Category: Wind Farms

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Stability Wind Farms

Grid forming converter in CIGRE 928 benchmark system

For both grid-following (GFL) and grid-forming (GFM) control, the same hardware structure is used, specifically an IGBT-based two-level voltage-sourced converter (VSC) with a 12 MW rating. This converter is connected to the plant via a 0.69/66 kV transformer, with the grid-side reactance of the LCL filter acting as the step-up transformer.

The converter control is configured to operate in grid-forming mode, using vector control in the dq-reference frame. The control system includes the following components: power calculation, power regulation, voltage regulation, current regulation, and pulse-width modulation (PWM). Additionally, an active damping control utilizing feedback from the series reactor current is considered.

Subsystems of grid-forming converter

Anti-aliasing filtering and sampling: The anti-aliasing filter is a second-order Butterworth filter, with a cut-off frequency set to half the sampling frequency. Sampling delays are approximated using a third-order Padé formula.

Park transformation: Grid-forming control is implemented in a reference frame synchronized to an arbitrary input, provided in per unit (p.u.), where the reference frequency (f*) is set to 1. In practical applications, the phase can be provided by a low-bandwidth phase-locked loop (PLL) that slowly tracks the system frequency.

Power calculation: Instantaneous active and reactive power are calculated based on voltage and current measurements taken at the LCL filter's output.

Power regulation: In GFM mode, power synchronization control generates the reference for the voltage angle, while reactive power control sets the reference for the voltage amplitude. Active and reactive power droop characteristics are combined with a simple low-pass filter.

Voltage regulation: The voltage is controlled through a proportional-integral (PI) regulator, ensuring zero steady-state error. Active damping is also incorporated to enhance noise rejection and system stability.

Current regulation: A proportional (P) controller manages the current, as the integral (I) component is unnecessary; the voltage regulator already handles tracking the voltage reference. Adding complexity to the current regulator may adversely affect converter stability.

PWM: The modulation block calculates the switching functions and sends pulse patterns to the converter's gate drivers. The PWM delay is also accounted for in this block.

Note: The proposed GFM control does not include DC voltage regulation. It is assumed that the GFM converter maintains a stable DC link voltage, such as from a battery energy storage system (BESS), so DC voltage control is unnecessary.

Benchmark grid forming converter control block diagram.

Grid-forming converter parameters

The electrical circuit for GFM unit is assumed to be the same as for GFL converter. The control structure is visualized in figure below.

List of parameters in the grid forming converter control system.

References

[1] Ł. Kocewiak, Ch. Buchhagen, R. Blasco-Gimenez, J. B. Kwon, M. Larsson, Y. Sun, X. Wang et al., “Multi-frequency stability of converter-based modern power systems,” Technical Brochure 928, Page(s) 1-147, CIGRE, March 2024.

Categories
Measurements Stability Wind Farms

Grid following converter in CIGRE 928 benchmark system

The primary objective of the benchmark model is to serve as a reference for studying interactions between converters and the grid. It provides a foundation for evaluating small-signal stability analysis methods and instability mitigation techniques. The model includes aggregated grid following (GFL) converters, interconnected via a medium-voltage (MV) cable network.

The model is inspired by a real-life AC cable-connected offshore wind power plant (PP). The GFL converter model is a part of benchmark system, introduced by CIGRE WG C4.49 and published in the CIGRE 928 technical brochure.

The model is developed in the dq-reference frame to simplify modelling and avoid coupling in the control system. The converter is based on a standard insulated-gate bipolar transistor (IGBT)-based two-level voltage source converter (VSC), rated at 12 MW, as typically seen in modern offshore wind turbines (WTs). To simplify the analysis, the mechanical system and its controllers are not included.

Converter control is designed as GFL unit, employing vector control in the dq-reference frame and a synchronous reference frame (SRF) phase-locked loop (PLL) for grid synchronization. The dq currents regulate the DC link voltage and either the voltage or reactive power at the converter terminals. Active damping control, using capacitor current feedback, is also incorporated to enhance stability.

