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.

Harmonics Wind Farms

Active filtering vs. passive filtering

Let us think about various sources of harmonic problems in large wind power plants (WPPs) and different ways of optimized harmonic mitigation methods. We discussed previously about harmonic problems such as sources of harmonic emission and amplification as well as harmonic stability which are commonly seen in large WPPs. Fortunately a significant variety of modern preventive and remedial harmonic mitigation methods in terms of passive and active filtering are possible.

Passive filtering

Three-phase harmonic filters utilized in the WPPs nowadays are shunt elements. They are intended to decrease the voltage distortions at the point of interest. From the grid code requirements point of view, a WPP voltage distortion is evaluated at the point of common coupling (PCC).
Nonlinear elements such as the power electronic converters, transformers, etc. generate harmonic currents or harmonic voltages inside the WPP as well as in the external network. The resultant harmonic current flows throughout system impedance. Passive harmonic filters reduce distortion by providing low impedance to the harmonic currents.
Typical shunt harmonic filters are presented in Fig. 1. Such filtering depending on the harmonic emission source can be installed either in the wind turbine circuit or somewhere at the WPP level (e.g. onshore substation, offshore substation, etc.).


  • Known state-of-the-art technology,
  • Relatively cheap solution,
  • High reliability due to simplicity in the build,
  • Effective if designed correctly.


  • Significant size especially for lower frequencies (for large WPPs the tuned frequencies are getting lower),
  • Additional losses,
  • Can cause some over-voltages during switching operations (e.g. energization),
  • Tuned only for specific frequencies (i.e. limited bandwidth),
  • Affected by uncertainties during the WPP design phase,
  • Cannot be easily re-tuned in the case of changing grid conditions during the operation of the WPP,
  • Uncertainties in terms of sizing due to lack of information from wind turbine manufacturers and TSOs during the design phase,
  • Size limitations during design due to e.g. limited space at offshore substation,
  • Long lead-time because of custom-made reactors.

Active filtering

All active filtering solutions employ power electronic converters for the absorption (e.g. harmonic compensation) or suppression (e.g. active damping) of harmonics. Nowadays large WPPs are already equipped with a number of grid connected converters either as a part of the wind turbines or as some sort of FACTS devices. In that case, the implementation of active filtering technique would only mean the retuning of the converter controller in order to meet with controlled harmonic levels.
The converter might be controlled adaptively or otherwise to suppress the selected critical harmonic components. From this perspective there is no need to interfere with the WPP design but it entails to providing additional control features. Such issues could be specified on a contractual level and required to be provided as an add-on together with the product.
Connecting all possible active filtering methods together with state-of-the-art passive filtering methods an optimized hybrid solution can be obtained.


  • Already existing technologies such as STATCOMS can be utilized for the active filtering at the PCC,
  • Active tuning might be permissible even during the operation,
  • Almost unlimited control potential (e.g. selective harmonic compensation, wide band high-pass active filtering, etc.),
  • Network impedance changes during operation could be addressed,
  • Control method can be tuned for each of WPPs independently taking into consideration grid code issues as well as WPP structure,
  • Negligible losses for series connected active filters such as wind turbines,
  • Reduces risk due to uncertainties related with lack of information from manufacturers (e.g. models) and TSOs (e.g. harmonic background, models, etc.).


  • Recent technology; not commonly applied in WPPs,
  • May suffer from harmonic stability problems,
  • Improved bandwidth and increased switching frequency is needed,
  • Component sizing issues and limited DC-link voltage utilization.

[1] Ł. H. Kocewiak, "Harmonics in Large Offshore Wind Farms," PhD Thesis, Aalborg University, Aalborg, 2012.
[2] Ł. H. Kocewiak, S. K. Chaudhary, B. Hesselbæk, "Harmonic Mitigation Methods in Large Offshore Wind Power Plants," in Proc. of The 12th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, London, UK, 22-24 October 2013, 443-448.

Harmonics Wind Farms

Harmonic mitigation methods in wind power plants

There are various techniques for dealing with the harmonic problem in large wind power plants (WPPs) depending upon the nature and source of the problem.
Large offshore WPPs are characterized by complex structures including wide application of power electronic devices in wind turbines, FACTS devices and/or HVDC transmission. Moreover, there is a large amount of passive components such as filters, cable arrays, transformers, transmission cables, and shunt compensation equipment. Consequently, there are many potential sources of harmonic problems, and simultaneously many ways of dealing with them [1].
Primarily there are two methods of harmonic mitigation in a WPP: (i) avoiding harmonic resonance by design and (ii) design and use of filters [2]. A good design involves system layout, component selection and controller tuning with the aim of avoiding potential resonance conditions in the WPP.

Harmonic mitigation methods
Fig. 1 Harmonic mitigation methods in wind power plants.

