Tag: active filtering

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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
Stability

Converter-based benchmark system from CIGRE 928 technical brochure

Introduction

A small-scale model of an actual AC cable-connected offshore wind power plant (PP) is proposed to compare different stability analysis methods and strategies for mitigating instability in converter-based power systems. This post provides a more detailed description of the benchmark system proposed by CIGRE WG C4.49. The system features power generation units (PGUs) connected to an AC grid via an extensive offshore 66-kV array cable system, offshore step-up transformers, both offshore and onshore HVAC transmission cables, and an onshore step-up transformer.

The primary goal of this benchmark power system is to offer a reference model where interactions between converters, as well as between converters and the grid, can be studied. It is designed to support small-scale, easy-to-model system studies and serve as a foundation for evaluating various instability mitigation methods introduced in CIGRE 928 technical brochure.

The benchmark system includes either aggregated converters or a collection of individual converters linked by a complex MV cable network. The model is formulated in the dq-reference frame to facilitate stability analysis using both impedance and modal approaches, allowing for a direct comparison of their results.

Study cases

Case 1: Aggregated grid following converters connected to a Thevenin equivalent

The transmission system is modeled as a long HVAC cable connected to a simplified Thevenin grid equivalent. The 420 MW power plant is represented by two aggregated power generation units (PGUs) of 180 MW and 240 MW. In this scenario the converters operate in grid-following mode.

The parameters for this aggregated system were derived from a detailed system representation, ensuring that the dynamics of the converters and their interaction with the grid remain consistent between the two. While the original benchmark configuration falls well within the capabilities of the modal and impedance analysis techniques discussed in CIGRE technical brochure 928, a simplified reduced-order model was developed to make the benchmark more accessible for researchers exploring new stability analysis methods, as well as to present results more concisely and clearly.

In this reduced version, each group of 5 PGUs, totaling 20 or 15 PGUs per group, is simplified into an equivalent cable segment and a single PGU. While the reduced model aligns well with the detailed model in the low-frequency range, significant deviations appear at higher frequencies, particularly beyond 500 Hz, as the Nyquist frequency of 1475 Hz, associated with the PGU control system, is approached. Consequently, the reduced-order model is not recommended for studying high-frequency resonance phenomena, but it is suitable for analyzing control interactions in the lower frequency range.

Case 2: Aggregated grid forming converters connected to a Thevenin equivalent

The transmission system is modeled as a long HVAC cable connected to a simplified Thévenin grid equivalent. The 420 MW power plant is represented by two aggregated power generation units of 180 MW and 240 MW. In this scenario, the converters operate in grid-forming control mode.

Download benchmark system

In Matlab

Benchmark System – C4.49 in MatlabDownload

In PSCAD

Benchmark System – C4.49 in PSCADDownload

In PowerFactory

To be ready soon...

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.
[2] 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.

Categories
Measurements

PhD Course on  Harmonics in Power Electronics and Power Systems

Description:
This course provides a broad overview of power system harmonic problems, methods of analyzing, measuring and effectively mitigating them. Several extended simulation and data processing tools, among others DIgSILENT PowerFactory, Matlab/Simulink or LabVIEW are used to assess and study the harmonic distortion at different points of power networks.
The results of analytical investigation and simulations are validated against measurements applying sophisticated data processing techniques. Furthermore, deep understanding of hardware considerations regarding har- monic measurements in harsh industrial environment is given, using specialized equipment, for in- stance GPS-synchronized measuring instruments.

The course covers the following topics:

  • Power Quality definitions. Generation mechanism of power system harmonics. Harmonic indices.
  • Voltage vs. current distortion as well as parallel vs. series resonance in modern power systems. Point of Common Coupling (PCC).
  • Sources and effects of harmonic distortion.
  • Harmonic measuring instruments and measuring procedures in LV, MV and HV networks.
  • Mathematical tools and theories for analyzing distorted waveforms. Signal processing and uncertainty analysis.
  • Modelling of classical power system components. Harmonic analysis.
  • Modelling of grid-connected converters for harmonic analysis purposes and their application in modern power systems including e.g. offshore wind power plants.
  • Harmonic load-flow, frequency scan and time domain simulations. Linear and nonlinear analysis techniques.
  • Steady-state harmonics vs. harmonic stability. Small-signal representation, sequence and frequency coupling.
  • Software tools for harmonic analysis.
  • Precautionary (preventive) and corrective (remedial) harmonic mitigation techniques. Passive and active line filters. Filter design.

