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Harmonics Wind Farms

Wind Turbine Harmonic Model - Some considerations

Nowadays large offshore wind power plants (WPPs) are complex structures including wind turbines (WTs), array cable systems, and HVAC or HVDC offshore/onshore transmission systems. This represents new challenges to the industry in relation to prediction and mitigation of harmonic emission and propagation [1]. Due to increasing complexity of WPPs it is more and more important to appropriately address harmonic analysis of WTs as well as WPP on a system level by means of modelling during the design stage as well as harmonic evaluation during operation.

Harmonic current emissions from the WT are strongly dependent on the WT internal impedances as well as the external network frequency-dependent short circuit impedance. Unfortunately until now there has been no systematic approach to represent a WT from its harmonic performance perspective. This brings inconsistency in WT harmonic performance assessment, evaluation of background distortion in grid-connected WT and harmonic analysis of WPPs.

Due to the different approaches in electrical design taken by WT manufacturers it is convenient to represent WT harmonics in a generic way by means of a Thévenin equivalent circuit comprising an ideal voltage source and an equivalent impedance. Such an equivalent circuit is to be provided for each harmonic component of interest to be included in the model. Therefore using the WT harmonic model, as either Norton or Thévenin equivalent circuits, in simulations with commonly used engineering tools one can estimate the harmonic contribution to the system to which it is connected [2]. WTs as a part of a WPP system can be potentially considered as harmonic sources as well as harmonic mitigation units by means of active and passive filtering thus the structure of the harmonic model should reflect that behavior, e.g. harmonic source and equivalent impedance adjusted accordingly to active filter software settings, equivalent impedance adjustment if the WT passive harmonic filter is incorporated in it.

According to Thévenin's (or Norton’s) theorem any linear electrical network with voltage and current sources and only impedances can be replaced at the terminals of interest by an equivalent voltage source VTh in series connection (or an equivalent current source INo in parallel connection) with an equivalent impedance Zth (or ZNo, where ZTh = ZNo). Thévenin's theorem is dual to Norton's theorem and is widely used for circuit analysis simplification and to study the circuit initial-condition and steady-state response.

Wind Turbine Harmonic Model

[1] Ł. H. Kocewiak, J. Hjerrild, and C. Leth Bak, “Wind Turbine Converter Control Interaction with Complex Wind Farm Systems,” IET Renewable Power Generation, Vol. 7, No. 4, 2013.

[2] Ł. H. Kocewiak, C. Álvarez, P. Muszynski, J. Cassoli, L. Shuai, “Wind Turbine Harmonic Model and Its Application – Overview, Status and Outline of the New IEC Technical Report,” in Proc. The 14th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, 20-22 October 2015, Brussels, Belgium.

Categories
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.).

Pros

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

Cons

  • 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.

Pros

  • 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.).

Cons

  • 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.

Categories
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.

Categories
Harmonics

Wind turbine harmonic models

Harmonic emission is recognized as a power quality concern for modern variable-speed wind turbines. For this reason, relevant standards (e.g. IEC 61400-21) require the measurement of harmonics and their inclusion in the power quality certificates of wind turbines. Understanding the harmonic behavior of wind turbines is essential in order to analyze their effect on a grid to which they are connected. Wind turbines with power electronic converters are potential sources of harmonic distortion, and therefore knowledge of their harmonic current emissions is needed to predict wind farm behavior and to design reliable wind farms [1]. The emission of harmonic currents during the continuous operation in steady state of a wind turbine with a power electronic converter must be stated according to the standards.

Nowadays there is a lack of appropriate wind turbine model descriptions for harmonic analysis purposes in standards. It is shown in this paper how the harmonic model should be developed based on measurements. It is recommended to develop wind turbine harmonic models based on the Thevenin (equivalently Norton) approach. The best way is also to compare results with simulations however various aggregation techniques can change measurement results and this should be taken into consideration.

In model development the most crucial measurements are done at the grid-side converter AC terminals and after the main reactor. Based on these measurements the wind turbine harmonic model can be developed based on the Thevenin approach. Please note that the Thevenin approach is equal to the Norton approach in harmonic assessment in wind power plants. The model developed based on the measurements describes the wind turbine harmonic behavior.

In model development it is important to use measurements that can describe the grid-side converter harmonic behavior. The reactor current is the most reasonable choice as well as the voltage at the converter AC terminals or the voltage after the series reactor. Unfortunately both measurement places can introduce some uncertainties.

