Wind & Solar Integration Workshop 2022
Harmonic distortion prediction method in a meshed transmission grid with distributed harmonic emission sources – Eastern Danish transmission grid case study
The Eastern Danish transmission grid is part of the Nordic synchronous area, includes the main Danish island of Zealand and the islands of Lolland, Falster and Moen. The grid includes 400 kV, 220 kV and 132 kV lines and is connected to Southern Sweden via the 400 kV and 132 kV cables, Germany via the Kontek HVDC connection and Kriegers Flak Combined Grid Solution, and Western Denmark via the Great Belt HVDC connection. The power quality (PQ) measurements show the 3rd, 5th, 7th, 11th, 13th, 23rd and 25th harmonic voltages of significance with the magnitudes below the IEC planning levels and varying with variations of the operation conditions of the grid.
The grid will undergo expansion due to connection of large wind power plants and the Bornholm Energy Island as well as part of the 400 kV overhead lines will be replaced with 400 kV underground cables. Such changes of the 400 kV grid arise needs of predicting the harmonic voltage distortion in a future grid using simulations to secure an adequate power quality. The outcome shall support investment decisions and harmonic mitigation for the grid which is not yet established and differs from the present grid.
The simulation method uses the harmonic Norton and Thevenin equivalents with numerically tuned magnitudes and phase angles which represent distributed harmonic emission sources. The goal of the numerical tuning is to match the measured harmonic voltage distortion for as many possible various operation conditions in the present transmission grid. The numerical tuning results in an empirically defined set of magnitudes and phase angles of the harmonic Norton and Thevenin equivalents. The harmonic magnitudes and phase angles are defined with reference to the base-frequency current and voltage magnitudes and phase angles and locked when conducting assessment of the future grid.
The various operation conditions in the present transmission grid include different power transports and harmonic filter operation status of the domestic and foreign HVDC converter stations and (n-L), L ≥ 0, operation conditions of the passive grid. Using the empirically defined set of the harmonic Norton and Thevenin equivalents, the harmonic voltage distortion for the future grid expansion is simulated for the (n-M+N) operation condition with M is the number of removed lines and N is the number of added lines.
The paper will explain a theoretical background of the presented method using the Eastern Danish transmission grid model as a case study and demonstrate the validation of the harmonic simulation model using the measured harmonic voltage distortion.
Further, the paper will briefly present a future development within the harmonic assessment in Denmark as part of an industrial PhD project by Energinet and Aalborg University. This work shall result in a guideline for prediction of the harmonic distortion in a meshed grid how analytical approach can replace observation studies with many numerical simulations.
Design algorithm of harmonic filters in a meshed transmission grid with distributed harmonic emission – Eastern Danish transmission grid case study
The Danish transmission grid undergoes a tremendous reinforcement due to integration of more renewables, stronger market coupling and connection of the Energy Islands. The political target is that new transmission lines shall, to the most technically feasible extent, be established as underground cables (UGC) and part of overhead lines substituted with UGC. Adequate power quality and harmonic mitigation are among technical challenges of the transmission grid with an increasing share of UGC. The Danish experience has shown that especially 400 kV UGC can cause resonance conditions, which coincide with harmonic orders of the harmonic distortion. This has led to significant increase of the harmonic voltage distortion. Further grid expansion may “reshape” the harmonic impedance characteristics within the range of characteristic harmonic orders, herby either suppressing or increasing the harmonic voltage distortion.
The Danish TSO, Energinet, has developed a simulation model of the Danish 400 kV transmission grid for harmonic assessment. The simulation model was successfully validated from the power quality measurements of the harmonic voltage distortion in the present-stage grid. The simulation model is applied for prediction of the harmonic voltage distortion in future grid stages by simulations.
An assessment of a new 400 kV UGC in the Eastern Danish transmission grid has revealed a significant increase of the 7th harmonic voltage distortion with a risk of violation of the IEC planning level in several substations. A proposed mitigation solution is establishment of a harmonic filter with the right properties, rating and location in the transmission grid.
The presentation will explain the algorithm developed by Energinet for automated design of harmonic filters in the meshed grids by simulations using the Eastern Danish transmission grid with the new 400 kV UGC as a case study. Challenges of the harmonic filter design in the meshed transmission grid include that the harmonic filter may, on the one hand, dampen a specific harmonic order in the given substation, and, on the other hand, be inefficient or even increase other harmonic orders in the other substations. Such phenomena are due to “reshape” of the harmonic impedance characteristics in different substations of the meshed grid when connecting a harmonic filter. Therefore, harmonic mitigation becomes a challenging task in the meshed transmission grid.
Such phenomena have not only been simulated for the future grid, but also measured in the present 400 kV grid of Eastern Denmark which strengthens confidence of this presentation.
The Opportunity of Grid-Forming Converters in the Wide Area Control of Power Systems
In conventional wide-area control schemes, the actuators are typically the power system stabilizers of synchronous machines. The control action, in this case, can not directly act upon the power-angle relationship of the machines. In this context, the grid-forming control for power converters offers the opportunity of an effective wide-area control: grid-forming converters present in fact a direct formulation of the power-angle control law, and therefore the possibility of directly acting upon it. The paper proposes an extension of the grid-forming control, including a specific additional control action based on a remote frequency signal. The proposed concept is proved with a simple representative system, and then demonstrated with two application cases, the standard IEEE 39-bus system and the European power system. The results indicate that the proposed participation of grid-forming converters in the wide-area control leads to a considerable improvement of the dynamic characteristics of the system.
Resistance is futile - Implementing automated renewable trading in competitive energy markets
The recent introduction of five-minute settlements into the Australian National Electricity Market (NEM) in October 2021 saw a massive increase in the number of offers submitted to the market at every five-minute Dispatch Interval. While many of these offers were from non-renewable generators, a significant proportion was now from Variable Renewable Energy (VRE) Generators that had rarely submitted offers to the market before the market change.
VRE generators use a wide range of approaches to actively manage their market risks, including internal and external multi-generator operations centres, advanced Machine Learning optimizers and simple single generator revenue optimizers.
This paper will discuss the advantages and risks of all three approaches, how each has operated in the market, and how strategies adapt to the dynamic competitive energy and ancillary markets.
Surprisingly, the simple and usually lower-cost option of a simple revenue optimization seems to provide a low risk, reliable and robust alternative for VRE generators and can also be combined with other systems to meet VRE compliance and other operational priorities generators.
More sophisticated systems are required for multi-modal and Battery Energy Storage Systems (BESS) as the forecast horizon needs to consider an entire trading day operations due to the physical constraints of the Lithium-Ion technology. However, the same revenue optimization approach can work for other battery technologies, such as supercapacitors, that do not have the cycle constraints of Lithium-Ion and are especially suited to providing frequency and contingency ancillary services.
Impact of smart grid technologies on the distribution network in Uganda: A case study
In light of rapidly growing energy demand, distribution network operators face significant challenges in maintaining a stable and secure grid. The focus of this study is investigating the integration of photovoltaic and battery energy storage systems and the most cost-effective options for grid reinforcement; evaluate what role, if any, smart grid components can take place of standard network reinforcement measures to bring the grid back into compliance. Low voltage distribution networks from remote villages in Uganda were selected as a case study. A techno-economic analysis showed that traditional grid reinforcement measures are the most cost effective. However, when new operational measures are introduced, only minor changes in the network structure are required to operate within the allowed limits. In this paper planning, operational measurements as well as integration of innovative technologies will be discussed and analyzed. Results show that using new technologies can increase the hosting capacity of renewable energy resources and hence reduce investing and operation costs.
Development of a generic SSSC model in PF. Convergence between IT and OT practices
One of many challenges that wind power faces while integrating into the existing network is the inherent decoupling between its development time (e.g., a couple of years) compared to that of traditional transmission solutions which may span into decades
Grid Enhancing Technologies, such as Static Synchronous Series Compensators (SSSC), offer the possibility to re-couple the timeframes by unlocking unused spare capacity, however, the availability of such devices within commercial electrical simulators is scarce or even non-existent
Moreover, detailed vendor models are specific to the incumbent Original Equipment Manufacturer (OEM) and might not be the best approach during the early necessary assessment stage. This fact ends up imposing another burden on the transmission network planners which sometimes discourages them from including such technologies in their solution portfolio
The following paper presents the results of the development methodology for a generic SSSC applied on the DIgSILENT PowerFactory simulator. The methodology was jointly elaborated by a TSO and an OEM, and can be expanded to include other commercial simulators (e.g., INTEGRAL, PSLF) since it's based upon test driven development and equations derived from fundamental AC circuit theory
The main result of this work is an integrated model for the SSSC on the DIgSILENT PowerFactory simulator; still some features were also ported to the INTEGRAL simulator. This model is useful for representing such devices on both steady state and dynamic RMS simulations.
An intelligent and automatic control of renewable production for flexible access of large offshore wind parks
Massive integration of renewable generation calls for more flexibility, from grid development and design, towards system operation.
A new meshed offshore 220kV AC network, the Modular Offshore Grid (MOG) has been built to allow the connection of additional 1000MW of offshore wind parks to the Belgian onshore transmission system.
Considering the intermittent nature of the power generation and the probability of loss of one element (e.g. undersea cable) it has been chosen to design the evacuation network with a permanent admissible transmission loading capability not sufficient to ensure the N-1 compliancy in all conditions.
To guarantee the full N-1 compliancy, the design has been completed with the application of the principles explained in this paper.
The choice was dictated considering the low probability of loss of the assets at the same time of full wind production and allowed a considerable reduction in investment cost by avoiding the installation of an additional undersea export cable.
The implemented solution is an intelligent dedicated algorithm running on the EMS system and using the available information from the SCADA system (no additional communication means are used) together with a set of dedicated assets to maximize the available transfer capabilities and operational procedures. The solution is based on:
- Measures to handle system operation
The algorithm defines and executes preventive and curative actions in a fully automatic way by taking into account the next possible incidents.
- Measures to optimize the capabilities of the assets
The thermal capability of the undersea cables is optimized by installation of a Real Time Thermal Rating/Distributed Temperature Sensing (RTTR/DTS) system.
- Measures to handle maintenances and the reaction of the wind parks
The algorithm considers the planned maintenance of the different assets in the MOG and possible wrong reaction of the wind parks to the active power set points.
- Measures to protect the assets
An inverse time overload (back-up) protection is implemented to avoid persistent thermal overloads and damages of the undersea cables in case that the modulation algorithm and/or the automatic control of the injections fails (e.g. software or communication system issue).
Regulatory and contractual agreements between ELIA and the wind power park owners complement the technical solutions implemented.
The implemented solution is an example on how intelligent solutions can allow the optimization of the usage of the asset with a considerable reduction of the societal costs when massive amount of intermittent renewable resources is integrated in the transmission and distribution system.
This paper presents, with focus on return of experience, the original and advanced development of an intelligent automatic control of the production of the MOG wind parks to handle incidents and maintenances on the MOG network, implemented and tested by ELIA.
Challenges for integration of renewable energy in public grid
Integrating renewable energy is one of the current and future subjects for the energy transition to CO2-neutral energy generation. New requirements, including grid and converter perspectives, must be considered as renewable power generation is carried out with power electronic converters. Inverter-based resources (IBRs) must meet grid connection regulations, forecasting options, time scheduling, storage options, and voltage quality. In this article, some essential requirements from the manufacturing perspective are described, and their testing procedure is displayed with the validation of simulation models. This paper further highlights the SICAM Power Plant Controller (PPC) for photovoltaic plants, enabling the plant operator to create generation forecasts for the next 2-3 days of power and energy. To be grid-code compliant, the PPC includes several functions to meet requirements, and its start-up sequence is also described. The system operator gains a grid-compliant model validated against site measurements. The procedures have been successfully applied to several IBR projects. Previews of new concepts to reach the future demands of integrating IBRs into the public grid are presented. Possible impacts on the power generation and components are assessed, and necessary countermeasures are applied with the influence and feasible solution for energy trading with IBRs. Future aspects of supra-harmonics and their impacts on public grids are described and presented as an outlook. This paper highlights the challenges of integrating IBRs and lessons learned based on project executions.
Impedance Shaping for the Control of PV Inverters Aimed to Support Compliance of Harmonics Requirements
Integration of PV Inverters is associated to stability and power quality concerns. Power electronic based inverters when interfaced with power system (PS) may trigger resonance on multiple factors, due to inverters output filters, poor controller stability, lack of control bandwidth and grid structure. They can disrupt converter stability, reduce power quality, amplify existing resonance points and interrupt normal PS operation. Hence, grid operators may impose stringent criteria in terms of voltage harmonics limit in the connection point of large PV (or wind) plants.
During the planning stages for new PV plant, PS studies are typically performed to focus on the compliance. Existing tools provide frequency-domain analysis features able to represent harmonic interactions based on the Nyquist criteria. For that, aside models of passive components of the PS, the PV inverters manufacturer should provide impedance/admittance models that represent the harmonics behavior. For example, a Norton Equivalent (NE) may be employed to represent the harmonic interactions of a PV inverter: NE source represents background PWM generation and NE parallel impedance/admittance is the component that represents the closed-loop interactions with the circuit.
In the method described above, closed-loop operation of the PV inverters have a potential freedom degree to modify the response of the inverters in order to achieve compliance in the whole harmonics range (50th harmonic could be an upper bound) e.g. if the worst case scenario (for a given harmonic) occurs for a highly inductive external grid impedance, the aggregated action of PV inverters may target to damp their output filter capacitances, so the whole series resonance as seen at the connection point is mitigated.
This work will show the controller design techniques able to shape the harmonic impedance for PV string inverters. Output LC filter configuration and large control bandwidth (i.e. fast switching frequency) are beneficial that permits different control strategies: for example, depending on the constraint, one can target for a more inductive/capacitive/resistive behavior.
From a design perspective, this technique should consider constraints: stability properties of the PV inverter should not be affected; “local” frequency actions targeting particular harmonics are needed. Some capabilities for inverters with a large control bandwidth are studied, and both the above mentioned constraints can be met by relying on the resonant controller principles. By using the inverter output current and/or capacitor voltage as main states of the control system, the design of resonant filters to shape the aimed harmonic behavior is achieved. Implementation of the proposed technique will be explained in the paper. It will show the mathematical derivation technique and the achieved performances. Eventually, a study using our product parameters together with an ongoing project field data will prove the benefits of the technique.
Tuning of power plant voltage and reactive power controllers considering equivalent short circuit ratio
The increasing focus on reduction of green house gases and security of supply is causing a large-scale replacement of fossil fuel generators with wind, solar and hybrid power plants (WSHPP) in many transmission grids. For areas with a high penetration of inverter based resources (IBR), the equivalent short circuit ratio can be used to predict the stability of the aggregated systems . The short circuit ratio can also be used to tune the power plant controllers to get the desired response for reactive power and voltage  . For a single WSHPP connected to a remote area which can be considered as a Thévenin equivalent, tuning based on the short circuit ratio gives a good prediction of the setpoint step response and the disturbance response of a full plant. Tuning of a WSHPP in a high penetration area without knowledge of the other WSHPP in the vicinity of the plant, however, is not straightforward because
- The dynamics of the setpoint response of a single plant will be very different from the common response to a network event.
