Time-domain simulations show the occurrence of oscillatory instability in the system, with VSM and synchronous machines swinging against each other and progressively diverging. Comprehensive modal analysis is performed for a deeper grasp of the system dynamics: the reasons of the instability are identified as the lack of damping in the system and the swinging interaction between grid-forming converter and synchronous machines. The opportunity of different corrective actions is analyzed and discussed, recognizing PSS-like control and active power phase compensation as the most suitable methods for the stabilization of the system.
The integration of wind and renewables into the generation mix has become a vital part of achieving global targets to reduce CO2 emissions and create a cleaner electricity grid. Many electricity grids have high levels of congestion that prevent bulk transfers of renewable energy to demand centres. These have usually been mitigated with uprating lines or building new lines. The timing associated with completing network reinforcements results in existing wind generators and other renewable generation being constrained and slows the new connections. These problems are usually a result of the large scale nature of these solutions and the inflexibility of traditional grid solutions to meet customers’ needs in terms of scale and speed. Added to that, planning the needs of the future electricity grid is becoming more difficult due to the mismatch in timing of grid project completion dates, generation connection dates as well as shifting patterns on the load side driven by the proliferation of electric vehicles, behind the meter solar generation and energy efficiency improvements. Planning the future grid to take account of this uncertainty requires flexible solutions that can minimise the overall investment risk and provide short, medium and long term certainty for operators and customers. Flexible solutions can provide renewable generation customers, such as wind farms, with greater certainty around the timing of connections and access rights to the grid through faster mitigation of grid constraints.
TSOs and DSOs are investigating solutions to resolve network constraints allowing wind to integrate faster. The most innovative transmission companies are using leading edge smart grid technology to support and augment the traditional grid reinforcements. One particular group of technologies that is being adopted by TSOs in Australia, North America and Europe is the Modular Power Flow Control (MPFC) devices. MPFC devices are being adopted at a fast rate because they are flexible, fast to deploy and redeploy, can be scaled up or down to meet network needs and provide a wide range of functionality. The latest in the MPFC devices, the Modular Static Synchronous Series Compensator (MSSSC), has rapidly transitioned from a world-first pilot installation in 2017 to being a widely adopted technology with projects being deployed around the world in 2019/20.
This paper explains the grid constraints that occur at transmission and distribution level that reduce wind and renewable generation and how MPFC can be used to create additional capacity quickly in ways not previously deemed cost effective or possible. The author will demonstrate this through description of the development of MPFC technology, focusing on the operation of the MSSSC. By highlighting real applications around the world the author will explain the business case for TSOs and DSOs to help resolve wind generation constraints, enable renewable generation gain access quickly and help meet climate targets
This paper reports on an investigation into future hydrogen technologies for use in renewable energy conversion schemes. The study detail main system components for example, fuel cells and electrolysers. A concept scheme based on offshore renewables and their potential application in the North Sea as part of an integrated European electricity market is discussed. A case scenario of producing hydrogen on site where electricity is produced by offshore renewables for better optimization and integration of surplus energy is studied along with deep sea storage options and the efficiency of on-site conversion and compression. The efficiency of using high voltage AC and DC transmission lines to send electricity to land is compared against a hydrogen gas transmission. Investigations into the possibilities of electrical and hydrogen storage on site is made.
Surplus energy in flexible grids is wasted to ensure that every consumer have enough instantaneous electrical delivery. In this case, the integration and optimization of renewables must be improved. There are many storage options ongoing around the world to reduce losses generated by renewables or increase plant effectiveness or utilization. To achieve this, we need to produce the most efficient and reliable system of producing and transmitting electricity to customer. This work will present an option of creating hydrogen as a storage potential on site where electricity is produced from wind, solar, wave or tidal energy resources.
The paper will discuss the system components, their models, less mechanisms and control requirements, and provide detailed technical assessment.