Both passive and active filtering could be used for harmonic mitigation. It is recognized that passive filtering is the state-of-the-art technology. However, it requires extensive knowledge of the system during the WPP design phase. In many cases information about the system is uncertain and over-sizing of passive filters may take place to cover uncertainties and risks.
Due to the fact that more and more power electronic equipment (e.g. wind turbines with grid connected converter, STATCOMs, HVDC, etc.) is being utilised in WPPs, active filtering appears to be an interesting solution.
Active filtering can be implemented at the converter control level, thereby avoiding or reducing the need for installing expensive passive filters. Moreover, active filter controllers could be tuned and re-tuned, sometimes adaptively, to overcome the uncertainties faced during the WPP design phase [3].
A comparison between passive and active filters including major factors is presented in Table 1. It can be easily seen that there is a potential in active filtering and the technology is improving.

Table 1 Comparison between passive and active filtering technology.

Indices Passive filters Active filters
Technology Known Improving
Reliability High Medium
Effectiveness Medium Good
Engineering time Large Medium
Power electronics No Yes
Energy storage Large Small
EMI No Yes
Control circuit No Yes
Voltage regulation No Yes
Dynamic response Slow Fast
Cost Low High

Considering the different attributes, probably hybrid solutions involving both the passive and the active filters at various locations, as shown in Fig. 1, would be the most beneficial for effective harmonic mitigation scheme. In order to optimize the WPP design from harmonic emission and stability perspective some more studies and research is required [4]. The hybrid solutions would comprise of:

  1. Passive filtering at the wind turbine level:
    • trap filters designed for carrier group harmonics filtering,
    • high-pass filters for high frequency content,
    • detuned C-type filters with limited bandwidth, etc.
  2. Active filtering at the wind turbine level:
    • selective harmonic compensation,
    • high-pass active filtering,
    • harmonic rejection capability,
    • active notch filters, etc.
  3. Active filtering in groups of wind turbines:
    • carrier signals de-synchronization,
    • phase shifter transformer groups, etc.
  4. Passive filtering at the WPP level – 4b) onshore or 4a) offshore:
    • detuned C-type filters,
    • double-tuned filter, etc.
  5. Active filtering at the WPP level:
    • shunt connected FACTS devices,
    • HVDC link, etc.

[1] V. Akhmatov, J. Nygaard Nielsen, J. Thisted, E. Grøndahl, P. Egedal, M. Nørtoft Frydensbjerg, and K. Høj Jensen, "Siemens Wind Power 3.6 MW Wind Turbines for Large Offshore Wind Farms," in Proc. 7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms, 26-27 May 2008, pp. 494-497.
[2] M. Bradt, B. Badrzadeh, E. Camm, D. Mueller, J. Schoene, T. Siebert, T. Smith, M. Starke, and R. Walling, “Harmonics and resonance issues in wind power plants,” 2011 IEEE PES General Meeting, Jul. 2011.
[3] Ł. H. Kocewiak, "Harmonics in Large Offshore Wind Farms," PhD Thesis, Aalborg University, Aalborg, 2012.
[4] P. Brogan, "The stability of multiple, high power, active front end voltage sourced converters when connected to wind farm collector systems," in EPE Wind Energy Chapter Seminar, Stafford, 2010, pp. 1-6.

Harmonics Wind Farms

Harmonic problems in wind power plants

Harmonics has always been of special concern in power system studies. In the past the power system comprised mainly of passive components with relatively linear operating range and synchronous generators. Harmonic analysis of such systems is the state-of-the art right now.
The wind turbines are nowadays mainly connected together into a collector system through a widespread network of medium voltage (MV) sub-sea cables. The voltage is then stepped up and the wind power plant (WPP) is connected to the power grid through long high voltage (HV) cables which constitute the HVAC or HVDC transmission system. Such configuration is still being challenging to the industry from harmonic generation, propagation and stability perspective [1].
The presence of harmonics inside the WPP is a nuisance as it leads to higher current and voltage levels in the system. Consequently, the system loss is higher system, and there is higher component stress. Moreover, if there is series or parallel resonance points in the WPP, the resonating harmonics may get amplified and then that can be destructive. The resonance can be series or parallel type as shown in Fig. 1. Besides, there are other issues with harmonic interference and power quality [2].

Harmonic problems in wind farms
Fig. 1 Harmonic problems in wind power plants.