- The inverter-based plants in the area will often have different controller types with different settings.
- A detailed model may not be available for older plants
- It is difficult to predict, how the system is going to develop over the lifetime of the production plants.
Common response can be seen only with wide dynamic model of the network or during real life operation. Sharing the large network model might not be possible due to non-disclosure agreements and compatibility issues. With equivalent short circuit ratio based tuning the common response is taken into account.
The analysis of the commissioning phase tests and model validation are becoming more difficult. Conventionally the analysis of the voltage control performance is done by changing the voltage reference of the commissioned plant. However, with fast voltage control in parallel WSHPP this will lead to parallel WSHPP acting against the voltage change. From the studied plant point of view the background network seems to be stronger than in reality. This can lead to misinterpretation which might lead to too fast tuning.
The content of the paper is
- General discussion around the success criteria for the response of a single WSHPP and the aggregated response from all WSHPP
- Analysis of the impact of ESCR based voltage controller tuning on the response and tuning of plant controllers
- Network voltage stability and damping of electromechanical oscillations
- Proposal of methods for grid connection studies voltage controller tuning in high IBR penetration networks
- Case study in two actual wind power plants in Finland.
 Cigre TB 671: Connection of wind farms to weak AC networks
 Martinez, J., Kjær, et.al (2011, March). Comparison of two voltage control strategies for a wind power plant.
 Lund, T., et.al (2020, November). Challenges and solutions for integration of wind power in weak grid areas with high inverter penetration.
Stakeholder engaged energy systems modelling: three Canadian case studies
Meeting Canada’s emission reduction targets requires a fundamental shift, not only in the infrastructure underpinning the supply and delivery of energy services, but also in the institutional frameworks that govern policy and investment decisions. Stakeholders’ engagement in the energy transition is gaining momentum, but a communication gap between experts and decision makers is impeding the impact of model-based decarbonization analyses. This represents a substantial missed opportunity. We present three case studies that adopt a two-pronged strategy to co-create and co-deliver model-based insights. The first prong entails the development of an integrated energy modelling suite that provides a holistic perspective of energy systems that spans sectors, spatial-temporal scales, and energy vectors. The second prong entails a model implementation process, in which stakeholders and researchers co-develop ‘Scenario Bundles’ to analyze a particular project, policy, or target through a series of collaborative activities. We present three case studies, at the federal, inter-provincial and municipal scales respectively, that apply these two prongs in distinct stakeholder-driven modelling projects. We conclude with a critical analysis of the role of modelling and stakeholder engagement in effective decision making.
New Pathways to Future Grid Compliance for Wind Power Plants
Over the last decade, the rapid growth of wind power is driving an increase of efforts in defining the frameworks and rules for the connection of wind power plants (WPPs) through the grid codes and standards, making testing/validation more demanding and complex for wind turbine (WT) manufacturers and WPP developers. Traditionally, WT manufacturers carry out grid compliance tests at full-scale prototype turbines whereas WPP developers and Transmission System Operators (TSOs) mainly perform studies for new WPPs based on offline RMS and EMT simulations. Until now, these strategies have been sufficient for ensuring grid compliance of WTs and WPPs, however moving forward the wind sector needs new testing methods to meet the increasing WT capacity, WPP complexity, and deployment pace to achieve society’s desired sustainability targets. In this paper, the emerging strategies that target these challenges are presented as 1) subsystems and component testing of WTs (described in IEC 61400-21-4) and 2) hardware-in-the-loop (HiL) real time simulation for WT/WPP control and protection appraisals (future 61400-21-5). Finally, a future outlook is given on the importance of addressing model validation and how these strategies can be complemented with other innovative ideas such as artificial intelligence/machine learning, data-driven dynamic modelling, digital twin technology, and software-in-the-loop (SiL) real time simulation.
Analysis of Transient Stability of Generator Groups in the Future Power System
This paper investigates essential factors of influence on transient stability risks within transmission grids and suitable countermeasures based on a simple grid model. The increasing penetration of renewable energy sources (RES) in the context of the European energy transition leads to highly changed feed-in and power flow situations in the continental European (CE) electricity grid. The installed wind and PV plants are connected to the grid via converters instead of synchronous machines like conventional power plants. This leads to the fact that both amount and distribution of available inertia highly depend on the feed-in situation. Furthermore, expected future distribution of generation capacities and growing international power trading volumes result in increased power transits.
Dynamic grid simulations of future scenarios show that transient stability problems can affect coherent generator groups. This means that entire grid regions can lose synchronism with the rest of the grid, which ultimately results in the affected area being disconnected by distance protection. This type of transient stability strongly depends on external factors such as load flow scenario, level and distribution of inertia, dynamic reactive power reserves and dynamic load behavior. The investigations are based on RMS simulations with a simple 8-node grid model. This model represents the most important characteristics of the German power grid. All faults are assumed to be three-phase faults on transmission lines in the north-south transfer region. For each scenario, the critical fault clearing time (CCT) is used to evaluate the criticality of a grid scenario and the effectiveness of countermeasures.
WTG manufacturer's experience with subsystem and component validation for Wind Turbines
Conventionally, wind turbine (WT) manufacturers are performing grid compliance tests at the WT level primarily in the field to verify the WT capabilities and performances against certain grid code requirements and validate simulation models accuracy and performance. Such compliance tests are becoming more challenging and numerous due to the growing number of relevant grid code requirements and the rapid growth of WT’s size and rating, which in turn require more powerful test equipment and larger sites to test such turbines. Upcoming standards such as IEC 61400-21-4 and FGW AK KEZ and other industry initiatives aim to address these challenges through the replacement of site-specific tests with tests performed on WT components and subsystems in a controlled test bench environment. This paper presents the experience gathered so far on various test benches including nacelle, electrical generation, converter, and real time digital simulator test benches. The results of these different test benches brought important lessons regarding the transferability of the results through the evaluation and comparison with one another and benchmarking against site specific tests to assess their accuracy in representing the performance, capability, and functionality at WT level. Finally, a long-term grid compliance strategy, revolved around on moving part of the testing campaign to subsystem and component testing and modelling, is presented with a proposal of new test bench and acceptance strategies for the industry.
Performance comparison of probabilistic and artificial neural network models for long-sequence generation of wind speed forecasts
This paper presents a new method for generating long-sequence wind speed time-series forecasts for purposes of offshore wind farm asset and operations planning. Our goal is to develop a planning decision support tool with which wind farm planners and operators can make informed decisions for development and operation of future offshore wind assets considering revenue and power generation yield, as well as operation and maintenance expenditures.The proposed methodology should be computationally efficient and should be able to reliably generate accurate wind speed time-series forecasts for the required planning timescale. In this paper, we used an Autoregressive Moving average model as benchmark to evaluate and compare the performance of four different artificial neural network models namely, uni-variate Long-Short Term Memory (LSTM), uni-variate hybrid one dimensional convolutional neural network with LSTM (1D-CNN-LSTM), multivariate Long-short term memory and multivariate hybrid 1D-CNN-LSTM architectures. The performance evaluation is delivered through the statistical comparison of the metrics, RMSE, MAE and MAPE, and the final selection of the outperforming model is supported using Diebold-Mariano statistic test. Experiments consists of applying different types of pre-processing to the wind speed dataset and the modification of models' architecture to include either a batch normalization or a drop-out regularization layer are realized to aid in the selection of the most suitable model engineering. Results suggest the uni-variate hybrid 1D-CNN-LSTM is able to deliver short-term prediction for longer timescales while maintaining a suitable degree of accuracy.
Short-Circuit Currents from Wind Turbines with Full-Scale Inverters
An important aspect of any wind power plant integration analysis is the short-circuit contribution. Reliable results can only be obtained by considering every aspect of the wind turbine behaviour that can impact its short-circuit current injection. This information is required to accurately represent the wind plant in power system models used by the system operators, to properly size circuit breakers and for protection coordination within the wind power. This task is often challenging not only due to the inherent differences between the synchronous generating units and wind turbine generators (WTGs) but also due to the variability of the WTG types and settings. This paper presents results of detailed short circuit analysis performed on hundreds of types and configuration variants of Vestas WTGs with full-scale converters. The WTG current contribution during the 1st, 3rd and 5th cycles for faults varying between 0.9pu and 0.01pu residual voltage were obtained using detailed WTG models. Various statistical techniques were used to assess the variability in the short circuit contributions and generalise the most appropriate values for the minimum and maximum transient contributions for a given WTG type and configuration. Finally, the paper dwells on the techniques used to obtain the equivalent positive and negative sequence impedances for the WTG for various fault conditions and turbine configuration and provides recommendations on the appropriate use of these impedances for wind power plant integration analysis.
OffsH2ore island grid for hydrogen production - Electrical simulations on how to reach grid stability
Green hydrogen produced sustainable from renewable energies is expected to become a key pillar for the decarbonization of our energy systems. For countries like Germany for instance, where the large-scale production of green hydrogen can be demanding due to land use competition or other challenges related to the development of renewable energy projects on land, import strategies for green hydrogen are being devised. However, hydrogen production at sea using offshore wind power with its high full-load hours offers the opportunity to cover the entire value chain at a national level. At the same time, the further growth of offshore wind capacity can be decoupled from grid expansion. Within the project “OffsH2ore”, the production of hydrogen from offshore wind in an island configuration is investigated. In this summary paper, the electrical components as well as the overall grid stability of the island system is discussed in more detail. First, the idea and theoretical realization of hydrogen production in this project is briefly described. In the following chapter, the modelling of the island system in the grid calculation software PowerFactory is presented. Thereby, the focus is set on the control of the wind turbines and a possible reactive power compensation system. Subsequently, the stability of the island is considered under different fault scenarios and different generation capacities. Finally, conclusions and recommendations for future investigations are presented.
High resolution load and renewables time series generation for prospective frequency studies
The electrical grid frequency must be continuously maintained close to its nominal value of 50Hz to ensure a safe operation of the grid and its components. Transmission Systems Operators are responsible for monitoring this stability in real-time and anticipating future challenges such as the growing share of renewable generation. Prospective studies regarding frequency quality require access to generation and demand data sampled at a very small time step (~ second). However, available data for long-term studies are very often 1-hour time series. The methodology described in this article explains how to turn 1-hour time series into realistic 10-second time series for load and photovoltaic and wind generation. The general idea consists in building a 10-second trend time series with the same hourly energy as the original data and then adding noise with zero mean on each hour. This noise is computed with an autoregressive model fitted for each type of generation or load, thanks to high-resolution historical measurements on a limited set of assets. The model calibration requires analyzing relevant variables (installed capacity, load factor, etc.) that impact its results. The assessment of the quality of this method is addressed through different technical indicators.
Damping of low-frequency oscillations using a PV & BESS installation: practical design case
This paper presents and compares different POD controller options for a hybrid PV+BESS installation, starting from the layout of a real plant in Mexico. The POD controllers are designed using the pole placement method and identification techniques, based on selected recorded signals. The different POD controllers (PV, BESS and PV+BESS) are integrated in the main Power Plant Controller (PPC) and their performance is tested using a representative test system. Case studies show that coordinated action of the PV+BESS can significantly increase the damping support for the system, particularly in scenarios with limited plant capability. Sensitivity analysis is also presented in this work to evaluate the impact of delays of communication channels on the POD performances and the impact of operating with different oscillation frequencies.
Island Grid Operation of a Modified Mobile Generator - Test and Optimization in a Living Lab with High PV Penetration
In the research project LINDA, a concept for local island grid operation with decentralized energy resources (DER) was developed. One part of the follow-up project LINDA 2.0, is the development of a hybrid mobile generator consisting of a battery storage system and a grid forming inverter. In order to derive a control strategy for this hybrid mobile generator, a conventional mobile generator (diesel generator set) is equipped with a load bank. The load bank allows the infeed of DER during the island grid operation in the distribution grid. With conventional mobile generators the island grid frequency is set to 51.7 Hz in order to disconnect all DER. The infeed of DER would destabilize the island grid especially when the generation exceeds the load in the grid as the mobile generator cannot absorb reverse power flows. Since this modified generator is going to be used for the auxiliary supply of real low voltage grids during regular and planned maintenance work, it was tested and optimized in a living lab.
The living lab consists of a PV-system (378, 558 and 738 kWp) and the mobile generator with the load bank. The mobile generator has a nominal apparent power of 275 kVA. Since the mobile generator itself cannot absorb reverse power flows, the load bank has to artificially increase the load stepwise. In order to have balancing energy in the event of load steps and to prevent the formation of deposits in the diesel engine due to an operation under low load conditions, a minimum output power is necessary.
During the living lab tests, the power of the PV-system, the frequency of the test setup as well as the minimum output power was changed and the influence on the system stability observed.
The tests show that a stable island grid operation with the modified mobile generator and the infeed of DER is possible. In further measurements during the living lab tests oscillations were observed. The grid inertia constant in the living lab is relatively low since the only rotating mass is the mobile generator. This affects the frequency and voltage stability of the system. The frequency and voltage are stimulated in the case of an in- or decrease of the PV-system power or a load step of the load bank. If then the system frequency exceeds 50.2 Hz and the PV-system starts to control the output power according to its standard (VDE-AR-N 4105), the island grid can become unstable and oscillate. This is described for grid following inverters in a guideline of the VDE FNN.
The full paper contains the evaluation and interpretation of the measurements in the living lab. The determined oscillations and the examined methods to prevent them as well as their results are included. The resulting modifications of the mobile generator as wells as measurements of first operations in the real grid are presented. The full paper proposes an algorithm for the mobile generator. This algorithm adjusts the system frequency according to the power fed in by the DER and the system state.
Influence of Power Oscillation Damping Assets Reactive Power Capacity on Damping Low-Frequency Power System Oscillations
Over the past ten years, the paradigm has shifted from conventional power generation to renewable generation. Large integration of these renewable energy sources (RES) into the power system poses challenges to system operators, leading them to put stringent requirements for their grid connection. Displacement of synchronous generators by RES reduces system inertia and consequently decreases the system damping capability of electromechanical oscillations. Poorly damped interarea oscillations reduce the transmission line’s capacities and may damage power system components. Hence, future grid codes will require wind and solar power plants to provide damping to the system. Several papers have proposed adding an auxiliary damping controller to the wind turbine control algorithm to damp the low-frequency oscillations (LFOs) by modulating active or reactive power. However, these studies have not mentioned if small power plants can damp LFOs in a multimachine system. Therefore, this paper investigates the influence of reactive power capacity on the damping of LFOs and its effects on optimal controller parameters using a simplified SVC model connected at the midpoint of the tie line of a two-area test system. A local feedback signal is selected as the input signal to the SVC damping controller. Controller parameters are optimized using the particle swarm optimization algorithm. Time-domain simulations performed in PowerFactory software demonstrate the damping behavior of the controller at different SVC ratings. The results show a minimum reactive power capacity is required for effective damping of power system oscillations.
