Instability Mitigation Measures in Modern Converter-Based Power Systems

Converter setpoint adjustment

The power generation unit frequency response changes depending on an operating point, e.g. active power setpoint, voltage setpoint. That is important for stability analysis within lower frequency range where the influence of phase-locked loop and controllers with lower dynamics, e.g. voltage control, is visible. Combination of specific setpoints and grid impedances, especially in case of low short-circuit ratio, can trigger unwanted instability. One can perform mapping of different operational points and schedule only the one not leading to any oscillatory behaviour. This mitigation measure can be applied within the design phase and during power plant operation. However, it is rather considered as corrective mitigation measure when any hardware changes are not available.

Converter control adjustment

Controller parameter tuning, digital delay compensation and pulse-width modulation filter resonance damping (or shifting) methods can be applied to improve the inherent stable operating region, all of which are digital modification that can be rapidly applied mostly without modification of hardware. Controller can be adapted (e.g. loop gain reduction, impedance shaping) to local grid conditions during all phases of the asset lifecycle, i.e. design, commissioning, operation, thus can be considered as preventive as well as corrective instability mitigation measure. However, much more flexibility in controller parameters adjustment as well as control structure modification is possible during the design phase. Adaptive control where the parameters are changed on the fly is also an attractive option, however complex to implement.

Internal and external operational scenario adjustment

Control interactions can often be avoided by changing the electrical topology. A change of the power plant internal electrical infrastructure will affect the short-circuit power and / or shift the resonance points in the system. Avoiding specific contingency operation cases can maintain the short-circuit ratio at a satisfactory high level. Moreover, if the outcome of the analysis demonstrates that a hazardous resonance point can be avoided in a specific power plant electrical infrastructure configuration, then this grid topology can be considered as an intermediate or final mitigation measure. The operational measures can only be chosen according to the individual situation and will always be very specific. Adjustment of operational scenarios within the power plant is rather considered as corrective mitigation measure, however, can be applied also during the design phase as a preventive measure.

All electrical components and subsystems are interconnected within the entire power system and contribute to a certain degree to assure stable operation. Thus, operational scenarios may not be changed only within the power plant internal electrical infrastructure but also within the power system to which it is connected. That also includes neighbouring power plants. In many cases the transmission system operator has much more flexibility to define operational philosophy focused on maximizing fault infeed and avoiding unwanted resonances, thus consequently improve the entire system robustness. Adjustment of operational scenarios within the grid is rather considered as corrective mitigation measure to address grid expansion or connection of new power plants. One of possible mitigation measures applied within the power system is inter-tripping to avoid contingency scenarios leading to converter instability.

Additional passive filter

Contrary to described earlier operational measures and control adjustment, a high-voltage passive filter can be added to alter the resonance frequency of the power grid at the point of interest. There is a large variety of passive filters being able to improve damping within harmonic frequency range. Installation of a passive filter is considered as preventive mitigation measure. It is much easier to add the passive filter during the design phase. Moreover, passive filters provide extra damping which together with stability enhancement can also reduce transient overvoltage and harmonics within specified frequency range.

Additional active power electronic equipment

The control interaction is a phenomenon caused by the active participation of power generation unit converters, which can occur with e.g. purely damped plant, weak grid, resonant network. This is because the converter control loops play an important role in defining the overall impedance of power plant. The impedance of each power plant can be reshaped by the addition of supplementary shunt-connected converters, such as a static synchronous compensators to provide extra resistive damping at the hazardous resonance frequency via a damping controller. As this type of mitigation requires additional equipment it is considered as preventive mitigation measure. However, static synchronous compensator are more often installed with renewable-based power plants and additional damping can be provided on already existing assets, thus in that case active damper one can consider also as corrective instability mitigation technique. Moreover, active damper control scheme can be adjusted on already operating assets to address potential power grid topology changes.

Power system voltage stiffness increase

In conventional power systems characterized by extensive use of synchronous machines the short-circuit ratio is typically calculated to measure the strength of the grid to which converter-based power plant is connected. Any attempt to improve short-circuit ratio by increasing the fault level at the power plant point of common coupling can improve the stability. It can be achieved by (i) avoiding severe contingency operation, (ii) improving transmission system capabilities by adding e.g. extra lines, (iii) installing synchronous condensers characterized by short-circuit current contribution. Increasing the fault level can improve the system robustness within the low-frequency range as it reflects the system impedance at the fundamental frequency. This mitigation measure is preventive because can be much easier initiated during the planning and design phases. Due to the complexity in power system modifications to improve short-circuit ratio, it is difficult to think about it as a corrective measure, unless one could easy avoid severe contingency operation leading to low short-circuit ratio. For modern power systems characterized by extensive use of power electronic equipment the voltage stiffness increase can be achieved by dedicated control loops if grid-following converters or use of grid-forming converters to improve the stability.


PhD Course on  Harmonics in Power Electronics and Power Systems

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.
Methodology Stability Wind Farms

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


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.


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.


  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.


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.

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.


[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.

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.