Identification of the presence of harmonics in the system and potential resonance conditions are very critical for the design of a WPP. Measurement of harmonic content is an important element of the WPP and wind turbine evaluation process. Measurement of field data is also required to validate the theoretical analysis and numerical simulations. The measurement equipment should be carefully adjusted in order to record harmonics of interest with acceptable accuracy and precision.
The harmonic measurements should be carried out during continuous wind turbine normal operation, i.e. fault free operation complying with the description in the wind turbine manual excluding wind turbine start-up and shutdown as described in IEC 61400-21. Since different operational modes are characterized by different frequency response of the converter thereby affecting the harmonic emission, the operational modes should be considered, and any change in the mode should be noted during the measurement process [3].
It is also recommended to perform measurements when the wind turbines are not operational such that the harmonic background spectrum can be evaluated. The wind turbine during background measurements should neither inject nor absorb any harmonic current during this process.
Harmonic mitigation by design is affected by several uncertainties in different factors during the design of a WPP. Some of them are listed below:

  • Lack of accurate models provided by the manufacturers.
  • Component tolerances in the WPP model.
  • Wind turbine harmonic emission model uncertainties.
  • Phase angle between harmonics from different wind turbines and possible harmonic cancellation.
  • Different operating modes of the wind turbines (e.g. power production levels, wake effects, voltage control, etc.).
  • Lack of reliable information from TSOs and DSOs for the external network model.
  • Changes in the wind turbine converter controller affecting harmonic emission.
  • Linear model of WPP components (e.g. transformers, converters, cables, etc.).
  • Linear harmonic load flow calculation method excluding possible frequency coupling.

[1] Ł. H. Kocewiak, C. L. Bak, J. Hjerrild, "Wind Turbine Converter Control Interaction with Complex Wind Farm Systems," IET Renewable Power Generation, Vol. 7, No. 4, 2013.
[2] Ł. H. Kocewiak, S. K. Chaudhary, B. Hesselbæk, "Harmonic Mitigation Methods in Large Offshore Wind Power Plants," in Proc. of The 12th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, London, UK, 22-24 October 2013, 443-448.
[3] Ł. H. Kocewiak, "Harmonics in Large Offshore Wind Farms," PhD Thesis, Aalborg University, Aalborg, 2012.

Harmonics Wind Farms

Harmoniske svingninger i store havmølleparker

This time Danish abstract of the PhD report entitled "Harmonics in Large Offshore Wind Farms (Harmoniske Svingninger i Store Havmølleparker)". The project was defended on the 2nd of February in 2012 at Aalborg University, Denmark.

Antallet af vindmøller med frekvensomformer til nominel effekt i mw-klassen, der anvendes til store havmølleparker, er stærkt stigende. De er tilsluttet et udbredt og forgrenet mellemspændingskabelnet stort set uden egetforbrug og er tilsluttet transmissionsnettet ved hjælp af lange højspændingskabler. Det stiller vindmølleindustrien og netselskaberne over for nye udfordringer i forhold til at forstå harmoniske svingningers  karakter, udbredelse og virkning. Vindmøllebranchen udvikler sig hastigt. Det stiller branchen over for nye udfordringer, hvilket har medført gennemførelse af flere og flere forskningsprojekter, der omhandler analyse af harmoniske svingninger med særligt fokus på vindenergi, og det er grunden til, at dette projekt blev påbegyndt og gennemført med et positivt resultat. Virksomhedens erfaring fra tidligere havmølleprojekter i forbindelse med forskellige harmoniske aspekter har medført et behov for at udføre omfattende undersøgelser af harmoniske svingninger.

Forskningsprojektet blev til på branchens foranledning, og blev gennemført i et  i samarbejde med institut for Energiteknik, Aalborg Universitet.  I forbindelse med planlægningen af projektforløbet blev rammerne for projektet lagt ud fra en traditionel rationalistisk tilgang for at kunne levere viden og en dybere forståelse for forskellige aspekter (f.eks. målinger, databehandling, dataanalyse, modellering, modelanvendelse) i studier af harmoniske svingninger. På baggrund af disse rammer, blev rapportens opbygning fastlagt. Læseren kan dermed følge alle projektforløbets stadier startende med målinger, databehandling og –analyse og sluttende med modellering og modelanvendelse. Forskellige aspekter af tidsdomænevalidering, frekvensdomæne og af brugen af statistiske metoder nævnes i forbindelse med specifikke problemer.

Målinger udgør en vigtig del af industriel forskning. Derfor er dette projekt unikt samtidig med, at det tilfører den akademiske verden vigtig praksis-orienteret indsigt og vice versa. Det er bevist, at analyse af systemer som store havmølleparker indebærer mange aspekter, der omhandler udvidede og mere præcise modeller, komplekse målekampagner og selvfølgelig bedre og mere anvendelige databehandlingsmodeller. Før de ovennævnte aspekter kan behandles, er det nødvendigt at have et pålideligt og robust målesystem til rådighed. Dette opnås gennem grundigt design af målesystemets hardware- og softwarelag.