Aspects of the relevance of hybrid power plants in control and stability of weak grids
Increased integration of renewable energy resources to overcome the current energy crisis and environmental issues has changed the overall power system structure. The current trend in power system is the increased utilization of Hybrid Power Plant (HPP) configurations in order to provide more flexibility, increased availability, and reduced variability through the combination of various sources. Therefore, this paper studies the possible contributions of hybrid power plants to support weak grids and maintain the desired stability criteria. In this regard, state-of-the-art literature is reviewed for control approaches in single technology power plants with a special focus on frequency and voltage stability challenges as well as the fault ride-through capabilities. Furthermore, the paper investigates the specific capabilities and challenges of HPP structures regarding the abovementioned stability criteria. Finally, the aspect of relevance of HPPs is discussed in the control and stability of modern power systems.
BALANCING GROUP OPERATION FOR WIND POWER PLANTS AND A PUMPED-STORAGE HYDRO GENERATOR TO MAXIMIZE EXPECTED REVENUE IN CONSIDERATION OF WIND POWER OUTPUT UNCERTAINTY
For using energy generated in wind farms (WFs) as the main power source in a power system, a sophisticated balancing operation scheme is required, since the generation is uncertain depending on wind conditions. This study focuses on an existing variable-speed pumped-storage hydro generator (PSHG), which does not require new installation costs and has a large capacity to compensate for the WF output, and proposes a balancing group (BG) scheme combining WF and PSHG. The proposed operational strategy aims to maximize the expected revenue obtained in the power market. In this scheme, the BG operator submits scheduled power supply in advance and adjusts the output of the PSHG according to the WF's output; the revenue is affected by the submitted output schedule, the power market price, the deviation of realized BG output from the scheduled one, and the imbalance charge paid for the deviation. In this scheme, the BG operator needs to plan the appropriate schedule under the uncertainty in WF output and market price, and by considering the operational constraints of the PSHG, i.e., the water storage capacity and the frequency of operation for switching pump-up/-down.
This paper proposes a planning scheme for the BG output schedule to maximize the expected revenue by estimating the uncertainty in WF output via probability density predictions and by referring to historical market price and imbalance charge trends. In this scheme, the discrepancy between the assumed and actual WF outputs is compensated for by the PSHG output changes in the operation phase. However, when the deviation becomes excessive, the PSHG tends not to be operated as scheduled, since the water storage reaches the operational upper or lower limits. Thus, it is important to reflect the effect of uncertainty causing such deviation in the scheduling procedure. The proposed scheme focuses on WF output scenarios derived from the predicted probability density and historical trends of WF output, and formulates the scheduling problem with scenario-based chance-constraints of the water storage transition.
The effectiveness of the proposed scheme is assessed based on the revenue, comparing with several operational schemes in numerical simulation using actual WF output data. The performance of these operations is evaluated under various conditions. Simulation results show that the proposed operation scheme achieves higher revenues than these comparative methods. The results suggest that the proposed scheme provides a promising way for a BG composed of WFs and PSHG by considering the uncertainty in the WF output based on the probability density predictions.
Evaluation Method of Congestion Frequency Considering Changes in Power Flow Conditions due to Wind Turbine Installation
Recently, the installation of large-scale wind turbines(WTs) has been promoted worldwide to use renewable energy sources(REs) as main power source. Generally, power generators select WT installation areas primarily considering wind conditions to maximize generation output of WTs. On the other hand, with the recent large introduction of REs, there is concern about the shortage of transmission line capacity i.e. grid congestion, in Japan. When grid congestion occur, WT's output might be curtailed and result in less power generation output than expected. Therefore, the area of WT installation need to be determined considering the frequency of grid congestion as well as the wind conditions to maximize WTs output. Currently in Japan, system operators disclose the actual data on power flow and operational capacity of transmission lines in bulk system, allowing power generators to evaluate the frequency of congestion before the introduction of new WTs in each area. However, only with these data, power generators cannot properly estimate the frequency of grid congestion after the introduction of new WTs, because it is difficult to estimate the change in power flow conditions caused by the generation output of new WTs. Especially, large-scale WT installations could significantly increase the frequency of grid congestion around the installation area.
Therefore, this study assumes that the system operator discloses the acceptable additional generation in each area without grid congestion based on past actual power flow data. Furthermore, using the parameter, the evaluation method is proposed for power generators to evaluate the frequency of grid congestion after the introduction of new WTs. Specifically at first, the system operator quantifies the increase in power flow of surrounded transmission lines due to the increase in generation output in each area by sensitivity analysis. Based on the sensitivity analysis and past actual power flow data, the acceptable additional generation for each area is calculated and disclosed as time series data for one year. Subsequently, the power generator calculates the frequency of grid congestion after the introduction of the new WTs based on the additional generation capacity with the expected generation output of the new WTs in each area.
To verify the usefulness of the proposed method, numerical experiments were conducted for several scenarios of introducing new WTs in a simplified power system model referring the northern Tohoku region of Japan. The generation output of WTs were simulated by estimating the wind conditions in each area based on past meteorological data. The results showed that the proposed method can properly estimate the frequency of grid congestion after the introduction of new WTs in evaluation scenario cases. The proposed method was confirmed to contribute to the maximization of WT generation output in future power system.
Analysis of harmonic voltage distortion simulation study results compared to actual levels measured at the 383 MW nearshore Windpark Fryslân
The Netherlands has ambitious targets for renewables, increasing offshore wind capacity from 3 GW now to 21 GW in 2030 and 70 GW in 2050. In 2021 the 383 MW nearshore Wind Park Fryslân (WPF), located in the IJsselmeer next to the Afsluitdijk, became fully operational. The windpark consists of 89 direct-drive, full converter WTGs of 4.3 MW each; 92 km. of 33 kV (three-core submarine) inter-array cabling; an onshore transformer station with two 33/110 kV, 210/275 MVA power transformers and two 110 kV (single core onshore) cable systems of 55 km to the grid connection point with TenneT TSO at substation Oudehaske. With its large, capacitive electrical infrastructure, WPF can be regarded as a 'worst case' for harmonic voltage distortion.
Extensive simulation studies have been done by WPF to assess the impact of the wind park on existing background harmonic distortion in the grid as well as the actual contribution to the harmonic distortion due to WTG emissions, using impedance loci to represent the transmission network. Notably, in the Netherlands, limits for wind park harmonic emissions are set based on the WTG emissions, while amplification of existing harmonics by the PPM electric infrastructure is the responsibility of TenneT TSO. The studies confirmed the expectation that the capacitive behaviour of the wind park electrical infrastructure, in combination with a weak inductive high-voltage grid, result in amplification of background harmonics. For WTG emissions, the simulations showed a significant contribution for certain harmonic orders, also dependent on specific inductive grid impedance ranges within the impedance loci. The question arises, to what extent the simulation results match with actual measurements.
WPF have received measurement data from TenneT for harmonic distortion and flicker (Pst) at TenneT’s 110/220 kV transformer substation Oudehaske, which is the Point of Evaluation for grid compliance of WPF. Using Windpark Fryslân as a case study, specific limitations of the existing approach for proving grid code compliance regarding harmonic voltage distortion have been shown by comparing measurement and simulations results. Specific limitations of the existing approach for proving grid code compliance regarding harmonic voltage distortion are identified. Since background harmonic distortion is currently not considered in load flow studies, small changes in assumptions (for example WTG impedance) can cause an amplification or damping of the background distortion of a certain harmonic order. This means that overall harmonic voltage distortion may be both under- and overestimated. To mitigate this risk, more precise impedance data could be made available by the grid operator. Also, background harmonic distortion at the PCC should be considered in harmonic studies to improve both the correctness and the accuracy of simulation results.
Utilisation of Synchronous Condensers for Improved Damping in Power Systems with High Renewable Penetration
With ever increasing variable renewable energy (VRE) and subsequent displacement of conventional synchronous generation
(CSG), system strength and inertia have been steadily declining in power system grids around the world. A mitigation to the
aforementioned problem is installation and operation of synchronous condensers (SC) for provision of system strength and
While the synchronous condenser is a mature and proven technology, the technology has not been fully utilised, particularly
in its ability to provide additional damping to system inter-area modes via Power Oscillation Damper (POD) controllers. Synchronous
condensers can provide inertia and modulation of terminal reactive power and voltage (via the excitation system) to
impact the system power flows such that it can provide a positive contribution to system damping.
This paper examines the implementation of a synchronous condenser in conjunction with a POD, in a network with high
renewable generation, to dampen inter-area modes. Firstly, we investigate a method to determine the ideal location and monitoring
Bus of a synchronous condenser, for damping of inter-area modes. Subsequently, the inertia time constant and POD settings
are optimised to provide adequate modulation of the SC terminal voltage and reactive power for increased damping of interarea
modes. Finally, large and small-signal disturbances are considered with and without the POD to demonstrate the device
Performance optimisation of a grid forming battery energy storage system
The Australian National Electricity Market (NEM) has witnessed the large-scale integration of solar and wind inverter-based resources (IBR) into the bulk power supply system. Predominantly these IBRs have been based on grid following control strategies which have known performance limitations under weak grid conditions and may require system strength remediation. IBRs using grid forming (GFM) controls have the capability to provide this service but introduce additional dynamic characteristics as a result, requiring a careful selection of control parameters.
This paper presents the application and control setting selection of a Virtual Synchronous Generator based GFM Battery Energy Storage System (BESS) project connecting to the NEM. A discussion and demonstration of the capabilities of the plant is provided. An approach to optimising the GFM plant’s performance within the context of the local network and the constraints imposed by existing grid codes is also presented. The paper discusses the major trade-offs involved in the performance optimisation, followed by the presentation of an open loop method to efficiently tune plant performance with wide area simulation data.
dynOpt-En – Cloud-based predictive energy manager for supply- and demand-responsive energy source connection
Currently there is a strong trend toward multi-generator systems with integration of renewable energies. Since the energy supply of regenerative energy sources is not congruent with the energy demand, shortages and surpluses occur with a time lag. Therefore, storage and the dynamically optimized use of different energy sources are of utmost importance. Intelligent balancing of supply and demand is an essential prerequisite, especially for sector coupling between heat and electricity.
In the dynOpt-En project (www.dynopt.de), an energy manager was therefore implemented that uses a prediction algorithm to optimize heat generation in buildings and neighborhoods on the basis of high-resolution meter and sensor data. For this purpose, the optimization software continuously calculates forecasts (e.g. for the next 36 hours) from the meter data available for energy billing in combination with weather forecasts and other sensor data. These are used to coordinate the operating times of various heat generators, such as heat pumps, CHP units, peak load boilers combined with PV or solar heating systems, optimized according to costs or CO2 emissions. Due to the consistent design of the energy manager as an online service, hardly any on-site installations are necessary. All data is available via data interface (web API).
The special features of the dynOpt energy manager are the algorithms for prediction and optimization, which require no complex individual programming and only a small number of sensors. Via a gateway, the data from the meters already available for consumption billing are transferred to the energy manager in 15-minute resolution. For this purpose, either newly developed gateways can be used as a plug-and-play solution or already installed data collectors and controllers can be integrated. Energy sources can then be switched in a time-optimized manner via the gateway. This does not replace the existing controller of the heat generator, but merely sets the operating requirement via interfaces intended for this purpose, such as the SG-Ready interface of a heat pump. The same applies to CHPs or, for example, gas boilers.
dynOpt-En is primarily intended for commercial properties such as multi-family houses and quarters in which several components such as heat pumps, PV systems, solar heating systems, gas boilers or CHP units are operated in conjunction with each other. Furthermore, dynOpt-En provides monitoring of energy consumption and costs. In this way, malfunctions can be detected promptly and signaled via a clear visualization. A cloud-based portal was implemented for commissioning and parameterization of the energy manager. Results are presented for 3 properties where the energy manager was tested. Furthermore, the performance of the energy manager was proven by means of annual simulations.
Correlation between Global and Local RoCoFs and their Relevance for Robustness Requirements of Generation Units
The system split on the 8th of January 2021 (separation of Balkan) has once again shown that system splits present a particular challenge for the stable operation of the continental European (CE) power system. As a result of a system split, potentially high rates of change of frequencies (RoCoF) occur in the subsequent separated systems. The global RoCoF in the center of inertia (COI) depends on the available inertia (rotating energy) on the one hand and the active power imbalance of the separated systems on the other. Locally, the RoCoFs can significantly differ from the global RoCoF in the COI. Usually, the simulation and evaluation of events like system splits are carried out using single busbar models, in which the separated systems are aggregated to one node. By doing so, only the average global RoCoF of a system can be evaluated. However, this approach cannot provide information about different local RoCoFs in the system, which can significantly differ from global gradients, especially close to the system separation line.
This paper presents the results of a detailed study that investigates the local and global RoCoFs following system splits. One goal of this paper is the analysis of the relation between local and global RoCoFs to derive recommendations for robustness requirements for grid users. For this purpose, time domain (RMS) simulations of three system split scenarios in CE are performed using DIgSILENT PowerFactory. The CE grid model has been extended by dynamic models for Renewable Energy Sources, frequency dependent loads as well as Limited Frequency Sensitive Mode - Overfrequency and the Frequency Containment Reserve.
One investigated system split scenario is a variation of the system split of 2006, focusing on the overfrequency grid island, which includes Germany. The simulation results confirm that local RoCoFs differ significantly depending on the proximity to the separation line and the power imbalance. The ratio of local to global RoCoFs, determined over an average window of 150 ms, is as high as 4.5 in this scenario. While for a window time of 150 ms a global RoCoF150 ms of 1.18 Hz/s in the separated system is observed, the local RoCoFs range between 0.1 Hz/s and 5.36 Hz/s. With increasing averaging window sizes, the global RoCoF500 ms is 1.07 Hz/s. The local RoCoFs over an averaging window of 500 ms are ranging from 0.4 Hz/s to 1.99 Hz/s. Hence, the distribution of local RoCoFs is more homogeneous with increasing time after the contingency and with larger averaging windows.
The results of the different system split scenarios are comprehensively investigated and evaluated in the presented paper. Thereby, the focus is on defining robust requirements for an upcoming extension/amendment of Grid Codes. Furthermore, the influences on RoCoFs and the plausibility of the results are highlighted and discussed. Finally, a proposal for robust requirements regarding withstand capabilities against RoCoF for generation units is discussed.
Comparison of Current-Limitation Approaches for Grid-Forming Converters enabling Fault-Right-Through Operation in Converter-Driven Power Grids
This paper addresses the issue of limiting currents of grid-forming converter systems in the event of a grid fault. Because power electronics are not capable of feeding in an unlimited amount of fault current, the control method must ensure that the current limits are not exceeded. The most trivial approach is to switch to current-controlled mode in case of a grid fault. This makes the fault current very easily controllable but can also lead to instabilities under certain circumstances, as this work shows. A second approach, which allows the grid-forming properties to be kept, is compared. The simulation results are obtained through experiments in a Hardware-in-the-Loop (HIL) test system. The analysation of the results yields to a better understanding of requirements for grid-forming current limitations for Fault-Right-Through (FRT) operations.