The Australian National Electricity Market (NEM) has experienced a dramatic transformation due to the rapid uptake of both small and large-scale renewable energy generation, along with the closure of multiple coal power plants. The Australian state, South Australia, epitomises this transformation as the share of renewable energy generation in the state grew from 14% in 2009 to 47% in 2018, dominated by wind power. Combined with limited interstate transmission capacity, the state's power system has experienced instantaneous penetration from wind generation in excess of 120% - the highest for any major power system in the world. This high presence of non-synchronous generation reduces the system strength that has traditionally been provided by synchronous generators. The ESCRI-SA 30MW Battery Energy Storage System (BESS) installed on the lower Yorke Peninsula in 2018, near the end of a long 132kV single-circuit radial feeder, is a Grid Forming BESS built on ABB's Virtual Synchronous Generator platform, which strengthens the grid by providing inertia, high fault current and fast power injection, as well as competitive market services on the NEM. It is also capable of seamlessly transitioning into islanded operation when faults occur on the upstream feeder, thus ensuring continuity of supply to local customers by operating an islanded power system with the nearby 91 MW Wattle Point Wind Farm and distributed solar PV. ESCRI-SA addresses an array of challenges, and reinforces how BESS with Virtual Synchronous Generator technology, can provide multiple value streams to utilities, consumers and the broader electricity market, as well as enabling large scale power systems to operate at 100% renewable energy.
Renewable energy resources, for example, wind and solar, are highly dynamic and intermittent compared to more traditional generation sources, which imposes increasing challenges to the electrical network operator in terms of effectively managing the resource to maximize energy transfer and maintaining system stability. Therefore, transient energy storage systems (TESSs), like batteries with fast charging/discharging capabilities, are suitable candidates to improve the availability and reliability of connected renewable systems. The Sodium-nickel Chloride (NaNiCl2) battery has good power and energy densities, and is a potential candidate for TESSs. The internal operating temperature of the NaNiCl2 battery is from 270℃ to 350℃ which is way above any external ambient temperature, thus the battery is not suspectable to local ambient temperature variations as is the case for other electrochemical storage technologies, for example, Lithium-ion (Li-ion). The molten liquid sodium (Na) requires thermal management systems, such as electric heaters and insulation to manage internal temperatures within the battery pack. This paper, a new model for the thermal analysis of a NaNiCl2 battery is proposed and validated via tests. The model includes cell thermoneutral potential considerations, an interesting feature that results in battery cooling during charging. The model is used to simulated the thermal performance of a NaNiCl2 based TESS system providing fast frequency support.
Presentation is part of a proposed session discussed and agreed with Thomas Ackermann. Session title: Latest developments of IEC TC 88 Wind Power Generation standards in relation to grid connection requirements.
Installed wind power capacity has been rapidly increasing for at least last decade. Furthermore, the 100% renewable-based power systems start to be a realistic target in the coming future. To achieve that there is a need to continue productive dialog among wind power industry stakeholders and maintain a standard technology platform to assure consistent wind energy sector development strategy.
The IEC Technical committee (TC) 88 is responsible for the international standardization in the field of wind energy generation systems. It covers wind turbines, onshore and offshore wind power plants, as well as their interaction with the electrical system. One of the most important aspect to assure the target of future 100% renewable-based power systems is the integration of wind power into the grid. Therefore, a series of IEC standard within 61400-21 as well as 61400-27 has been proposed to harmonize the integration of wind turbines and wind power plants into the power system.
The standards have been broadly used by wind turbine suppliers, wind power plant developers and operators as well as distribution and transmission system operators. For developers it important to develop and utilize a common platform of standards as it is the key factor to maintain high quality andassure high reliability of power delivered from wind.
IEC 61400-21-1 assures that wind turbine electrical performance is maintained and benchmarked across the entire supply chain. A standard way to evaluate the electrical characteristics of wind turbines is important for wind power plant electrical infrastructure design. IEC 61400-21-2 is still under development. However, it will provide a standard way to evaluate the electrical characteristics of wind power plants. IEC 61400-21-4 defines the standard harmonic wind turbine model, which can be used as the basis for harmonic analysis, resonance studies etc. of wind turbines and wind power plants. IEC 61400-27-1 is focused on verification of the electrical simulation models for wind turbines (and power plants in future). A standard way to model wind turbine for dynamic studies allows to understand the entire system behaviour and to easily address improvements or modifications to achieve desired performance. At this stage wind turbine positive-sequence models are standardized. However, the need to negative sequencerepresentation is increasing as more system operators would like to integrate this functionality. Furthermore, all mentionedstandards support the grid-connection process by assuring clear interfaces, common understanding, generic modelling approach as well as standard documentation format.