The converter system is linked to the grid through a 0.69/66 kV transformer, where the low-voltage reactance corresponds to the grid-side reactance of the LCL output filter.

Benchmark grid following converter control block diagram.

Subsystems of grid-following converter

The converter control system has been tuned to mimic the behavior of a generic converter model and has not yet been customized for the specific grid under study. As a result, various instabilities may arise in both the base case and during disturbance scenarios.

Anti-aliasing Filter and Sampling: the anti-aliasing filter is implemented using a second-order Butterworth filter, with the cutoff frequency set at half of the sampling frequency and the sampling delay is approximated using a third-order Padé approximation.

Park Transformation: the GFL control is implemented in a synchronous reference frame (SRF), with the phase determined by a phase-locked loop (PLL) that tracks the system frequency.

Power Calculation: instantaneous active and reactive power are calculated from voltage and current measurements taken at the output of the LCL filter.

DC Voltage Control: the DC voltage regulation is managed through a proportional-integral (PI) controller.

AC Voltage Control: the AC voltage regulation is implemented using a simple droop control method.

Reactive Power Control: the reactive power regulation is handled by a proportional-integral (PI) controller.

Phase-Locked Loop (PLL): the grid synchronization system uses a PLL, where the voltage’s q-component is filtered by a first-order low-pass filter and regulated by a PI controller, which provides the system’s angular frequency, which is then integrated to determine the phase for the Park transformation.

Current Control: the converter reactor current regulation is achieved using PI controllers with decoupling in the SRF, and active damping is incorporated to attenuate the capacitor current in the LCL filter, and an output voltage feed-forward component is added to the voltage reference.

Pulse Width Modulation (PWM): the modulation block computes the switching functions and provides the pulse patterns to the converter gate drivers and the PWM delay is also included.

Grid following converter parameters

List of parameters in the grid following converter electrical circuit.
List of parameters in the grid following converter control system.

References

[1] Ł. Kocewiak, R. Blasco-Gimenez, C. Buchhagen, J. B. Kwon, M. Larsson, Y. Sun, X. Wang, “Practical Aspects of Small-signal Stability Analysis and Instability Mitigation,” in Proc. The 21st Wind & Solar Integration Workshop, 12-14 October 2022, The Hauge, The Netherlands.
[2] Ł. Kocewiak, R. Blasco‐Giménez, C. Buchhagen, J. B. Kwon, M. Larsson, A. Schwanka Trevisan, Y. Sun, X. Wang, “Instability Mitigation Methods in Modern Converter-based Power Systems,” in Proc. The 20th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, 29-30 September 2021.
[3] Ł. Kocewiak, R. Blasco‐Giménez, C. Buchhagen, J. B. Kwon, Y. Sun, A. Schwanka Trevisan, M. Larsson, X. Wang, “Overview, Status and Outline of Stability Analysis in Converter‐based Power Systems,” in Proc. The 19th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, 11-12 November 2020.

Categories
Methodology Stability Wind Farms

CIGRE C4.49: Multi-frequency stability of converter-based modern power systems

Background

Nowadays, it is seen that the rapid transformation of power systems from conventional with high natural damping, short-circuit current and natural inertia to power-electronic-based with limited damping, fault infeed and inertia may trigger unstable operation, if not investigated carefully. Moreover, the electrical infrastructure is becoming more complex due to the introduction of long high voltage alternating current (HVAC) cables, high voltage direct current (HVDC) connections, widespread penetration of renewable energy sources, e.g. photovoltaic (PV) plants, wind power plants (PPs), and offshore electrical network development. This power system transformation creates challenges such as operational coordination of grid-connected converters and small-signal stability assurance both in the sub-synchronous and harmonic (super-synchronous) frequency regions.