Markov Mixture Distribution Modelling for Downscaling and Global, Diffuse and Beam Component Separation
High resolution global, beam and diffuse horizontal irradiance data is useful for estimating the output from tilted photovoltaic panels. However, access to high resolution global, beam and diffuse irradiance data is limited for most locations as the cost for, and complexity of installation and maintenance, for irradiance component measurements is high. Also, most models for downscaling global horizontal irradiance are based on downscaling that variable alone. This paper presents a Markov mixture distribution model for estimating minute resolution global, beam and diffuse irradiance from hour resolution global horizontal irradiance. The model is tested on data from Norrk\oping, Sweden and Oahu, Hawaii, USA. The universal applicability of the model, i.e. being trained for one location and being applicable to another, is discussed as well.
Oversizing Co-located Wind and Solar Parks to Increase the Capacity Factor
This paper presents a study on oversizing existing wind parks with solar PV parks. Oversizing is done by installing larger capacity than stipulated in the connection agreement with the system operator. We use two years of observed wind power output from 264 operational wind parks across Sweden and modeled solar power output. The results show that the required curtailment and the increasing PV capacity has a non-linear relationship, where curtailment losses are small at first but increase quickly after installing more than roughly 50 % of the nominal contracted export capacity. The average curtailment losses largely depend on the anti-correlation between the resources and the latitude location, where sites with stronger anti-correlation and higher latitude location in general have less curtailment losses. The curtailed power is largely a function of the overinstalled PV, where storages should be rated at approximately 35 % less than the overinstalled PV in order to capture at least 90 % of the curtailed power. Furthermore, the autocorrelation of the curtailed energy is largely unaffected by the amount of overinstalled PV and reaches a low correlation after approximately four hours.
A Methodology to Improve the Predictability of Wind Energy Generation with Preliminary Confirmatory Evidence from Great Britain
This paper presents a methodology to improve the short-run predictability of both onshore and offshore wind energy generation using data from the British power grid. It is first noted that the level of predictability is deficient. The analysis proceeds by investigating the nature of the forecast errors. It is observed that the forecasts do not fully reflect expected meteorological conditions. Following this finding, a machine learning approach known as ARCH/ARMAX is used to predict wind energy generation. The model’s exogenous inputs are the forecasted wind energy generation levels, the aggregate generation projections reported by the wind farm operators, and simulated meteorological conditions. The formulation also uses over 20 ARMA terms that reflect the autoregressive nature of wind energy generation. The model is estimated using 30-minute data from December 7, 2018, through December 31, 2021. The model is evaluated using out-of-sample data from January 1, 2022 – April 15, 2022. The out-of-sample period-ahead predictions are more accurate than those based on the existing methods over the same period.
A novel modular combinable Hardware-in-the-Loop platform for stability investigations of converter-driven power grids
The most important aspect for future renewable electricity grids is to ensure at least the same power quality and supply reliability requirements as today’s grids. Therefore, it is necessary that the mostly converter-based generators fulfill the present grid codes and the correct behaviour for all contingencies is proved. This paper introduces a modular combinable Hardware-in-the-Loop platform for testing converter system from early development stages up to holistic systems. These experiments supplement the results of detailed modelling in simulation programs and provide realistic investigation of converter-driven grids. As an application of the test platform a stability analysis of two converters with different grid-forming control methods are examined. The test platform enables the investigation of future large converter-driven grids and research on future challenges on grid stability, protection devices and grid codes requirements.
Review and comparison of single and dual active bridge converters for MVDC-connected wind turbines
A key component for all-DC wind farms is the DC/DC converter. The converter must have multi-megawatt power capability, a high step-up ratio, provide galvanic isolation, and operate efficiently while being able to fit in the wind turbine nacelle. The single active bridge (SAB) and dual active bridge (DAB) converters in standalone or cascaded configuration are promising topologies that have the potential to meet these requirements. This paper reviews the operation and control of these converters, and compares their volume, weight, and efficiency for a 15 MW wind turbine with 80 kV DC connection. The results show that the standalone topologies are significantly smaller and lighter than their cascaded counterparts. However, all topologies fit inside the wind turbine nacelle. The SAB designs are the most efficient and robust, as they use diodes in the output bridge. The DAB topologies have the advantage of bidirectional power flow at the cost of additional switches and losses. The standalone DAB requires series-connected switches in the output bridge, which may difficult to implement. The cascaded topologies offer higher reliability without significantly increasing losses, making them the most attractive option for future DC wind turbines.
PQ4Wind – A Novel Test Bench For Component-Level Wind Turbine Converter Testing
The Fraunhofer-Institute for Wind Energy Systems IWES is developing a new multi-megawatt testing facility for low voltage wind turbine converters. The new facility focuses on examining the frequency converters’ electrical behaviour in a wide frequency range from fundamental frequency up to the 200th harmonic. This paper presents an overview of the project, the design principles of the facility and select preliminary simulations of its grid emulation capabilities. The simulations comprise of a demonstration of the grid emulator’s voltage control and the emulation of both linear and non-linear frequency-dependent grid impedances. The simulations exhibit promising results of the new testing facility regarding accuracy of emulation and usable frequency range.
Challenges in analyzing green hydrogen scenario pathways for the transition years - a meta-study for Germany
Hydrogen plays a key role in the deep decarbonization of energy systems. In this study, the demand for hydrogen and its derivatives in five major German energy system studies and a total of nine scenarios is surveyed by a meta-study approach. Special focus was set on the transition year 2030, which shows higher uncertainty in rank and size of sectoral demands than later transition years. In 2030 as well as 2045 the industry sector is generally seen as the most important sector with demand ranging from 12 to 66 TWh in 2030. For other sectors, the rank and demand differ strongly in 2030 and show no correlation to the target electrification rate of the scenarios. This applies to the overall demand as well, ranging from 14 to 123 TWh in 2030. In general, uncertainty and heterogeneity are higher for demands in 2045 than in 2030. Additionally, a structured overview of methodological differences between studies and results is presented and result reporting guidelines are provided to facilitate future scenario comparisons. A data annex to the study is provided.
Integrating a higher share of renewable energy in the southern region transmission network of India: Impact and recommendations
This paper highlights the renewable energy (RE) potential in India’s Southern region (SR), estimated primarily on the basis of resource availability, land use and land cover, and proximity to grid infrastructure. It also projects the additional RE generation capacity required, apart from the existing and planned conventional and RE generation capacity, to meet the electricity demand in SR for the year 2030 (as estimated by the Central Electricity Authority). The study involved performing power-flow analyses for the SR transmission network through detailed transmission-network modelling in the ETAP software. Eight different scenarios—chosen to capture the variations in SR demand and in solar and wind generation patterns—were considered. Our study finds that SR has a solar potential of 329 GW and a wind potential of 188 GW, spread over an area of 6,654 km2 and 39,405 km2, respectively. It estimates that to match the projected demand, another 53 GW of RE capacity needs to be installed, requiring additional transmission infrastructure capacity—22,735 MVA of transformation capacity and 5,642 ckm of transmission lines. The study also projects that an investment of around ₹12,867 crore ($1.62 billion) would be needed for strengthening and upgrading the region’s transmission infrastructure.
Towards full electrical certification of wind turbines on test benches - Experiences gained from the HiL-GridCoP project
Today's power generation plants must meet a set of strict standards to be allowed to be connected to the power grid. For wind turbines, the required grid compliance has been verified mainly by well-established field measurements. But, since such tests typically take a long time, depend on good wind conditions, and can only be replicated to a limited extent, more and more manufacturers are relying on test bench measurements, such as those that can be performed at the Dynamic Nacelle Testing Laboratory (DyNaLab) at Fraunhofer IWES. While at the beginning of development, the industry was rather skeptical about such test facilities, latest projects that have focused on the testing of a complete nacelle were able to prove the functionality of the test methods and create increasing acceptance among customers and system operators.
To reduce design and test cycles and the associated costs even further, Fraunhofer IWES and the project partners Nordex Energy SE & Co. KG and Vestas Wind Systems A/S have developed and validated new test methods within the Hardware-in-the-loop Grid Compliance Test Bench (HiL-GridCoP) project. The test bench aims to perform an electrical certification of wind turbines by testing a reduced system, consisting of the generator, the converter, and the transformer plus control system. Within the project, the test procedure and the 9 MW hardware-in-the-loop grid compliance test bench were developed by Fraunhofer IWES, whereas the project partners carried out filed tests for validation. It was possible to reproduce the recorded behavior of wind turbines with a doubly-fed induction generator (DFIG), from which the requirements for the test procedures could be derived.
Due to the special torque characteristics and initial high short-circuit currents of turbines equipped with a DFIG, the performed tests present a major challenge for converter-based test benches. Since up to now, only simulative studies have demonstrated the comparability of test bench measurements with field tests when using DFIGs, the project represents a novelty in the era of new grid compliance test facilities and brings the industry one step closer to full electrical certification based on test bench measurements.
The paper describes the test bench structure and the virtual mechanical model of the turbine and discusses the utilized impedance emulation principle in detail. It allows the simulation of various inductive-resistive impedances on the grid side, and thus, represents a core element of HiL-GridCoP. Besides strong and weak grids, it can also be used to simulate the impedances of a fault-ride-through container for modeling symmetrical and asymmetrical voltage dips in the field. In this context, it is shown that a validation of the container parameters is important for the comparison of test bench and field tests, and it is discussed which limitations may be caused by test bench components such as transformers and how the occurring effects can be minimized.
Transmission Reform in the US
In this paper, we report on two analyses that support the need to reform processes for transmission and generator interconnection planning in the US. The first develops methodologies to quantify the multiple benefits of transmission and applies these methodologies in two sample transmission projects. Using 40 years of weather data, a methodology is developed to examine resilience to extreme weather events. Extensive sensitivity analysis is conducted across renewable energy and fossil-fuelled generator levels, gas prices, and load growth rates, to examine risk mitigation. Loss-of-load expectation analysis is used to examine resource adequacy benefits. Adjusted production cost, emissions, and enablement of high-quality renewable resources are also quantified. These methodologies are applied to two types of example transmission projects in ERCOT (Electric Reliability Council of Texas) to demonstrate a value-stacking approach. The second investigates cost savings for new generator interconnections through a proactive planned approach. A proactive planned approach can study larger numbers of projects or even look forward to anticipate projects due to public policies and may be able to right-size transmission facilities for the longer term rather than sequentially building many small facilities. In this analysis, we compare generator interconnection costs across three different levels of proactiveness in MISO (Midcontinent ISO) and SPP (Southwest Power Pool) to understand the importance of a proactive, planned approach for generator interconnection.
Density Integration Approach for Probabilistic Prediction of Wind Power Generation Based on Ensemble Weather Forecast
Wind power generation is highly expected as an element that will constitute a part of the dominant power resources for achieving carbon neutrality. In power systems that must constantly balance supply against demand, prediction of wind power output based on various measurement data and consideration of its uncertainty are further important to realize compensation for output fluctuation through operation of reserve power sources and energy storage systems. In particular, the framework of probability density forecasting, which derives a conditional probability distribution of the forthcoming wind power output based on the latest observable information, is attractive from the perspective of providing rich information not only on the expected value of power generation output but also on its uncertainty, and various approaches have been studied. Among them, multiple scenarios representing uncertainties in atmospheric behavior derived from the ensemble weather forecast will provide useful information for the precise prediction of the probability distribution of wind power output with forecast time horizon longer than a few hours, although frequent forecast updates are difficult due to the computational costs required by the numerical weather model.
This study discusses an approach to realize highly accurate and frequently updatable short-term probability density prediction of wind power output by focusing on fresh observations, while taking advantage of ensemble weather forecasts, which are computationally expensive and not easily updated frequently. The approach focuses on the spatio-temporal information of horizontal wind speed represented by individual ensemble members, which is updated only a several times a day, and applies a machine learning method, i.e., gradient boosting, to derive multiple sequences of probability densities for the wind power plant output; these sequences derived from individual members are dynamically weighted and integrated according to its current plausibility by matching them with the latest actual power generation at the required prediction timing. In particular, the proposed scheme introduces a generalized class of the mixture model for achieving the flexible integration framework of the multiple probability density prediction results. The characteristics of the proposed framework is evaluated through benchmarking experiments on data sets collected at several real-world wind power plants.
ANALYSIS AND MITIGATION OF SUBMODULE CAPACITOR OVERVOLTAGE FOR MMC-BASED GRID EMULATOR UNDER LVRT TEST
The modular multilevel converter (MMC)-based grid emulator is recently reported as a promising approach for grid-code compliance testing of next-generation wind turbines. This paper reveals the overvoltage on the submodule (SM) capacitor of the MMC-based grid emulator during the low-voltage ride-through (LVRT) test, and proposes an approach to address such issue. First, a simplified average model is developed to quantify the maximum and minimum SM capacitor voltage during the LVRT test. Then, a SM capacitor energy balancing approach is proposed to mitigate the transient overvoltage, which limits the SM capacitor voltage within 60%~110% of the rated voltage. Simulation results have confirmed the correctness of the theoretical analysis.
Employing wind farms in grid restoration processes - a field testing
As the energy transition is underway the penetration of the power grid with distributed generation units (DGUs) providing renewable energy (RE) increases continuously. These DGUs typically exhibit very volatile feed-in, that stems from varying availability of solar radiation or wind. In order to remedy the volatility of RE generation, hybrid plants employing both wind and solar power as well as storage solutions have gained popularity.
Although grid codes are designed to enable RE integration under regular circumstances, operators also face challenges when employing RE in exceptional or critical situations, such as black-outs. Hence ancillary services are at the moment largely requested from conventional power plants and controllable loads such as pumped storage plants. Grid supporting services from DGUs are only sometimes employed, e.g. to support utility conservation by restricting power feed-in, or for voltage containment in high and extra-high voltage grids
In the research we present here we will argue, that wind power plants (WPPs) are technically able to provide valuable ancillary services to the power grid in critical situations, and support this assessment with a field test using a wind farm, with a total installed power of 51.9 MW and 22 wind turbines. Our approach is two-fold:
First we describe a farm control mode, that is designed to support the grid operator in critical grid situations and frequency containment during grid restoration, and present both simulation results as well as measurements in a number of test case scenarios. The WPPs farm control unit (FCU) has an integrated P(f)-curve with a variable f-Setpoint and can be operated with a throttled feed-in, hence providing up- and downward power reserve to cover drops and peaks in the frequency. A careful adjustment of the WPPs’ P(f) characteristic and its parameters is necessary to avoid instabilities in the grid. We provide evidence, that frequency restoration can be sped up this way. This control mode can be triggered from the grid operation control centre directly and is hence available in any kind of grid situation.
Secondly we suggest a communication infrastructure that provides the ability to generate and store forecasts of the available active power on-site the WPP, hence providing the forecast to the grid operator directly via the FCU. The locally available forecasts are generated from most recent sensor data available on-site the wind farm using local machine learning models, where the forecasting horizon is optimized for a few hours ahead, in order optimally support grid restoration.
The presented research has been carried out in the joint research project SysAnDUk (FKZ 03EI4004A) funded by the German Federal Ministry for Economic Affairs and Climate Action.