Organizer: Professor Claus Leth Bak
Lecturers: Christian Frank Flytkjær from Energinet and Łukasz H. Kocewiak from Ørsted

Harmonic current of 6-pulse rectifier supplying a resistive load
Figure 1 Harmonic current of 6-pulse rectifier supplying a resistive load.
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.

Categories
Harmonics Wind Farms

Active Filtering Functionality in Wind Turbines Connected to Wind Power Plant Offhore Network

Active Filtering (AF) functionality can be understood very broadly. A number of technical solutions could be introduced in grid-tied converters functionality depending on the expected outcome.

1           Local resonance damping

The Wind Turbine (WT) is connected to an offshore array cable system within Wind Power Plant (WPP) electrical infrastructure. The aim of AF is to mitigate or damp internal resonances within the WT Low-Voltage (LV) circuit. It could be mainly resonances caused by shunt-connected Pulse Width Modulation (PWM) filters in connection to series inductance. That would allow other converters (e.g. WTs) in the same power system not to be affected by undamped resonances. In this case the WT is acting as virtual damping circuit.

2           Local harmonic current compensation

It can be seen that WTs inject harmonics into the system to which are connected. The amount of injected current is of course dependent on the system impedance. In that case even small voltage distortion imposed by the Voltage Source Converter (VSC) can cause unacceptable excessive distortion level caused by a resonance circuit within the offshore electrical infrastructure. Furthermore due to already existing harmonic voltage distortion in the system where the WT is connected harmonic current can also flow into the WT internal circuit from the network. The task of AF would be to control the current flow between the WT converter and the external network and e.g. reduce it to minimum.

3           Local voltage distortion mitigation

As mentioned above even small voltage distortion introduced by WT’s grid-tied converter can lead to excessive harmonic current flow in the resonant network to which the WT is connected. The goal of AF would be to improve and minimise as much as possible the voltage distortion level at the converter terminals caused by the power electronics non-linarites as well as limited harmonic rejection capability of the controller. The equivalent voltage source of the VSC would tend to be as less distorted as possible.

4           Unity amplification factor

In many cases when a new plant (e.g. STATCOM) is connected to already existing power systems it is strongly desired that the new plant will not change the harmonic profile after the connection of the existing before system. This can be obtained fulfilling two objectives (i) no harmonic contribution/injection, (ii) no changes in the system impedance at the Point of Connection (POC). The second objective is related to unity amplification factor at POC which can be achieved by AF so the newly connected impedance in not visible to the existing network. Such requirement can be put also to WTs.

5           Resonance damping at the remote bus

Having a number of WTs in WPPs allows also looking on AF in more global way. WTs could be programmed to mitigate prominent resonances in the WPP offshore network, e.g. Offshore Grid Entry Point (OGEP) in the UK or Point of Common Coupling (PCC) in Germany. This would optimize the overall system damping leading to robustness increase of grid-tied converters as well as lower harmonic voltage distortion level. The WTs would operate in groups or clusters and could be understood from electrical infrastructure perspective as an equivalent damped filter.

6           Harmonic compensation at the remote bus

Nowadays it is more and more challenging to meet demanding grid-code requirements, especially in resonant offshore networks with low damping. Therefore the AF functionality in WTs leading to keep the harmonic voltage distortion level at e.g. PCC as specified in the grid-code is critical to assure continuous WPP active power production. WTs could act in groups or individually to achieve that objective, e.g. the 7th harmonic equal or lower than 0.5% at OGEP in the UK or harmonic current injection from all WTs cannot cause higher voltage distortion incremental higher than 0.1% in Germany.

7           Converter controller passivity

In modern WPPs the industry is facing more and more diversification in utilized power electronic devices and their controls. One of examples would be HVDC-connected WPP with multi-vendor WT configuration or HVAC-connected system employed simultaneously with WTs and STATCOMs. That creates even more challenges considering grid-tied converter interaction issues such as stability. Furthermore more complex control structures imposed by AF functionality application requires more focus at the early stage of the WPP system design. Therefore one of potential requirements to the suppliers would be to assure converter passivity within specified frequency range where e.g. where AF is applied. This would secure the robustness of the overall system operation.