Stability – Łukasz Kocewiak

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

Review of existing literature regarding subject related stability issues including state-of-the-art converter stability aspects.

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.

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.

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.

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.

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.

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 20^th 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 19^th 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.

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 9^th 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]:

Converter parametrization
Power grid operational measures
Passive filter placement
Active damper
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. 7^th 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 15^th 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 17^th 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 19^th 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.