Design Considerations and Test Results of a Grid-Forming DFIG WTG
Grid-forming inverter-based resources are gaining attention as a viable mechanism for supporting significantly higher penetration of renewables as conventional synchronous generators are being retired or displaced in bulk power systems . Because grid-forming resources emulate some of the physical characteristics of synchronous machines via control algorithms, this technology may help alleviate growing concerns related to decreasing inertia and reducing system strength in power grids. This paper is related to recent technology development of dual-fed wind-turbine technology with grid-forming capability.
Residential building with rooftop solar PV system, battery storage and electric vehicle charging: Environmental impact and energy matching assessments for a multi-family house in a Swedish city
In this paper, environmental impact and energy matching assessments for a residential building with a rooftop photovoltaic (PV) system, battery energy storage system (BESS) and electric vehicles (EV) charging load are conducted. This paper studies a real multi-family house with a rooftop PV system in a city located on the west-coast of Sweden, as a case study. The environmental impact parameter assessed in this study is CO2 equivalent (CO2-eq) emissions. It should be noted that the CO2-eq emission assessment takes into account the whole life cycle, not only the operational processes. The assessments consider both the household and transport energy demands for the building’s residents.
Results show that, CO2-eq emissions from the building electricity usage are increased by 1.65 ton/year with the integration of PV-BESS system. This is because the Swedish electricity mix has a lower CO2-eq emissions than the PV-BESS system. The total CO2-eq emissions from the transport needs of the building’s residents are significantly decreased, by 32.9 ton/year, if they switch from fossil-fuel-powered cars to EVs. However, the integration of EVs increases the power demand significantly which could be problematic for the power system. In such scenario, the highly-utilized distributed PV systems, enhanced by BESS, can be a low-carbon solution to address the increased power demand challenges coming from transport electrification.
The Optimal Control with Implicit Phase Coordination of a Collective of Wind Turbines
The following paper details an optimal control algorithm that coordinates a small collective of wind turbines. The algorithm consists of a reduced order model (ROM) of the wind turbine collective, a discretization of the resulting state equations using a collocation method, and an optimization formulation that guides the collective’s behavior. In order to validate this algorithm, the paper provides results from a scenario where two separate three turbine collectives are connected via a transmission line. Combined with energy storage, each collective delivers a constant amount of power to the grid while simultaneously coordinating their performance to bound any excessive fluctuations in the voltage. This scenario suggests that a collective of wind turbines, combined with energy storage, can be coordinated to provide a constant amount of power with consistent voltage to the grid in spite of rapidly changing wind conditions.
Transient operating strategies for Solar Heat Supported Solid Oxide Electrolysis systems for hydrogen production
Countries worldwide target to replace fossil-based energy
and material resources with renewable alternatives as far as possible. This comes with two challenges. First, the high share of intermittent renewables which requires efficient and cost competitive long-term energy storage on a large scale. Second, renewable energy needs to be accessible by hard-to-electrify sectors like transport and heavy industry. For both challenges, green hydrogen is considered as a viable solution. One of the most efficient technologies for producing green hydrogen is electrolysis based on Solid Oxide Cells (SOC). Due to their high operating temperatures and enhanced kinetics, SOCs can achieve considerably higher efficiencies than other electrolysis technologies, such as Proton Exchange Membrane (PEMEL) and Alkaline Electrolysis (AEL). Besides, SOC electrolysis (SOEL) offers the possibility to supply part of the energy demand by industrial waste heat or by renewable sources, such as solar thermal energy.
At the German Aerospace Center (DLR), a simulation framework was developed with the aim to investigate the transient behaviour of SOC-based process systems. Within this framework, a transient model of a 100 kW SOC reactor with multiple stacks was developed. It was validated with experimental results obtained in a DLR testing environment designed for such reactors. Within the DLR project Solar Heat Supported Solid Oxide Cell Electrolysis (SOL), simulations and experiments are being used to investigate the coupling of solar heat with a SOEL-system. The main goals are to develop system concepts and demonstrate these on a real coupled system for green hydrogen production.
In this contribution, a solar-SOEL coupled system concept is introduced and analysed for its capability to cope with typical intermittences in solar irradiance. The main focus is laid on the transient behaviour of the SOC reactor during variation of different operating parameters, such as the applied current, the feed gases’ temperatures and the reactant conversion. Additionally, it has to be assured that the limits of safe operation are not exceeded. Results will be presented showing that the current variation has the strongest effect on the stacks’ temperature, yielding relevant temperature gradients, especially in endothermic operation. Whereas by increasing the reactant conversion during endothermic operation, it was possible to reduce thermal stress in the stacks, while increasing the hydrogen output as well as the system efficiency. It will be presented how these effects can be combined and utilized for the development of control and operating strategies that aim at improving system performance. This will be exemplary illustrated for improving system performance during a period of overclouding and thus reduced heat supply.
Grid forming and grid following comparison for an offshore wind farm connected via a HVAC cable
Due to the ever increasing size of wind turbines and progressing offshore capabilities, a wind farms are being positioned further offshore. For distances longer than 50 km, it is common to use HVDC transmission systems, mainly in that these have less losses and the power flow is fully controlled. However, HVAC is a highly mature and cheaper technology, thus should not be disregarded, especially if proper reactive power compensation is installed adequate converter control strategy is implemented.
This study, which was done as part of a PhD thesis, has the objective of comparing the stability and robustness of two converter control units connected to the onshore grid via a HVAC cable, when deployed in an aggregated wind farm model. These strategies are the grid forming Virtual Synchronous Machine (VSM) and the grid following Standard Vector Current Control (SVCC).export , where different grid strengths are tested, varying the SCR.
To ensure a fair comparison, a framework was followed. Firstly, both controllers were tested via computer-based simulations. The exact same hardware and same system parameters were used and also, both systems were tuned following the same methodology, with identical minimum dynamic performance requirements. Several operating points were analysed for power (between 0 p.u. and 1 p.u.) and voltage (0.9 p.u. to 1.1 p.u.). The export cable was assigned a fixed length of 50 km with the comparison being performed in both frequency and time domains. were
The first study completed was to vary power and voltage operating points. For the VSM, and looking at disk margins and pole-zero maps, it was seen that the gain and phase margins are almost identical, which suggests this controller is robust to different operating points. The lowest gains were seen at the grid frequency, around 50 Hz. On the contrary, in the SVCC case, the results suggest that the system suffers with an increase in power penetration or decrease in voltage as the gain and phase margins magnitude diminishes significantly thus, this controller does not offer the same robustness when changing operating points. When looking at varying the SCR, the SVCC was shown to be highly sensitive to grid strength whereas VSM maintains close gain and phase margins for different SCR levels. Therefore, VSM seems more suited to sustain weak grids. With regards to the time domain analysis, for both voltage and frequency drops, it was seen that the SVCC power loop responds much faster to a disturbance when compared to the VSM, albeit with a higher overshoot.
In this study, two converter control strategies are compared. It was seen that stability is achieved for both, using different power and voltage set points with various SCR. However, after further analysis was performed, VSM has shown to be more robust to all kinds of disturbances and grid strengths, being therefore, a more robust technology for this specific system.
Automated data integration of residential and commercial PV systems into DSO SCADA utilizing IEC 61850 compliant comprehensive data model
As a vital part of the ongoing energy transition, an increasing number of residential as well as small and medium-sized commercial PV systems are and will be installed in the distribution grids. In terms of PV integration, the physical integration of grid-interconnected PV systems often stands in the focus. However, system integration on the information level must be handled as well regarding the scalability of decentralised energy systems. It is not feasible to manually integrate millions of decentralised PV systems into the utility grid management systems. Hence, a model-driven automation approach should be considered to standardise the integration process of distributed PV systems.
This paper investigates this issue at the data model and communication level. Two highlights are: 1) the utilisation of an IEC 61850 compliant comprehensive data model for advanced use cases, such as PV curtailment and application of PV forecast; 2) an IEC 61850 model-based automation framework as the data model interface for an experimental distribution grid control centre. The proposed concept has been validated at different testing levels and could assist DSOs in handling the controllability and scalability of distributed PV systems, realise the high capacity utilisation scenarios of future distribution grids and reduce grid reinforcement investments.
Towards Standardized Testing Procedures for Inertia Provision of Grid Forming Inverters
Grid Forming (GFM) Inverters and their capabilities are critical to enable growing penetration of Distributed Energy Resources (DER) into the electrical grid. The electrical inertia brought by GFM inverters to the network can replace or supplement the inertia of rotating machines that is a key element for power system stability. The German research project “Netzregelung 2.0” (“Grid Control 2.0”) investigated the operation of power systems with GFM inverters and developed inverter controls and possible grid code compliance testing approaches. First outcomes regarding the testing approaches for inertia provision are presented. A major portion of grid-forming control approaches can be described following the well-known equations of synchronous machines and thus can also be characterized in their frequency behaviour with reference to the first order swing equation of a rotating system, mainly characterized by the acceleration time constant (Ta) and damping (D). The accurate determination of these values is important for system operators and future ancillary service markets. Measurements were performed based on existing and upcoming standards and guidelines with a special focus on parameter determination and measurement uncertainty.
Did wind cause the price rise in European electricity market in 2021? – an econometric analysis using multi regression model of spot market prices -
The European electricity markets have seen price rises to levels not seen before since 2021, even before Russia’s invasion of Ukraine, and it was especially evident in the autumn of 2021. Some media argued the cause of the price rises in European markets was low wind, such as “Energy Prices in Europe Hit Records After Wind Stops Blowing” (The Wall Street Journal) and “Europe’s renewables generated energy crisis” (Financial Review).
This paper clears that what factors affected the electricity market prices in European countries using multiple regression analysis to evaluate whether the argues were correct. Multiple regression analysis is one of the well-known methods in econometrics that quantitatively and objectively measure the impacts of explanatory variables on an explained variable. However, a simple regression model is not sufficient for causal inference because the model cannot explain causal relationships, but rather correlations among factors. To apply regression analysis to causal inference, it is necessary to create an explanatory model that takes confounding factors into account. Therefore, the author employed the ERM (Expanded Regression Model) in STATA ver.17, a kind of the method of instrumental variables and can express confounding factors and/or a reverse causality.
The model using the STATA ERM consists of approx. 15 variables including output powers from various technologies, gas market prices, volumes in gas storage, temperatures and so on (although it is market-dependent, as some countries do not have nuclear and/or hydro power plants in their generation mix). The explanatory model is described by two linear equations combined by a confounding factor. Here, the output power of gas turbines was chosen to the confounding factor because gas turbines are dispatchable generators that depend on the output from the other generators, including VRE (Variable Renewable Energy).
Multiple regression analysis using the ERM on some European electricity markets including Denmark, Germany, France, and Spain, revealed that the impact of wind on market prices was quite little, whileinfluence of gas market prices was the most dominant, especially in the second half of 2021, when the price rises were remarkable. Thus, it became clear that there was no objective and scientific evidence in the narrative that many people believe, “low wind caused the high electricity price”.
Offshore Network Development Plans for Europe - Joint Efforts
The European offshore RES strategy from November 2020 aims at 300 GW offshore wind capacity by 2050 in European waters, breaking down to first 60 GW by 2030.
In the following two years, groups of several countries and European Regions join forces on political level to agree on joint ambitions, as the task is too big to tackle nationally alone.
In the summer of 2022, a new European Regulation has entered into force, organizing joint infrastructure planning for the European sea basins, as this is crucial to achieve the ambitions.
The new TEN-E regulation EU 2022/869 requests that new offshore network development plans for each European sea basin are to be produced for the three time-horizons 2030, 2040 and 2050. This must be a joint effort of Governments, ENTSO-E and the European Commission, each having to solve a part of this task.
The contribution to this conference will describe how ENTSO-E and its member TSOs jointly prepare for solving this task and what is deemed necessary for a seamless international development across waters, mainland, energy systems and - sectors.
Multilateral TSO-investigations into possible pathways are addressed, including in relation to projects currently under development.
AREAL POWER PLANT: AGGREGATION SYSTEM TO CONTROL A MULTITUDE OF DISTRIBUTED GENERATORS DURING POWER SYSTEM RESTORATION – DEMONSTRATION RESULTS
Power system restoration (PSR) is an unlikely but critical event, that requires new approaches to cope with increasing shares of distributed generators (DG) like photovoltaic (PV), especially in low- and medium-voltage grids. One possible solution to tackle this challenge, a so called “Flächenkraftwerk” / areal power plant (APP), will be examined in its prototype stage. The core idea behind an APP is the provision of large(r) scale power plant characteristics with aggregated DG especially during PSR. For testing the APP it has been deployed in a virtual laboratory (virtual lab) including PV emulators which are capable to simulate active power behaviour as well as ICT aspects. Furthermore, the virtual lab has been manually interlinked with a proven training system for control centre personnel to allow the look and feel of a real restoration situation. The main result of two extreme scenarios, a worst case scenario with no DG remote controllability and a best case scenario with partial controllability, shows the APP’s capability to allow a higher re-supply of loads if the assumed high PV feed-ins occur. Therefore, the APP could be a cornerstone of future PSR in PV dominated regions if the DG’s bidirectional remote controllability increases.
Power quality in a solar-wind hybrid park – preliminary results from a one-year metering campaign
A concern from a grid owner perspective is how so-called hybrid power park (HPP) will influence the power quality (PQ) in their distribution grid. This project studies how PQ and on-load tap-changer operations are affected by a solar-wind HPP located in a Swedish municipality. PQ measurements were performed in ten-minute intervals during 2021 for relevant PQ parameters in wind and photovoltiaics (PV) power parks and a low-voltage substation all connected to the same 10/130 kV transformer and compared to two other load-dominated transformers. Results show that ‘events’ (i.e., deviations from PQ standards) were more common when measured at the connection points of the parks (130-132 events) than in a load-dominated transformer (70 events). Events are slightly more likely when PV power generation dominates. However, only a few (sag) events may require the grid operator to act. Overall there are less tap changing operations in the transformer in which the HPP is connected compared to in a load-dominated transformer. This is probably due to a broader bandwidth in the tap-changer of the HPP transformer, which may cause future problems downstream. In conclusion, the grid operator should presently not be worried that the HPP will increase PQ issues.
Sharing the network infrastructure between renewable sources with different technologies as a way to increase the connection possibilities of the power system
The aim of this article is to present the possibility of connecting two RES facilities with different technologies to the grid, while keeping the value of the connection power of the first one at an unchanged level. There are a number of low-power renewable energy installations in operation in Poland, connected on the basis of appropriate agreements concluded with grid operators. The most widespread here are wind farms operating individually or in small groups. The connection points are located on medium voltage (MV) lines. The owners of these RES facilities often make efforts to increase the power and number of their installations, which is associated with applying for an increase in the value of the connection capacity.
Grid operators issue decisions in which they often refuse to enhance contracts for connecting new sources. The operators explain that connecting another installation would require changes and modernization of the power grid infrastructure, which are not included in the investment plans of these companies for the coming years.
In such a situation, a reasonable solution is to expand the existing RES installation with a facility with a different electricity generation technology. With the use of an appropriate control system, this solution is completely neutral for the network operation safety and does not cause changes in the power flow previously adopted as the basis for issuing connection conditions for a single technology facility.