If one would like to develop the model based on measurements after the series reactor the reactor impedance should be included in the Thevenin impedance. Unfortunately it is not so easy (especially for lower frequencies) to measure the frequency dependent impedance of the reactor. Small errors/uncertainties in the reactor measurements can introduce significant errors in model development, especially for harmonics with low magnitude. Therefore measurements at the grid-side converter AC terminals seem to be more reasonable because the reactor impedance is not needed in the Thevenin impedance. In case of measurements directly at the converter terminals only the internal converter impedance specified by the control structure is required. However such measurements also introduce some uncertainties. Please note that in high power density wind turbines with LV converters there is a need to use several parallel connected converters with sharing reactors. Such converters introduce a certain degree of unbalance which can affect internal harmonic current flow between converter modules. Even if coupled sharing reactors are designed to limit the current imbalance some asymmetry in harmonic generation between converter modules can be seen. The current flowing from the converter to the grid can be assessed based on measurements of all converter modules.

As it was mentioned earlier the converter internal impedance is strongly dependent on control structures applied by different manufacturers. Most of nowadays wind turbines are based on fast current control loop which has the most significant impact on the converter frequency response. Even if in theory the fundamental frequency controller is represented in the same way in natural/stationary reference frame still the controller transfer function may significantly vary if the current control is implemented in stationary or synchronous reference frame [4]. Please note that also harmonic compensation and switching frequency can affect the internal converter impedance. Therefore the internal impedance is kept as a trading secret by the wind turbine and converter units manufacturers.

The problems mentioned above cannot be avoided. Therefore it is recommended to perform simultaneous measurements in both locations (i.e. converter AC terminals and between the series reactor and the wind turbine transformer) and develop two independent models based on two datasets. Later the models can be compared.

In order to avoid any aggregation errors during the calculation of the Thevenin equivalent harmonic voltage sources it is recommended to apply harmonics directly from the Fourier decomposition (i.e. from the 10-cycle window). Later the obtained results (i.e. Thevenin equivalent harmonic sources) could be aggregated according to the methods recommended above. According to IEC 61400-21 there is a need to have at least nine 10min time-series of instantaneous measurements for each power bin. Based on experience it can be said that one month of measurements should be absolutely enough.

At the end it is worth to emphasize the the harmonic assessment approach presented in the IEC 61400-21 standard concerning measurements and power quality assessment in wind turbines assumes measurements of 10-minute harmonic current generated by a wind turbine for frequencies up to 50 times the fundamental frequency of the grid [2], [3]. It has to be emphasized that the most popular standard concerning measurements and power quality assessment of grid-connected wind turbines refers only to current harmonic components without any phase information. Therefore it impossible to evaluate if the harmonic current is flowing into the wind turbine and is mainly caused by background distortions or is caused by the grid-side converter and is flowing from the wind turbine to the grid.

Sometimes based on the power quality report from IEC 61400-21 the wind turbine is modeled as an ideal current source which can cause significant errors in harmonic analysis of wind farms. The harmonic source can be modeled only as an ideal current source for component where the internal converter impedance is equal to infinity. This can happen only for controlled frequencies (e.g. harmonic compensation) and the current value is equal to the reference signal in the control loop [4], [5].

[1] "Wind Turbine Generator Systems – Measurement and Assessment of Power Quality Characteristics of Grid Connected Wind Turbines," IEC 61400-21, 2008.
[2] H. Emanuel, M. Schellschmidt, S. Wachtel, and S. Adloff, "Power quality measurements of wind energy converters with full-scale converter according to IEC 61400-21," in International Conference on Electrical Power Quality and Utilisation, Lodz, 2009, pp. 1-7.
[3] A. Morales, X. Robe, and M. J. C, "Assessment of Wind Power Quality: Implementation of IEC61400-21 Procedures," in International Conference on Renewable Energy and Power Quality, Zaragoza, 2005, pp. 1-7.
[4] Ł. H. Kocewiak, J. Hjerrild, and C. Leth Bak, "Wind Turbine Control Impact on Stability of Wind Farms Based on Real-Life Systems Analysis," in Proc. EWEA 2012 - Europe's Primier Wind Energy Event, Copenhagen, 16-19 April 2012, pp. 1-8.
[5] Ł. H. Kocewiak, "Harmonics in large offshore wind farms," PhD Thesis, 2012, pp. 332, 978-87-92846-04-4.

Categories
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.