I rapporten forklares det, at det er meget vigtigt at kende typen af de harmoniske svingninger, der genereres i store havmølleparker for at kunne anvende de rigtige databehandlingsteknikker. Tids-/frekvensanalyse baseret på multiresolution wavelettransformation bruges til at udføre tids-/frekvensdomæneanalyser, som kan bidrage til at definere de harmoniske svingningers oprindelse og observere korttidsvariationer. Ikke-parametrisk spektralanalyse anvendes på interpolerede signaler tilpasset de varierende elsystemfrekvenser. Forskellige databehandlingsteknikker er præsenteret og anvendt afhængig af signalet (dvs. om det er stationært eller ikke-stationært) eller typen af harmoniske svingninger (dvs. spline resampling eller direkte spektralanalyse). På baggrund af grundig analyse af målinger ses det, at visse harmoniske komponenter, der dannes på netsiden af omformeren i vindmøllen påvirkes af to faste frekvenser, dvs. af elsystemets grundfrekvens og basisbærefrekvenssignalet. Derfor er målinger af harmoniske svingninger udført primært med kommercielle spændingskvalitetsmålere i nogen grad utilstrækkelige, og den efterfølgende vurdering af resultaterne kan derfor være misvisende.

Forskellige statistiske værktøjer er anvendt til at analysere oprindelsen og karakteren af forskellige harmoniske komponenter. En omfattende sammenligning af harmoniske spændinger og strømme baseret på en vurdering af den sandsynlige fordeling samt passende statistiske beregninger (f.eks. middel, varians, sandsynlig tæthedsfunktion mv.) anvendes. En sådan tilgang giver et bedre overblik og en bedre sammenligning af harmoniske komponenters variationer og forekomst.

Flere frekvensdomænemetoder til beskrivelse af vindmølleparker bestående af flere komponenter såsom vindmøller, transformere, kabler mv. beskrives og sammenlignes. Det forklares, at store havmølleparker kan producere yderligere uønskede resonanser i lavfrekvensområdet. Dette kan have en betydelig indflydelse på systemets generelle stabilitet. Derfor er analyse og designoptimering af store havmølleparker mere komplekst end analyse og designoptimering af små landmølleparker.

I dag er vindmøller komplekse anlæg udstyret med den nyeste teknologi. Derfor er analyse af harmoniske svingninger i sådanne anlæg ikke så ligetil. På grund af vindmøllernes kompleksitet kan man ved studier af harmoniske svingninger fokusere på flere forskellige aspekter såsom reguleringsstrategi, moduleringsteknik, omformerdesign og hardwareimplementering.

Forskellige reguleringsstrategier er blevet overvejet sammen med deres indflydelse på dannelsen af harmoniske svingninger og generel systemstabilitet. Analyser er hovedsaglig udført i frekvensdomænet. En analyse går ud på at finde ud af, hvordan forskellige komponenter i reguleringskonceptet (f.eks. filtre, kontrolenheder mv.) kan påvirke styringen og dens evne til at udkompensere harmoniske svingninger. Reguleringsstrategiernes indflydelse på mølleparkens generelle stabilitet er ligeledes blevet grundigt undersøgt. Egnede stabilitetsindeks er foreslået og anvendt i flere konkrete cases.

Omhyggeligt modelerede ækvivalenter af store vindmølleparker i frekvensdomænet sammen med møllernes frekvensrespons giver et godt overblik over, hvordan store havmølleparker reagerer ved forskellige frekvenser. En sådan tilgang har vist gode resultater i forbindelse med studier af eksisterende mølleparker.

Da harmoniske svingninger i vindmøller og vindmølleparker har forskellig oprindelse og er af forskellige typer, kan det være problematisk at sammenligne dem. Derfor er selektiv validering af specifikke frekvenskomponenter til tider mere anvendelig. Det blev observeret, at sammenligning af resultater i frekvensdomænet og tidsdomænet og anvendelse af statistiske metoder er nøglen til forståelse af resultaterne.

På baggrund af de præsenterede studier kan det ses, at store havmølleparker sammenlignet med typiske landmølleparker kan generere flere uønskede resonansscenarier. Uønskede resonanser kan påvirke mølleparkens generelle stabilitet og ydelse (f.eks. kan harmonisk resonans anslåsog forstærkes). Derfor er det meget vigtigt at analysere mølleparker grundigt, især store havmølleparker, også ud fra et harmonisk perspektiv.

Denne erhvervsPhD fokuserer på at finde frem til de bedst mulige metoder til at gennemføre forskellige harmoniske studier af havmølleparker, herunder en række forhold som ikke før er blevet overvejet. Anvendelse af nye metoder og en udvidelse af rækken af modeller bidrager til at opnå den højere rådighed, der er nødvendig på havmøllerparker, hvis de skal fungere som store kraftværker i det elektriske system.