In the case of a wind power plant, the second source may be a photovoltaic installation. Such a source system can be called a dual technology system or a hybrid system. The main advantage of the dual technology system is the reduction of the demand for connection power resulting from the natural diversity of generation possibilities in various technologies (weather, time of day, season) and good controllability of photovoltaic technologies, thanks to which the resultant maximum output power of the hybrid system may remain unchanged.
Considering wind generation as a priority at a given connection point, it is possible, in the technical, economic and legal sense, to make available periodically free connection capacity of another installation, provided that it will adjust its instantaneous power so that the resultant generated power does not exceed the value of the contractual connection capacity. originally connected RES installation. Such a solution of course requires the investor to accept the necessary, periodic limitation of the value of generated power, implemented by a dedicated control system.
The purpose of this article is to determine the parameters and numerical indicators that would be characteristic of the operation of low-power RES sources sets consisting of two technologically different installations. The analysis uses the generation characteristics of real sources, and the obtained results seem to be an attractive solution for investors interested in hybrid RES systems.
Application of Phase Shifting Transformer (PST) for Blackstart and Stable Operation of Offshore Wind Farm with Diode-Rectifier Unit HVDC link
This paper describes a new method for operating the offshore wind farm (OWF) with diode-rectifier unit (DRU) HVDC (offshore side), where a medium voltage (MV) submarine cable is in parallel operation with DRU-HVDC link. In order to avoid uncontrolled current flow through the MV submarine cable, a phase shifting transformer (PST) is applied on the onshore side of the MV submarine cable. The application of PST is to ensure the smooth blackstart and stable operation of the OWF and DRU-HVDC link. Both static and dynamic behaviors of the proposed method are presented in this paper and the simulation results validate the proposed method.
An Optimal Charging Scheduler-Based Charging Strategy of a Real Life Hybrid Energy Storage For A Solar And Wind Equipped Student Residence
With increasing renewable energy penetration globally, Energy Storage Systems (ESS) act as a vital component for transforming the current energy sector. In the form of Grid connected ESS, Lithium-Ion Battery (LIB) technology is presently the most popular form of ESS, especially because of its fast response capability, efficiency, and reducing market prices, but is not always preferred for long-term storage, due to its relatively shorter lifetime. A Redox Flow Battery (RFB) on the other hand has a higher lifetime and better long-term storage capability, but has a higher upfront cost and reduced round trip efficiency. A Hybrid ESS (HESS) consisting of LIB and RFB offers the advantages of both the technologies, thus making the individual ESS more economical and flexible to use while also improving its cycle lifetime. Such a grid-connected HESS is planned and installed for a student residence at Bruchsal accommodating 150 students and equipped with 220 kWp photovoltaics (PV) and 10.5 kWp wind-power.
In order to control this conglomerate, an Energy Management System (EMS) is deployed which not only controls but also optimizes its operations in real-time. The EMS aims at achieving multiple objectives which include reducing the ESS aging, operating the system at reduced losses, and the most important, improving building self-sufficiency. This paper focuses on the charging strategy of the HESS which is optimized in two folds. First the HESS is operated with a fixed priority-based strategy where the operation efficiency of the High Energy ESS, i.e. RFB is improved. Secondly, based on generation and consumption forecasts of the setup the EMS optimizes the charging of the High Power ESS, i.e. LIB. With the forecasts available, the EMS strategically schedules delayed charging of the individual ESS, which avoids longer relaxation periods at higher SOC and thus the aging caused due to it. Additionally, the optimization algorithm iteratively searches and operates with the possible optimal operation point of the ESS, where conversion losses are minimal under the given circumstances. Results of real-life operation of the setup based on these operation strategies are provided in this work.
Grid-Forming Synchronverter-based Control Method with Current Limiting Method for Grid-Side Converters of Converter-based Generation Plants
This paper presents a grid-forming control method for grid-side converters of converter-based generation plants. It is based on the Synchronverter and includes, among other things, current limiting. Current limitation is an essential criterion for grid-forming control methods to be applied in real systems. This control method is validated using a Matlab Simulink EMT simulation. The simulation model includes the converter of a laboratory-scale test bed of a wind power plant, which was presented last year. In the simulation, all cycle and dead times occurring in the real plant are realistically reproduced. The studies are being carried out as part of a doctorate.
The control method developed is based on the self-synchronising Synchronverter presented by Qing Chang Zhong in 2014. In order to offer a possibility of providing primary control power comparable to a synchronous generator-based power plant, the droop gain was separated from the damping coefficient, as proposed by Tarek Younis 2018 for single-phase converters. This also enables the implementation of a deadband in the active power/frequency characteristic. The implementation of current limiting is also based on an idea by Tarek Younis, which he presented for the first time in 2019 for single-phase converters. Here, a fast current control loop is subordinated to the existing Synchronverter control. The current control loop can guarantee the provided current without hard controller switching in the event of a rapidly changing grid state with regard to grid voltage amplitude or phase. In contrast to Younis, PI controllers in the park-transformed mode are used instead of PR controllers for simpler parameterisation. Further changes, e.g. with regard to the switching logic, are presented in detail in the paper.
Eye2Sky: A network of All-Sky Imagers (ASI) enabling accurate and high-resolution very short-term forecasts of solar irradiance
Eye2Sky is a network of all-sky imagers (ASI) installed and operated by DLR in northwest Germany. At 30 different locations, high-resolution fisheye images of the sky are taken every 30 seconds. At 10 locations, the images are complementedby radiation and meteorological measurements. The Eye2Sky network covers about 100x100 km² centered at the city of Oldenburg. It has a low ASI density in rural areas and a high density in city of Oldenburg, thus providing an almost complete coverage of the city.
The images and measurements from Eye2Sky are the basis for ongoing nowcast experiments at DLR. These experiments investigate the potential of very short-term, high resolution and accurate predictions of solar irradiance in the upcoming minutes. The benefit of ground-based cameras is that small clouds and cloud fronts, responsible for short-term fluctuations, are better resolved compared to satellite images or products from numerical weather predictions models. The latter represent the standard data source for solar power forecasts.
Due to the limited field of view of an ASI, the nowcast horizon and the spatial coverage for a single camera system are also limited. The average nowcast horizon of 15 minutes and the covered radius of 4 km (depending on cloud conditions) is therefore extended by combining multiple ASIs of the Eye2Sky network.
In addition, a multiple data source approach using on-site irradiance measurements and satellite-based irradiance nowcasts together with ASI based nowcasts contributes to further improvements in accuracy, nowcast horizon and spatial coverage.
We are currently engaged in an intensive exchange with stakeholders interested in increasing the accuracy of solar irradiance predictions especially in the very short-term range. Feedback from stakeholders on needs and requirements will support us to adapt nowcasting strategies to specific applications.
Here we would like to introduce the Eye2Sky network and its potential for the very short-term forecast horizon. Feedback from stakeholders like grid operators, PV plant operators, forecast providers or market traders is very welcome.
Efficient Validation Framework for Integration of Hydrogen Based Technologies in Electric Distribution Grid.
The backbone of a neutral greenhouse gas energy system in the near future will be the electric infrastructure in combination with the gas grid infrastructure and in particular the coupling components between the two grids. Due to the fact that in a renewable energy driven system, the electricity production is dependent on the weather (solar radiation, wind) and not anymore on the actual demand, sector coupling elements and flexibilities in the energy system are going to play an important role. Sector coupling technologies between the electric, gas and heat infrastructure are for example electrolyser and fuel cell based combined heat and power systems (CHPs). To aid the collective heat demands of settlements or household, the deployment of heat pumps and CHPs at distribution power grid level might be an option. To adapt and analyse the possible changes in the energy system infrastructure, rigorous frameworks in particular from the technical perspective are being developed by universities, research institutes and industries. The frameworks span from feasibility studies via simulations to experimental tests in physical realm.
Performing simulation studies provides an intensive overview on the impacts, advantages, operational management, and shortcomings in the electric grid which may arise as a result of sector coupling by installing for example CHPs. However, such studies might fall short of indicating the dynamic behaviour at the component level and its effects at the system level due to the adopted simplifications or considered assumptions while executing the simulation studies. On the contrary, physical testing on a large scale on an actual system is most of the times not feasible and even not possible considering the sensitivity of the systems.
This paper presents a framework which is able to bridge the gap between simulation studies and physical testing and tries to combine the advantages of the two approaches. Therefore, the set-up of the developed framework for testing the integration of hydrogen-based components like CHPs in a low voltage distribution grid is explained in detail. The proposed power hardware-in-the-loop (PHIL) framework is viable for demonstration of grid dynamics and subsequently the grid connected component response. The structure for such a cyber physical system is presented along with the preliminary simulation results of a system consisting of a representative distribution grid, a photovoltaic system, a CHP system and a residential household representing the consumer. The CHP model is based on empirical values obtained through the laboratory tests. The validation framework is to be implemented in the grid-lab (Emulation Centre for Networked Energy Systems). The study emphasizes the contribution of PHIL application for a fast and efficient validation, inclusion and characterization of sector coupling technologies for instance hydrogen-based components in the energy system.
POSSIBILITY OF EXPORTING HYDROGEN IN NEPAL
The leap of Nepal’s hydropower energy generation due to increased capacity of hydropower plants leads to a significant energy surplus. For proper utilization of power, surplus generation can be bridged with electrification of major sectors such as transportation, house hold heat supply, industries and export. It can be used to replace fossil fuels and achieve energy independence. Utilization of surplus hydropower and renewable energy gives an opportunity to produce green hydrogen at marginal costs. Hydrogen is a carbon neutral solution with multi-faceted applications in the field of heat, power and energy storage. Energy models to determine the installation capacities of hydrogen plants is very important to plan projects and visualize results. This paper is an observational study where different scenarios of surplus energy and the potential to produce hydrogen are explored. Estimation of size, cost and hydrogen generation for different scenarios of surplus are analysed using excel model. The data simulated and projected are taken from NEA, IRENA, and ADB etc. The simulated data of electricity generation profile is used. From the results of this paper, a size of hydrogen system and its economic constraints can be speculated for different case of Nepal’s surplus energy.
Separating renewables growth from SF6 use – a 'mission possible'.
Worldwide, growth of renewables generation is a major driver for network extension and reinforcement. Electrical switchgear is a key component in electrical transmission and distribution networks. It is part of electrical installations like substations but also one of the key components in wind and solar PV plants.
For decades, SF6 has been used as protective gas in switchgear. Due to the extreme global warming potential (GWP about 23000) and its long life in atmosphere, in most applications, SF6 use already has been banned by regulation (Kyoto Protocol and the European F-Gas regulation). In electrical switchgear, technically and economically viable alternatives have been lacking, though, and SF6 use has been steadily growing over the last decades.
In a business-as-usual scenario, SF6 use is going to grow further. One of the key drivers is the expected strong growth of renewables and the resulting needs for network extension and reinforcement. Currently, the contribution of SF6 to overall greenhouse gas emissions is limited. With successful decarbonisation of our economies this share will become more and more relevant.
Recently, manufacturers introduced several alternatives to SF6 for a range of electrical switchgear and related equipment covering a wide range of applications. The status, technical and economical characteristics of the alternatives differ from SF6 but depending on the use case, a variety of viable SF6 free solutions exists. The range is going to be expanded rapidly by equipment manufacturers, in terms of specifications as well as in terms of diversity of industry offerings. Accordingly, regulation is going to be tightened, in Europe as well as in parts of the US.
Due to lacking experience and various concerns, end user engagement in deploying SF6 free alternatives is still limited. In the regulated network industries, policy instruments may effectively stimulate introduction of alternatives. Manufacturers and independent project developers in the renewables industry are not directly regulated. Application of SF6 free solutions in this sector will very much depend on gaining knowledge and on management decisions. From a public perception perspective, of course, late adoption of alternatives may be problematic.
The paper and presentation provide an outlook on the spectrum, characteristics and potential of alternatives to SF6 in electrical switchgear with a focus on renewables technologies like wind and solar PV installations. They will illustrate the need for alternatives as well as realistic scenarios for their introduction.
Power-to-Methane via H2O/CO2 Co-Electrolysis Integration: A Conceptual Performance Assessment on Methanation Off-gas Recirculation using Exergy Methods
Electrolytic Power-to-Methane processes have been a motive of interest in the recent years due to the benefit of producing sustainable methane from energy sources like solar and wind. Solid oxide electrolysis cell (SOEC) reactors have emerged as a promising option because of their high electrical efficiency and the potential for direct H2O/CO2 co-electrolysis for syngas production. Co-electrolysis operation is still understudied, for this reason, modelling methods can lead to improvements in process efficiency and overall carbon utilization.
This contribution presents a system concept modelling study of a co-electrolysis SOEC coupled with a methanation reactor considering three system configurations. The base case consists of an open system with no recirculation. The other two systems assess the recirculation of off-gas to either the SOEC or the methanation reactor. The study was performed using an inhouse component-oriented modelling framework written in python which is experimentally supported by the research group’s extensive experimental facilities. This approach ensures smooth implementation and modification of simplified process system configurations, while producing reliable and reproducible fluid and thermodynamic process calculations. The results presented quantify the advantages and disadvantages of the two configurations in terms of yield, and exergy efficiency – upon optimization of key operational variables.
Integration of BESS for Grid Code Complaint Operation of Transmission Networks in case of Contingencies using PSS/E and Matlab
Increase in deployment of wind and solar energy power plants is leading to deterioration of frequency regulation performance of electricity grids. The situation is set to become worse as existing conventional power plants after becoming obsolete get replaced by renewable generation. Improvement in performance and efficiency of Battery Energy Storage Systems (BESS) coupled with a reduction in cost has enabled BESS to help in responding to variability in generation and demand thereby providing renewable energy integration support. BESS have very fast ramp up capabilities and can play an important role in enhancing frequency regulation performance during contingent conditions in power systems which are nowadays operated close to their stability limits. This can help in deferring the upgradation of transmission system for some years. This paper envisages the integration of Grid Scale BESS to ensure adequate reserves for Grid Code compliant operation in case of Contingencies in 3048-Bus transmission network of an Indian province with more than 25 GW installed capacity. Dynamic simulations are carried out by implementing real time bi-directional communication between Power System Simulator for Engineering (PSS/E) and Matlab to observe the improvement in system frequency regulation performance when Grid Scale BESS is partitioned and placed at optimal locations.
A Short-term Wind Power Output Forecasting Model based on the Enhanced Gradient Boosting Machine (GBM) Algorithms for High Wind Power Penetrations
With rising concerns of climate change, there has been a worldwide trend of establishing policies regarding net zero emissions and sustainability. According to Korea's 2050 Carbon Neutral Strategy, the government aims to decarbonize the country’s economic structure and increase penetration of renewable energies. Statistics also show that wind power generation in Korea has been increasing steadily over the years. However, the intermittent nature of wind power remains an obstacle in predicting wind power outputs. Therefore, accuracy in wind power forecasts must be improved to facilitate larger integration of renewables to existing electrical grids. In this paper, we propose the the implementation of a short-term wind power output forecasting model based on the enhanced Gradient Boosting Machine (GBM) algorithms for high wind power penetrations. GBM is an effective machine learning algorithm which improves its performance by combining previously learned weak learners to form a strong learner. A 15-minute cycle of measured data from Jeju’s wind farms is applied to the model as the input data. The results include scatter plots and line graphs depicting the outcome of prediction data by the GBM model and real data.