Motivation

The increased use of power electronic converters in modern electrical systems creates challenges w.r.t. power system stability assurance but also simultaneously provides wide range of power system performance and stability enhancement solutions. Better understanding about the application of various instability mitigation methods, including impact on power system performance, use depending on instability root cause, implementation methodology, is needed. Power system operators, operators of renewable PPs, transmission solution developers, renewable generation developers, academic units and original equipment manufacturers expect coordinated effort to understand when and how to apply specific mitigation measures.

Therefore, the overview, status and outline of instability mitigation methods in converter-based modern power systems is needed. Thus, the CIGRE C4.49 working group entitled “Multi-frequency stability of
converter-based modern power systems” was established. The instability phenomena, instability root cause and suggests optimal mitigation measures are investigated within the working group. Moreover, guidelines regarding the general approach how to choose optimal instability mitigation method will be suggested in the technical brochure.

Scope

  1. Review of existing literature regarding subject related stability issues including state-of-the-art converter stability aspects.
  2. Definition of stability phenomenon to be covered within the technical brochure.
    • Stability effects above the fundamental frequency, i.e. harmonic stability.
    • Small-signal stability below the fundamental frequency, i.e. sub-synchronous stability.
    • Clarification of definitions to avoid misinterpretation with steady-state harmonics and classical harmonic propagation analysis.
    • Symptoms and root causes of sub-synchronous and harmonic stability phenomenon.
    • Examples of sub-synchronous and harmonic stability phenomena observed and their impact on wider power systems.
  3. The impact of grid-connected converter controllers on sub-synchronous and harmonic stability phenomenon.
    • Classification of typical controllers used in modern converters.
    • Evaluation of various control loops and techniques and their impact on stability, e.g. voltage control, current control, phase-locked loop.
    • Frequency range of interest and controller interactions/couplings.
  4. Overview of linear modelling and analysis methods to perform small-signal stability studies, e.g.
    • Classical control theory approach of linear time-invariant systems, i.e. compensator and plant interactions, and possible general extension to linear time varying systems including e.g. linear time-varying periodic systems.
    • Impedance-based stability criterion.
    • Advantages and disadvantages of single-input single-output and multiple-input multiple-output representation.
    • Relevant stability evaluation methods, e.g. eigenvalue analysis, Nyquist criterion.
  5. Other analysis techniques.
    • Time-domain numerical simulations of linear and non-linear systems.
    • Frequency and sequence coupling investigation.
    • Stability of non-linear dissipative dynamic systems including e.g. limit cycle and bifurcation theory investigation.
  6. Description of mitigation methods to overcome sub-synchronous and harmonic stability issues, e. g.
    • Clear evaluation criteria and minimal requirements regarding the stability indices, e. g. stability margins, damping.
    • Recommendations to address plant resonance profile at early stage during the grid-connected converter controller design.
    • Converter coordination guidelines in modern power systems to avoid potential instability, e. g. passivity requirements.
    • Mitigation measures incorporated in the grid-connected converter control (e.g. active damping) or within the power system electrical infrastructure (e.g. passive damping), also at later stage of project development or during operation.
  7. Guidelines on general approach to such studies and the availability as well as choice of tools. Identification of limitations with the available analysis tools and suggestion of possible areas for development.

References

Ł. Kocewiak, R. Blasco‐Giménez, C. Buchhagen, J. B. Kwon, M. Larsson, A. Schwanka Trevisan, Y. Sun, X. Wang, “Instability Mitigation Methods in Modern Converter-based Power Systems,” in Proc. The 20th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, 29-30 September 2021.

Ł. Kocewiak, R. Blasco‐Giménez, C. Buchhagen, J. B. Kwon, Y. Sun, A. Schwanka Trevisan, M. Larsson, X. Wang, “Overview, Status and Outline of Stability Analysis in Converter‐based Power Systems,” in Proc. The 19th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, 11-12 November 2020.

