Categories
Stability

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.

Categories
Measurements

PhD Course on  Harmonics in Power Electronics and Power Systems

Description:
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.
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.