The utilization of fossil fuels such as coal, oil and natural gas have played a significant role in the design of the energy supply system and its mechanisms of operation. The high environmental impact and the confined existence of these energy sources are the cause of their gradual substitution with sustainable and renewable technologies. Share of variable renewable generation, like wind and solar is expected to increase and become major energy resources towards the fossil fuel free energy system. The increase of the renewable energy in the grid brings new conditions to the energy system as it comes with the cost of variability in production and uncertainty in the relevant forecasts. Power imbalances due to uncertainty of the renewable energy generation, such as wind power, poses major challenge for the reliability and security of power system.
The authors of this paper analysed the balancing requirements for future Danish power system with large share of renewables. High imbalances can be prognosed by power system due to forecast error when the share of renewables is high. The requirements for the power system to balance these imbalances for simulated European scenarios of 2020, 2030 and 2050 are studied in this paper. This is accomplished by modelling Intra-Hour balancing model for Danish network which simulates the creation of power imbalance in the system caused due to Day-Ahead VRE forecast error and imbalance due to hour shift in the generation. The model returns an optimized (in terms of minimizing the social economic cost) plan of the actions required from generators in order to counteract the aforementioned imbalances. The optimization procedure for reduction of Intra-Hour imbalance where dispatch is done in the form of deterministic economic dispatch with one hour optimization horizon. Technical limits such as ramp rates limitations and the maximum generation limit are taken into account by the optimization algorithm. Finally, the Real-time imbalance is calculated and the required automatic reserves are quantified. The scope of this paper is to investigate how the increased variability and uncertainty from offshore and onshore wind power plants in North Sea affects the balancing of the Danish power system and need for reserves. The contribution of this paper are as follows:
Variable renewable energy (VRE) sources like wind and solar power are having more and more share in energy systems in all over the world. To accommodate these VRE, grid development and reinforcement is also required, which is expensive and time consuming. Combining wind and solar resources to optimally use the grid infrastructure can be a viable option to reduce grid cost.
Reduction of cost of energy for onshore wind farms is reaching the minimum value. Power curtailment – previously an unthinkable economic burden for VRE – looks viable scenario in the future power systems with large share of VRE in power grids. At the same time, prices for photovoltaics (PV) and storage are sharply reducing.
These reasons together is increasing global attention on utility-scale hybrid power plants (HPP). There are only 3 HPPs with capacity more than 50 MW – namely, Vattenfall’s 64 MW Haringvliet, The Netherlands; 78.8 MW Kavithal project, India; 60 MW Kennedy Energy Park, Australia. India has come out with National Wind-Solar Hybrid Policy and intends to launch 2.5 GW auction.
Another major advantages of combining these generation sources is optimal utilization of electrical infrastructure inside the HPP. For example, either connecting the resources at AC connection point or connecting different resources at the DC bus of wind turbine, can be different options to connect the generation sources. However, different types of configurations have different control complexities and control architecture.
An AC connected wind farm, PV farm and battery energy storage system (BESS) comprising HPP has a wind park controller, PV park controller and BESS controller. However, an HPP controller acts as supervisory control to send different dispatch signals to individual controllers. Whereas, in an HPP where PV panels and Battery storage are connected to the DC bus of the wind turbine through DC-DC converters, the control architecture is quite different. In such an architecture, there may be no specific advantage to have separate wind park controller, PV park controller and BESS controller, rather a single HPP controller can be sufficient.
Dynamic modelling of all these different configurations need to be developed in order to perform studies such as ancillary services, stability etc. It should be noted that the dynamic model also differs for different purpose of studies, namely, active power support, reactive power support, fault ride through, frequency support etc. To the best of authors' knowledge, there is serious lack of literature regarding dynamic modelling of utility scale grid connected HPP. This paper demonstrates different HPP electrical topologies along with control architecture. The paper discusses different control strategies for different studies. The paper discusses different dynamic models for these different electrical configurations. The performances of these dynamic models are simulated for different delays, weather conditions and set point changes.
