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 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.