Temporary Overvoltages and their Impact on Grid Security - Final Results from the Joint Research Project OVRTuere
The stability and security of electricity grids is of central importance for society and the economy. One potential technical risk, which grid operators have been giving increasing attention, is that of the potential large-scale tripping of generation plants due to temporary overvoltages. This paper makes recommendations about dealing with temporary overvoltages in the German power system, taking the increasing displacement of synchronous machines by inverter-based plants into consideration.
Industrial Flexibility Options: Requirements for the Regulatory Framework of the Future European Procurement of System Services
With the shutdown of conventional power plants as part of the decarbonisation of the energy system, new capacities for system services are needed. Therefore, according to the EU's Electricity Balancing Guideline from 2017, Demand Response is to be integrated non-discriminatorily into the joint European mechanism for procurement of system services, which is currently being developed.
In Germany, the procurement of system services was last adjusted in 2020, so that flexibility options available at short notice can participate recently. This offers a new marketing option for industrial flexibility options. Expected revenues in this balancing energy market are higher than in the continuous intraday market. Nevertheless, the introduction of the balancing energy market has not fulfilled its objective of increasing competition and the costs for balancing energy are immense. However, complex analyses are necessary for optimised marketing in the balancing energy market, compared to the intraday market, where the attractiveness of flexibility marketing is also increasing.
In this paper, it is shown, that the prequalification mechanism for participation in the german energy balancing markets is currently too high a hurdle for industrial demand response. Within the framework of European procurement, it must be possible to intensively integrate modern virtual power plants.
Successful Renewable Integration into the Indian Grid in 2030- Estimating Balancing & Storage Capacity Requirements
The World including India is witnessing a steady and constant shift of contribution of Electrical Energy generated from Non-Renewable to Renewable Sources of Energy. World over countries are targeting aggressively to transition from Power generation to the Renewable Energy Sources. India too has established several targets for renewable energy (RE) capacity in the coming years as announced in September 2015, at the United Nations Climate Action Summit (COP 21), most notably the target of deploying 175 gigawatts (GW) of Renewable Energy generation capacity by 2022 and 450 Gigawatts (GW) by 2030 (The Later being revised to 500 GW in COP 26 ).
The Energy Transition for India would be challanging as bulk of its power comes from Coal Plants which have less ramping up and down facilites as compared to the Gas and Hydro plants (which are abundant in Europe) and hence the Renewable Integration here would be more challanging in compared to Europe and other western countries whose VRE (Varaible Renwable Energy) share is high. It is hence expected that if such a high amount of VRE is injected into the grid there might be a large amount of surplus available which would be needed to be curtailed unless there are enough energy storage facilities.
In this comprehensive study we have analysed the past data pertaining to Annual Daily load demand curve, Solar Energy pattern and Wind generation parrtern on hourly basis and tried to find out the surplus/storage and the balancing requirements which are needed to be in service by 2030 to successfully integrated VRE in the Indian Grid.
Forecasting for the Weather Driven Energy System – A new Task under IEA Wind
The energy system needs different forecasts for its operation than just the narrow wind power forecast. Therefore, the group behind the former IEA Wind Task 36 Forecasting for Wind Energy (running 6 years, from 2016-2021) now approaches the entire sector much broader, also reaching out to other IEA Technology Collaboration Programmes such as the ones for PV, hydropower, system integration, hydrogen etc. The existing Work Packages (WPs) are complemented by work streams in a matrix structure.
The Task is divided in three WPs according to the stakeholders: WP1 is mainly aimed at meteorologists, providing the weather forecast basis for the power forecasts. In WP2, the forecast service vendors are the main stakeholders, while the end users populate WP3. Since many of the activities we do are spanning several groups, we decided to have work streams perpendicular to the WPs. Some examples are the Atmospheric physics and modelling, Seasonal forecasting, Minute scale forecasting, State of the Art, Forecasting for underserved areas, Decision making under uncertainty (including uncertainty quantification), Extreme power system events, Data Science, Privacy, data markets and sharing, the Value of forecasting and Forecasting in the Design phase.
The new Task started in January 2022. Planned activities include 4 workshops on the state of the art in forecasting for the energy system plus related research issues, a workshop on seasonal forecasting with emphasis on Dunkelflaute, storage and hydro, a workshop on minute scale forecasting, and a workshop on extreme power system events. Each of those workshops will be recorded by a paper. Additionally, the Recommended Practice on Forecast Solution Selection will be updated to reflect the broader setup.
In short, the poster presents the results of the first six years of 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. It also shows the setup and work programme for the new Task 51 on Forecasting for the Weather Driven Energy System. For collaboration, please contact the author (email@example.com) and see the website at www.iea-wind.org/task51.
Evaluating Flicker Damping Capabilities of Wind Turbine Inverters with Grid-Following and Grid-Forming Controls
Evaluating Flicker Damping Capabilities of Wind Turbine Inverters with Grid-Following and Grid-Forming Controls
Laila Rezai, Florian Pöschke, Moritz Andrejewski, , Horst Schulte, Jens Fortmann
HTW Berlin, Germany
The variation of output power, due to wind speed variation, turbulence and wind gust, as well as periodic disturbances such as tower shadow and wind shear cause flicker emission in the grid during continuous operation of grid connected wind turbines. Pervious research was mainly focused on the flicker contribution of stall-controlled wind turbines only. In order to study flicker effect of the WT output voltage, This paper aims at analyzing both flicker contribution as well as damping capabilities of modern wind turbines, applying test cases of the newly finalized IEC 61400-21-4 on test bench measurements to converters with both grid-following and grid-forming control. A simulation model of a MW-level wind turbine with full converter is used to study the effect of the wind turbine control as well as grid-side converter control on the voltage flicker at the PCC (point of common coupling).
Analysis and Systematic Comparison of Concepts for Voltage Control with Inverted-Based Prosumer Devices
The energy transition with its penetration of renewable energy generation and increased electrification puts high stress to the electrical power grid. Especially the low voltage level with its rising shares of prosumer devices like PV systems and electric vehicles is affected by severe deviations of the bus voltage from the nominal values potentially violating the limits of the voltage band. In this paper several voltage control concepts are analysed with a focus on supply quality, additional grid stress, and losses. The concepts presented are implemented and submitted to a simulative investigation. Main results are a positive effect of all concepts on bus voltages with no concept being superior in all key figures. The reactive power feed-in with consideration of the grid status in form of a ????(????) presents the best trade-off for photovoltaic systems with regard to impact on critical voltage values and negative effect on the grid and prosumer devices. While a combination of ????(????) and ????(????) is the most appropriate option for consumption devices, voltage control concepts for battery storage systems do not seem to have any significant impact. Adjusting the installation specifications of grid operators based on these results could mitigate critical grid situations in congested grids.
Benefits from Islanding Hydrogen Production in Renewable Energy Systems
Transitioning to a net-zero economy demands analysis of how to produce clean hydrogen cheaply, and at scale. In a fully renewable energy system, production of electricity and green hydrogen may benefit from synergies, e.g., by reducing renewables curtailment. On the other hand, separating electricity and hydrogen production could offer advantages, since stand-alone green hydrogen plants can be placed far away from load-centres, and power electronics of DC to AC to DC conversion can be avoided. We quantify the potential synergies of "integrated" electricity and hydrogen production versus stand-alone ("islanded") production in systems entirely powered by wind and solar. We explore the trade-offs for several countries in a simple one-node model that enhances analytic understanding. Our findings show that integrated hydrogen production is cheapest if hydrogen demand is low (<20%) compared to electricity demand. If cost reductions for islanded electrolysers are counted, there is significant system benefit to islanding hydrogen production. We show that a moderate reduction to capital costs of islanded electrolysers of 15% reduces total system costs by up to 15%. These results are particularly relevant for hydrogen-exporting countries where hydrogen production may be larger than local electricity demand, and electrolysers should thus mainly be deployed in islanded settings.
Passivity-Based Analysis and Design for Selective Harmonic Resonant Control
This paper analyses the impedance passivity of selective harmonic resonant control for voltage-source converter. It is found that the phase-lead compensation of harmonic resonant controller may fail to eliminate the negative damping above certain harmonic frequency, depending on the size of time delay. Further, a trade-off between the internal stability margin of harmonic control and the impedance-passivity shaping is identified. To overcome these drawbacks, a feedforward control scheme is proposed in this work, which guarantees both impedance passivity and sufficient internal stability margin for the selective harmonic resonant control. Electromagnetic transient simulations verify the theoretical analyses and the effectiveness of the control method.
Type 5 Wind Turbine Technology: how synchronised, synchronous generation avoids uncertainties about inverter interoperability under IEEE 2800:2022
Degradation of system strength because of inverter-based resources (IBRs) is a major concern facing the zero-carbon transition. A new standard released this year, IEEE Standard 2800, attempts to codify the relationship between IBRs and the Transmission System Operator (TSO).
It is apparent from IEEE 2800:2022 that there remain fundamental problems with quantifying whether source impedance (the measure of “system strength” with which the standard is concerned) will present a problem for allowing an IBR to connect. This is “because of complex interdependencies between IBR and power system characteristics”. So developers are increasingly required to adopt mitigation options such as adding synchronous condensers or curtailing IBRs.
A proven Type 5 (synchronous) wind turbine exists and has been running at 0.5 MW scale in a 46 MW wind farm in New Zealand since 2006 and eight turbines in Scotland since 2013. The US National Renewable Energy Laboratory (NREL) is conducting a study of the impacts on grid reliability, stability, and resilience of Type 5 wind turbines. The project has both simulation and testing tasks and will result in proposing a variable generation solution that will help system operators and utilities address all reliability and most resilience challenges in the evolving grid.
EMT Modeling of Inverter-Based Resources for Grid Stability Analysis Using Vendor-Independent Interfaces
The stability of future power grids significantly depends on the behavior of Inverter-Based Resources (IBR) such as wind or PV power plants, HVDC, STATCOMS or large battery storage systems. IBR can produce extremely high dynamic behavior depending on the control software. Hence, Transmission Systems Operators (TSOs) face the challenge to get appropriate simulation models for IBRs, which allows impact analysis of the IBRs on power system stability. The authors have investigated two different methods of modelling the dynamic behavior of the converters. The idea was to setup and evaluate Electromagnetic Transient (EMT) simulation processes that contain a standard interface following the ‘ENTSO-E: Standardized control interface for HVDC SIL/HIL conformity tests’, which allows integration of original control structures of the converter to be analyzed. Thus, the results represent an important milestone towards universal standard interfaces for vendor- and software-independent simulations and tests of inverter-based systems.
1. Software in the Loop (SIL) Approach
The vendor-independent standard interface of the software in the loop (SIL) approach is formed by a shard library, e. g. a dynamic link library (DLL), that contains the original control software code of the modeled inverter. The inverter’s power hardware and the grid conditions are modeled with an EMT grid simulation tool, which in this case was PSCAD. By integrating the DLL into the PSCad model the true behavior of the analyzed inverter can be investigated without disclosing IP-relevant control structures.
2. Hardware in the Loop (HIL) Approach
In this case the vendor-independent interface is formed by a standardized fiber optic interface between the controller hardware of the inverter and a real-time simulator, which contains the models of the power electronics hardware and the grid conditions. Here, the interface was based on the ENTSO-E Standardized Control Interface for HVDC SIL/HIL conformity tests.
To evaluate the feasibility of the simulation methods the authors used a 500 kVA inverter as a reference. Voltage and current as well as internal control signals of the inverter were recorded during different study cases, such as FRT or RoCoF events. The measurements were later used to validate the simulation results.
In this paper we will share our experience gained by implementing the two modeling approaches and we will give advice for their applicability.
HVAC transmission design challenges for grid integration of offshore wind power plants
Offshore wind energy is identified as a significant pillar for the energy transition and the potential development will only continue to grow within the upcoming years. The size increase of wind power plants has led to higher complexity of offshore transmission infrastructure. Therefore, the early planning of the export system should be motivated according to a reduction of costs and uncertainties. This paper focuses on the technical and economic evaluation of an HVAC transmission system design. A tool is developed to analyse such systems and provide insight into the power losses, reactive power compensation and the total cost of the system while complying with the grid code requirements. The concept is applied on a 1000 MW wind power plant including various scenarios. The scenarios are defined based on modifying relevant parameters of the system such as e.g. wind power plant reactive support, transmission voltage, transmission length and location of reactive power compensation. Lastly, the cost differences between the scenarios are presented and the ultimate design is suggested in terms of the total cost. The key contribution of this work is a comprehensive approach for addressing design challenges and comparing different transmission solutions.
Unlocking the Potential of Renewables with Green Hydrogen
In this presentation, we describe the development of a model that allows us to configure a hybrid renewable and electrolysis plant to produce green hydrogen while minimizing the LCOH (Levelized Cost of Hydrogen). In essence the model enables the plant designer to explore the tradeoffs between sizes of various key components and the utilization of renewable electricity and electrolyzers that will result in optimal LCOH. It considers various parameters, such as electrolysis capex, efficiency, O&M cost as well as PV and Wind Plant production and costs and the resulting green H2 production. The model includes grid interactions (if any) as well as local BESS (Battery Energy Storage System) to manage transients and available renewables resource variability. The model will be utilized for a multi-GW size project to explore feasibility of competitive green hydrogen in a rich renewable resource location (both solar and wind) with green hydrogen export potential. A previous version of this model, which included PV and electrolyzers, was undertaken with First Solar and NEL. This model has since been extended to include the use of wind energy and energy storage.
A Reversed Impedance-Based Stability Criterion for IBR Grids
The existing impedance-based stability criterion is effective for analyzing local control interactions; however, it is difficult to scale the existing criterion to analyze wide-area control interactions among numerous IBRs through a complex power system network. The scaled version of the existing criterion requires the impedance response of each IBR in the system as well as of the network looking from all the IBRs. It is quite challenging to obtain all these impedance responses because of the computational effort and the requirement of separately scanning the impedance of the network from the IBRs. We propose a reversed criterion for the impedance-based stability analysis to address these problems. In contrast to the existing criterion, the reversed criterion analyzes the stability of a power system when an IBR is disconnected from the system. The reversed criterion estimates the impact of an IBR on the frequency and damping of power system oscillation modes using the impedance scans of only the IBR and the grid at its terminal. It can be sequentially applied at different IBRs to evaluate their impact on the power system stability. In addition to scalability, the reversed criterion gives flexibility to focus only on a few selected IBRs, depending on their rating, the magnitude of oscillations observed at their terminals, and the vendor support available for implementing stabilizing control system updates. The reversed criterion is demonstrated on a 14-bus power system with 100% IBRs.