As the share of renewable energy power generation is gradually increasing throughout the world, strict grid compliance requirements are being put into effect by different countries for the grid integration of these decentralized power generation plants. Grid connection requirements are being continuously updated either by grid operators or through designated working groups. Compliance verification in case of wind power plants not only involves the assessment of power generating units (wind turbines) but also involves the compliance verification of power plant controllers. In Germany, grid compliance of power plant controllers is verified through robust testing in a simulated system test setup, where a first order delay transfer function is used to replicate the behavior of a power plant. Time constants used for the plant model for active and reactive power control testing are predefined in technical guidelines part 3 issued from FGW e.V.
In Ireland, grid code compliance of wind power plants is verified through field tests as mentioned in “TSO & DSO Wind Farm Testing Protocol “issued by EirGrid. These tests mainly focus on the control response of the wind farms for active and reactive power setpoint commands as well as P(f) control. Hence, they provide the controller response behavior under actual circumstances involving physical wind power plants instead of an approximate plant model. This paper will focus on comparing these two different compliance verification strategies for windfarm controller by analyzing the test data from different measurement campaigns. Controller response behaviors will be discussed along with the analysis of the transfer function time constant.
The electrical power system is facing several challenges as the penetration of converter based renewable power increases, including a reduction of the synchronous based inertia, loss of converter synchronism and weakened grids. Grid forming converter controllers, especially the so-called Virtual Synchronous Machines (VSMs), are seen as a potential solution for some of these issues. VSM controllers mimic the behaviour of a synchronous machine to different degrees of detail. Of these implementations, those that do not use Phase Locked Loops (PLLs) or current loops have been shown to be advantageous. This paper presents several proposals for a Fault Ride Through controller for a VSM controlled converter without a PLL or vector current control loop, which are imposing limitations on the stability of the inverter in low Short Circuit Ratio grids. Also, a method to limit the power and voltage/reactive power references, considering the converter maximum current, is presented. This paper validates and shows the advantages and limitations for the proposed control structures through extensive simulations using MATLAB Simulink for different grid conditions applied to a wind turbine.
This paper analyses impact of different load types on voltage stability of a power system with wind power. To analyse voltage stability, dynamic as well as static static voltage stability assessment are performed. Dynamic model of wind turbine based on Type 4A IEC 61400-27-1 is used to develop an aggregated wind power plant model including the wind power collection system. Polynomial load model is used to represent different load types, namely, constant impedance, constant current and constant power loads. With respect to voltage stability assessment, eigenvalue analysis is used for static voltage stability assessment to determine stability of a particular operating point. Voltage collapse proximity indicator is used as dynamic voltage stability assessment tool to analyse the proximity of an operating point from voltage collapse. Impact of load type on voltage stability is discussed. Impact of reactive power support from wind power plant for the different load types is analysed and discussed. From the case studies performed, it is observed that effect of reactive power support from WPP varies depending on the load type. Results show that distance to instability for the same reactive power support from WPPs can be greater in case of voltage dependent loads than that of constant power loads.
The European Network Code Requirements for Generators (ENC RfG), published in 2016, introduced a set of binding grid connection requirements to all kind of generators that finally came into force in April 2019. Next to these so-called exhaustive requirements, fixed to all European Member States, the ENC RfG also introduced ranges on non-exhaustive requirements that had to be shaped by the Member States in terms of national implementations. More over, the ENC RfG has given a binding framework on compliance schemes both for the commissioning phase of generators and the compliance monitoring during their lifetime, including certificates, measurements and simulations. Hence, in all over Europe a full range of new grid codes is on stage, providing some challenges to all parties involved in the installation and operation of wind farms: to manufactures in terms of technology development, to project developers in terms of electrical planning and code compliant commissioning processes and to system operators with respect to their newly defined obligations on ensuring grid code conformity.
The paper will give a comprehensive overview on the technical requirements defined in the national grid codes based on the general ENC RfG provisions typically applicable to wind power installations with a special focus on codes where additional provisions have been put in place, like Over Voltage Ride Through (OVRT) requirements or islanding and/or black start capabilities, that exceed the regular scope of the ENC RfG. As well, temporary arrangements are outlined, that some Member States have established in order give to some transitional periods introducing the new ENC RfG requirements. The pictures will be completed by the general compliance regulations within the ENC RfG and by a synopsis on specific compliance schemes that have already been defined in some Member States or are in current discussion. Different schemes like testing, simulation, certification and inspections will be characterized and compared. A proposal to equipment certificates will be given. The paper will also include a report on the status quo on ongoing international standardization in terms of the testing standard CLC 50549-10 and the work within IEC-RE on a comprehensive certification scheme.