Categories
Measurements Stability Wind Farms

Instability mitigation methods in converter-based power systems

As the number of power-electronic-based power generation units (PGUs) and the power system infrastructure complexity are rapidly increasing, there is a need for carefully investing the system stability to assure robust and reliable operation. However, no commonly agreed methods are available for the analysis of potential sub-synchronous and harmonic (or super-synchronous) stability problems.

Hence, there is a need to provide a general overview of the topic, highlighting the root-cause of sub-synchronous and harmonic stability issues of grid-connected power electronic devices supported by state-of-the-art literature survey as well as industrial experience.

Instability in modern power systems

Several instability incidents related to control system in PGUs have been seen until know. Some of them are briefly summarized below.

1. Oscillations in PV systems with harmonic resonances [1]

Measurements of unstable PV PPs are documented in [1]. The paper takes a model-based approach to predict instability within the harmonic frequency range and improve robustness in a PV PP. The measurements showing instability are obtained from a commercially operating PV PP and are presented in Figure 1.

Figure 1 Measurement of unstable behavior of a PV PP with oscillations the point of connection at 549 Hz as shown in [1].

Furthermore, the studies clearly show that increase of power-electronic-based PGUs in electrical grids characterized by weak conditions increase the need for power converter controllers able to adapt to wide range of grid conditions. The need to change from classical current-controlled VSCs to a more adaptive approach is acknowledged.

2. Oscillations in wind PP with HVDC and harmonic resonances [2]

The paper [2] shows measurements and analysis of one instability incident that happened in German North Sea at an offshore wind PP, connected to onshore by a VSC HVDC system. After a switching operation of a cable an oscillatory behavior could be seen on the voltage waveform as shown in Figure 2. The instability was caused by a control interaction, most likely due to the WTs being sensitive to a grid resonance.

Figure 2 Measured voltage during the instability from [2].

The time domain and impedance-based analysis showed that the WT controllers in use had stability problems with a very poorly damped resonance at the frequency around the 9th harmonic. Due to the switching operation the resonance frequency drops from 600 Hz to around 450 Hz which caused the instability.

3. Oscillations in systems with Type 3 WTs and series compensation [3]

The paper [3] describes a sub-synchronous resonance observed in a wind PP in North China. The measured oscillations were around 6-8 Hz (see Figure 3) and driven by interaction between double-fed induction generators and series-compensated transmission lines.

Figure 3 Phase current at the 220-kV side reflecting sub-synchronous oscillations as reported in [3].

The system vulnerable to sub-synchronous oscillations was investigated using time-domain simulations and supported by eigenvalue-based analysis to understand the impact of grid parameters on the instability.

4. Oscillations in Type 4 WTs in weak grids [4]

The publication [4] presents the need to perform eigenvalue-based stability analysis to investigate sub-synchronous oscillations in an offshore wind PP. It is reported that the instability happened during PP contingency operation due to one export HVAC cable outage. Excessive power oscillations were measured as presented in Figure 4.

Figure 4 Measured reactive power oscillations due to sub-synchronous instability reported in [4].

The paper shows that the effective short-circuit ratio (SCR) at the MV terminal of the WT transformer dropped due to the contingency to 1.2-1.5. Such extreme weak grid conditions triggered WT controller instability.

Instability mitigation methods

Following the investigations and discussions of previous sections on instabilities and their root-causes, this section outlines recommended practices for risk mitigation. The following methods have been identified [5]:

  1. Converter parametrization
  2. Power grid operational measures
  3. Passive filter placement
  4. Active damper
  5. Converter setpoint adjustment
Figure 5 Instability mitigation methods in modern converter-based power systems.