On the Low Risk of SSR in Type III Wind Turbines Operating in Grid-Forming Control
We have shown in previous work that the risk of subsynchronous resonance (SSR) between wind power plant with Type III wind turbines and a series-compensated transmission line is low when the wind turbines are operated in grid-forming mode, instead of the standard grid-following mode. This paper explains the fundamental mechanism behind the improved damping characteristics by modeling the positive- and negative-sequence impedances of Type III wind turbines in grid-forming control mode. It is discovered that the grid-forming control naturally acts against the negative resistance behavior of Type III wind turbines at subsynchronous frequencies that results from an interaction between the rotor-side converter current controller and negative slip. The developed sequence impedance models and improved damping behavior are verified using PSCAD simulations of a 2.5-MW Type III grid-forming wind turbine. The modeling predictions are also supported by experimentally measuring the sequence impedance response of a 2.5-MW Type III wind turbine in grid-forming control mode.
Non-linear Stability Boundary Assessment of Offshore Wind Power Plants Under Large Grid Disturbances
Recent research has identified that the dynamics of the phase-locked loop (PLL) converter control contributes to grid-synchronisation instability when a wind turbine (WT) system is perturbed with large disturbances (i.e. severe grid faults). The metric for such stability assessment is the region of attraction (ROA), which signifies a subset of the system’s state-space in which all the trajectories converge to a stable equilibrium point. This paper reviews the performance of transient stability assessment methods such as the time-domain methods, i.e. time-domain simulations, and phase portrait analysis; and the analytical methods, i.e. equal-area criteria, and Lyapunov’s direct method, for estimating the ROA of a post-disturbance WT system. Additionally, some highlights of advanced methods for transient stability assessment, such as the sum-of-squares optimisation technique and machine learning methods, are also presented. The time-domain and analytical methods are applied on an equivalent swing equation model of
a WT, where it is seen that the time-domain methods are less complex and provides accurate ROA, accompanied by a high computation burden. On the other hand, the analytical methods are complex and provides either slightly optimistic or highly conservative estimates of the ROA; however, such methods are very fast due to closed-form solutions.
New Current Limiting Control for Grid-Forming Converter under Unbalanced Faults
In the future, most power systems will rely on con-
verter based generation. In order to guarantee stable operation
so-called grid-forming converters were developed. In contrast to
synchronous generators, the behaviour of converters is mainly
dependent on its control algorithm and not by its physics. This
offers a wide range of opportunities for shaping the control
in the desired manner. The disadvantage of low overcurrent
capability can be overcome by oversized power electronics or by
appropriate controls. In this paper we present a control scheme
based on the virtual synchronous machine with an approach for
overcurrent protection in positive- and negative-sequence. The
proposed approach can limit fault currents successfully and keeps
its grid-forming functionality during the fault. Furthermore pri-
oritization of active and reactive power in positive- and negative-
sequence during fault is possible and good transient stability can
be observed. The effectiveness is shown in MATLAB/Simulink
and laboratory test.
Frequency-Domain Stability Study of Converter-Based Power Systems
Grid integration studies of wind power plants usually focus on their impacts on voltage, frequency and transient stability according to traditional power system theories. In recent years, fast control of turbine converters and the resulting high-frequency interactions with the grid have drawn great attention. Impedance-based frequency-domain modeling and analysis is an effective method to study such interactions: variable-frequency impedance models are capable of capturing the fast dynamics of converters, thereby overcoming the limitations of fundamental-frequency models used in traditional power system studies. To apply the method, the grid is represented by a Thévenin equivalent circuit at the plant interconnect point, and the grid impedance forms a feedback loop with the plant output impedance. Accordingly, stability of the system can be determined by applying the Nyquist criterion to the ratio between the two impedances.
The impedance-based frequency-domain method reviewed above is effective for studying the stability at the grid interface of individual turbines or wind power plants. The method is also applicable to solar power and HVDC systems, including offshore wind power plants with HVDC transmission. However, the method does not consider coupling among multiple plants (or HVDC converters) through the grid network nor their simultaneous interactions with the grid. Such coupling may lead to new modes of interaction and instability that cannot be predicted by modeling one interface at a time. As the penetration of renewable generation continues to increase and the grid transits to converter-based power systems, it has become necessary to consider multiple renewable power plants at the same time and study their combined effects on overall grid system stability in the frequency range that is not covered by traditional system studies.
This paper describes the methodology and progress of an ongoing research project that addresses this need. The general framework adopted in the project is impedance-based frequency-domain modeling and analysis. Variable-frequency impedance models of turbines are also used to represent individual wind power plants. To account for the interconnection of multiple plants at different locations, the grid model is extended to an N-port Thévenin equivalence, based on which a system model resembling a multiple-input-multiple-output (MIMO) feedback loop is formulated. System stability is then determined by applying the generalized Nyquist criterion to the MIMO frequency-domain model. To identify possible modes of instability and the contribution of different wind power plants and network segments to each such mode, frequency-domain modal analysis is performed based on the eigenvalues and eigenvectors of the MIMO model. Based on the analyses, measures to improve system stability by modifying turbine control, optimizing network design, and dynamically adjusting system operation strategies are developed. An approximate representation of the ERCOT wind power system in the Panhandle area is used as an example to demonstrate the application of the developed methodology.
Optimal Sizing of Energy Storage to Enable Offshore Wind Farm Black Start Operation
The goal of transitioning toward 100% renewable energy sources (RES) poses serious challenges to the black start
service in electrical power systems. In the instance of a blackout, black start units must restore the power. Conventional black start sources are often taken out of operation to accommodate a larger share of RES and this jeopardises the resiliency of the grid. To replace conventional black start units, offshore wind farms (OWFs) can become future black start providers. However, a black start unit must meet stringent technical requirements, and
due to the variable power output of OWFs, it may be challenging to meet these requirements without external support.
Therefore, battery energy storage systems (BESSs) are a promising solution to support OWFs to satisfy black start requirements. In this paper, a probabilistic method is applied to determine the optimal BESS power output to support the OWF during black start operation. The considered black start technical requirements are taken from the British Transmission System Operator (TSO). The wind generation behavior is approximated with a Weibull distribution, and BESS power output is estimated considering a worst-case scenario logic. Finally, results are validated with a series of load-flow simulations to verify the time series generation of an OWF. The analysis conducted shows that the required power output from a BESS is dependent mainly on the size of the OWF and the availability requirement dictated by the TSO.
Obtaining flat initialization of complex Renewable Power Plant models
The transition from fossil fuels to renewable energy sources has triggered a tremendous transformation of the power generation sources away from synchronous to inverter-based resources (IBR). To safeguard the stable operation of the power systems with high shares of IBR, grid operators require electrical simulation models to reflect the power plant performance with very high fidelity under any operating condition. To accurately reflect real plant performance, electrical simulation models need to be either source code integrated, or a very close implementation of the algorithms executed on the plants. Utilities are interested in performing wide area studies using dynamic simulation models, which have historically been done using Root Mean Square (RMS) models. To achieve this, large network models comprising a variety of dynamic simulation models from different original equipment manufacturers (OEMs) are considered. If not well managed, such simulation models become very challenging to handle. Therefore, strict requirements on “flat-start” of models are commonplace. For RMS models this requires exact initialization of all state variables from the initial load-flow, and for electro-magnetic transient (EMT) models steady state shall be reached in few seconds. This paper focuses on the challenges to achieve flat-start touching not only on the dynamic model design, but also on the importance of departing from an accurate load-flow model. Recommendations to build simulation cases are provided as well as proposals for new functionalities in RMS and EMT simulation tools to overcome these obstacles.
Experience and EMT Study for Onsite Grid Forming Test Using BESS – RINGO Black Start Project
The idea of altering the energy mix in the French transmission network implies additional challenges related to the intermittent nature of renewable energy sources and their uneven geographical distribution. Integrating renewables in the power grid and making way for new uses of electricity requires innovative flexible solutions that might change the traditional way of tackling these challenges in the network. For that purpose, RTE (French TSO) is piloting the innovative RINGO project which deploys software-controlled battery energy storage system (BESS) in France used to optimize management of power flows in the transmission grid. Within the framework of the RINGO project, the black start capability of a grid-forming 12 MW/24 MWh BESS, installed in Vingeanne substation in eastern France, has been tested and confirmed among other functionalities and auxiliary services.
The RINGO black start test was performed in a network comprising the grid-forming BESS (manufactured by Nidec and owned by RTE) and a 10-MW wind park (owned by CNR and manufactured by Vestas) connected in a radial configuration through the network of the distribution system operator Enedis. In an undesirable event of a black out, the BESS should be capable of energizing the studied part of the grid by establishing the voltage and frequency reference, start the wind farm and resynchronize the islanded part of the grid with the main 63-kV network. This implies that the BESS is deployed in a bottom-up black-start strategy.
The on-site test consisted of mainly three sequences as highlighted in the circuit diagram of Fig. 1:
• TEST 1: Energization of HV passive elements that connects the BESS substation (in grid forming mode) to the CNR wind farm.
• TEST 2: Coupling of the 14.6-km cable to the CNR wind farm. Firstly, power supply from the BESS to the auxiliary systems of the wind farm. Then connection of the wind farm: five 2-MW wind turbine generators (WTGs) coupled sequentially one after another and charge the BESS during several hours.
• TEST 3: Coupling of the previously created separated network with the main RTE grid. This includes closing the AC breaker (BRK ZFONT) by means of synchro-phasor to the general RTE network followed by switching from UF mode to PQ mode of the BESS.
To the best of our knowledge, such black-start test (i.e. black-start operation of BESS and wind parks
owned by different parties via a network and resynchronization to the grid) is the world first
successful onsite test. Therefore, a thorough organization was a key point to avoid any on-site issue,
and coordination groups composed of the four main parties (Nidec/RTE/Enedis/CNR) were put in
place, with their principal responsibilities being preparation for the on-site test and performing EMT
dynamic simulation studies.
For the preparation of each of the above-listed tests, EMT dynamic studies were performed for the
following main reasons: to investigate the impact on the protection system, to ensure optimal and
stable performance between the BESS in grid forming mode and the 5 WTGs in grid following, and
to investigate any risks of equipment damage due to transient stress events. Therefore, several EMT
studies with manufacturer’s black-boxed model where conducted such as AC faults, control
instability, inrush currents during soft and hard energization of transformers in the system, phase
jump, frequency and voltage step changes during re-synchronization to the main grid, etc. An iterative
process was put in place in order to solve control interaction and interoperability issue.
The paper provides the applied methodology that led to the success of the on-site test and EMT
dynamic simulation studies conducted before and after the on-site black start experiments. In
addition, model validation against on-site measurement and lessons learned are provided. The present
paper aims to identify and outline the challenges related to the deployment of an independent gridforming
BESS in black start applications. The present work provides valuable insights for the success
of such future functionalities that involves multi-parties.
Performance Improvement of DFIG-based WECS with Non-Linear Load Penetration Using Additional DSTATCOM
The reactive power inadequacies associated with the Doubly-Fed Induction Generator-based
Wind Energy Conversion System (WECS), when connected to the AC grid with the proliferation
of non-linear load, results in poor power quality and deterioration in system performance. This
paper addresses this challenge using an additional distributed static compensator (DSTATCOM)
controlled by a novel adaptive algorithm. The proposed adaptive algorithm estimates the load
current's active and reactive weight components by considering changes associated with the
system. The proposed algorithm is simulated in MATLAB, and the results are validated using the
different case studies. The case studies verified the proposed algorithm's effectiveness in
improving the system's performance by mitigating PQ issues as per the IEEE 519-2014 and EN
50160 international standards.
The system under consideration:
A 4.5 MW DFIG-WECS is integrated into the AC grid in the presence of linear and non-linear
loads. The additional DSTATCOM with adaptive control has connected the shunt of the system
to improve its performance.
The additional reactive power is injected at the point of common coupling by the DSTATCOM to
improve the PQ of the system in the presence of linear (0.8-lag PF) and non-linear loads.
The proposed methodology can help enhance the performance of existing DFIG-WECS.
Harmonic Distortion Assessment at the Point of Connection for Compliance Verification
All grid interconnected plants are expected to be compliant to harmonic distortion limits specified by the relevant utilities. This paper discusses the process followed for verifying harmonic compliance through a ‘one-shot’ assessment as per CIGRE TB468. The main aim is to showcase the importance of quality of measured data and the inherent difficulties in the major assumptions of ‘One-shot’ assessment which led to project extension with potential cost over-runs. The discussion presented in this paper focuses on the non-compliant 5th harmonic order.
The data is collected over a duration of six hours at Point of Connection (PoC). The grid impedance data for three different operating scenarios, during the measurement period is provided by the utility.
The plant switching operation is in sequence as suggested in CIGRE TB468. When the wind farm is disconnected, the network background harmonics were measured. Based on the available measured data, the windfarm contribution is estimated at each harmonic order by considering the summation law as per IEC 61000-3-6.The detailed plant model is implemented in a simulation tool and the distortion levels of the plant with the measurement data and simulated data were analyzed to understand the gap in the results.
Based on the measured data, it is observed that the 5th harmonic contribution was more than the allocated individual harmonic limit. The simulated value based on the Wind Turbine Generator (WTG) harmonic model is within the harmonic distortion limits. As the wind farm contribution was observed to be more than the simulated values, it would jeopardize the operation of the wind farm if the required filters were not added within a stipulated time. Also, since the 5th harmonic distortion based on the simulation was less than the distortion extracted based on the measured data, justification on the correctness of the WTG harmonic model or the lack of it was to be provided.
Major Conclusion Drawn
One of the major assumptions of ‘one-shot’ assessment, the grid impedance, should not vary during the measurement period. Computation of wind farm contribution from considerably varying grid scenarios will not give satisfactory results.The harmonic model’s correctness is showcased through a simplified network theory approach. The 5th harmonic impedance of the plant was considerably higher than the grid impedance. By this even though there were issues during the measurement due to varying grid impedance, the data from two different timeframes as suggested in the CIGRE TB468 is not required to demonstrate the correctness of the WTG harmonic model. The current measured at PoC directly yields the wind farm contribution due to high impedance offered by the plant at 5th harmonic order. This resulted in avoiding proposal of filters and the correctness of the harmonic model was proven.
California’s Low Carbon Grid – An Update
California has an announced goal of a “carbon free” economy by 2045 supported by a dramatically expanded electric grid powered by renewable energy. It is roughly half way to that goal for the electric grid and meaningful electrification of the transportation and building sectors has begun to make a difference in electricity demand. This spring, the State saw several hours wherein net exports of carbon free renewable energy occurred on the seventy percent of the grid operated by the California Independent System Operator (CAISO). At the current rate of investment, within four years, the State’s electric grid, including the non-CAISO portion, will be completely carbon free for roughly 15% of the hours in the year.
Nevertheless, the journey still has a long way to go and significant hurdles remain. However the State is blessed with world class renewable resources, a strong economy and the political will to become “carbon free.” It could be progressing faster and could achieve the “100% goal” well in advance of 2045 with flat or even slightly declining retail electricity rates. This report will assess the technical, economic and political lessons learned to date and discuss how the State could accelerate achievement of an economy wide net zero carbon future.
Quicker Connection of Wind: A Case of Dynamic Line Rating to Connect Wind and Defer Grid Investment