The paper will end up with some recommendations on introducing lean, applicable and proven measures to introduce effective and efficient grid code compliance schemes on a standardized basis.
Lower costs and higher reliability of diode rectifiers (DRs) make them a promising alternative to be used in the offshore substation of HVdc-connected wind power plants. However, these advantages come along with lacking the grid forming capability of the offshore substation. Hence, the grid side converter of wind turbines (WTs) connected to a DR offshore substation must take over such responsibility besides controlling the WT output power. This paper proposes the use of a common synchronization reference (CSR), which is generated in the offshore HVdc substation and distributed among the WTs through the optical fibers embedded in the power cables. Using the proposed CSR, the conventional control of the WTs can be used with minor changes while equipping the WTs with grid-forming capability. The same infrastructure can be used to send signals to WTs for frequency support of the onshore grid.
As the proportion of wind and solar photovoltaics in an electrical grid extends into the 50-100% range a combination of additional long-distance high voltage transmission, demand management and local storage is required for stability. Pumped Hydro Energy Storage (PHES) constitutes 97% of electricity storage worldwide because of its low cost. However, it has been difficult for developers of energy project outside the hydro sector to identify sites due to the strong dependence on topography. We used Geographic Information System (GIS) approaches to develop a global atlas of off-river pumped hydro energy storage to overcome this difficulty. Protected areas and areas of high urban population density are excluded to reduce land use conflicts. We identified about 616,000 potentially feasible PHES sites around the world with storage potential of about 23 million Gigawatt-hours by using GIS analysis. This is more than one hundred times greater than required to support a 100% global renewable electricity system.
High-frequency resonance has become a common problem in high-voltage dc (HVDC) transmission systems based on the new modular multilevel converter (MMC) technology. The frequency of such resonance can range from close to 1 kHz to 3-4 kHz and changes with network configurations. The problem has affected HVDC systems for both bulk power transmission and offshore wind, with consequences that range from shutdown or inability to start of the system to physical damages to components such as surge arrestors and filter capacitors by the resulting high-magnitude, high-frequency voltage and current harmonics. Similar problems have also affected wind turbines and PV inverters.
This paper presents methods to model and solve such high-frequency resonances in HVDC and wind energy systems. Delay is identified as a common root cause and is shown to create a negative damping in converter output impedance over certain frequencies. One common source of delay is the time required to acquire measurements and execute different control functions. Digital pulse-width modulation (PWM) introduces an additional delay equal to 1/2 or 1/4 of a carrier period, depending on whether the PWM reference is updated once or twice over each carrier cycle. The paper presents the modeling of such delay by small-signal sequence impedances and uses the models to study its effects on system stability. The method is applied to type-IV and type-III turbines as well as HVDC converters based on MMC. In addition to full models that can predict the associated impedance and negative damping behavior accurately, simplified models are also presented to gain insights.
Based on the developed models, the paper further presents possible solutions to high-frequency resonances caused by control delays. The solutions are categorized into active and passive damping methods, and case studies are presented in each category to highlight the effectiveness and possible limitations. The example systems studied include type-III turbines connected to an overhead transmission line and an MMC-based HVDC converter for offshore wind. Detailed electromagnetic transient (EMT) simulation is used to validate the proposed models and system analysis results.
This paper analyzes the impact of Wind Turbine (WT) Voltage Source Converters on harmonic emissions levels and harmonic stability by performing two case studies for several Offshore Wind Power Plants (OWPPs) in the North-Sea. Firstly, it was investigated to what extent the expected harmonic emission levels at the Point of Connection (POC) are affected when the WT converters are modelled as active elements instead of constant current sources in PowerFactory. The impact of connecting an increasing amount of OWPPs to the same POC on the harmonic emission levels was studied as well. Secondly, the harmonic stability of the offshore network was assessed by utilizing both an impedance-based analytical model and a full model in PSCAD/EMTDC. The effect of connecting more OWPPs, and thus WT converters, on the harmonic stability was analyzed. The results of the harmonic stability assessment were verified with time-domain simulations in PSCAD/EMTDC.