References

[1] F. Ackermann et al., “Stability prediction and stability enhancement for large-scale PV Power plants,” in Proc. 7th International Symposium on Power Electronics for Distributed Generation Systems, 2016.
[2] C. Buchhagen et al, “Harmonic Stability – Practical Experience of a TSO,” in Proc. The 15th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, 2016.
[3] L. Wang et al., “Investigation of SSR in Practical DFIG-Based Wind Farms Connected to a Series-Compensated Power System,” IEEE Transactions on Power Systems, 2015.
[4] L. Shuai et al, “Eigenvalue-based Stability Analysis of Sub-synchronous Oscillation in an Offshore Wind Power Plant,” in Proc. The 17th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, 2018.
[5] Ł. Kocewiak, R. Blasco‐Giménez, C. Buchhagen, J. B. Kwon, Y. Sun, A. Schwanka Trevisan, M. Larsson, X. Wang, “Overview, Status and Outline of Stability Analysis in Converter‐based Power Systems,” in Proc. The 19th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, 11-12 November 2020.

Categories
Harmonics Wind Farms

Embedded Power Electronic Solutions in Offshore Wind Power Plants

There is a big potential of wide application of power electronic-based embedded converter systems (e.g. static synchronous compensator, battery energy storage system, active power filter). An optimized integration of aforementioned features can provide extensive functionality covering the following areas:

  1. Grid connection of renewable energy sources
    Wind power plants (especially offshore) are nowadays connected to the grid at remote locations, far from the main consumption centers. Active and reactive power control in battery energy storage systems assures robust operation and grid stability dynamically contributing to voltage and frequency control. It supports simultaneously the grid as well as the wind power plant, partially decoupling the dynamics of both.
  2. Wind fluctuation balancing and Load support
    Wind power is stochastic in nature. Active power from wind turbines, when set to maximum power point tracking, can therefore only be predicted subject to the accuracy of the forecasted wind. Likewise, there is continuous variation of the load demand. Any power system unbalance resulting from the variation of generation and/or load affects the system operation and leads as frequency variations. Battery energy storage systems can store energy from wind when there is excess generation and low demand and release the stored energy during periods of low generation and high demand. Moreover, battery energy storage systems can smoothen out fast ramping up/down of the wind power generation due to sudden fluctuations in the wind velocity. Thus, battery energy storage systems enable to optimize the mission profile and enable more predictable and reliable operation of the whole power system, including the wind power plant.
  3. Reserve capacity
    Battery energy storage systems can reduce the number of on-line generators in the system. It can provide the grid with the reserve capacity that is normally subject to limitations on power plant utilization. Battery energy storage systems serves as a dynamic power source. It can continuously support the grid with reactive power, and in the event of loss of generation, battery energy storage systems can supply active power into the grid until the grid is reconfigured (limited by the energy capacity and state of charge of the battery energy storage system).
  4. Ancillary services
    With higher and higher penetration of renewable energy sources, ancillary services to support the power system are becoming increasingly important, e.g. fast frequency response, virtual/synthetic inertia response, power oscillation damping. Frequency regulation service is often provided by generators having the spinning reserve when they are dispatched below their maximum output level. Battery energy storage system installation providing continuous grid support, such as for voltage control, supplies short-time real power at the lowest cost, thus making it the most attractive supplier.
  5. Emergency power
    In the event of a blackout, wind power plant internal/local loads and power system sensitive loads such as hospitals or power distribution areas, can be fed by a battery energy storage installation until emergency generators are started.
  6. Black start
    As the penetration of renewable energy sources increases, and as old thermal generation plants are phased out, there is an obvious need for new black start equipment in the power system grid. Battery energy storage systems can support generators that lack inherent black start capability. Battery energy storage systems can supply the power needed for safely controlled black starts. It keeps the frequency within range and controls the voltage throughout start-up.
  7. Active filtering
    Many modern industrial processes are, by nature, detrimental to power quality. At the same time, grid code requirements are becoming more stringent to address sensitive power electronic-based plants/loads. The modular multilevel converters have a high effective control bandwidth. This property can be used for active filtering of harmonics that are already present in the grid to compensate non-linear plants and improve the quality of power, as the power electronic interface in battery energy storage systems can inject harmonic currents into the grid with proper phase and amplitude to counteract the harmonic voltages. Furthermore, harmonic propagating through the system can be utilized to charge a battery energy storage system and consequently convert harmonics into the fundamental frequency.