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Harmonics Measurements Wind Farms

GPS disciplined oscillator for long-term synchronized harmonic measurements

Measurement process is one of the most important issues during wind turbine generator (WTG) and wind power plant (WPP) evaluation and requires careful approach. Accurate measurements of harmonic voltages and currents in offshore WPPs followed by proper data analysis are essential for harmonic emission evaluation. In harmonic measurements it is of great importance to specify appropriate measurement points and optimize data acquisition devices as well as sensors.

Measurement systems involving multiple devices often require accurate timing in order to secure event synchronization and correlation in long-term data acquisition. One of the ways to achieve this synchronization measurement units must synchronize their individual clocks in order share a common time base. In large offshore WPPs distributed clock synchronization becomes necessary. Distributed clock synchronization in WPPs requires devices synchronized to a GPS satellite because of significant distances between measurement units [1].

Measurement System (Disciplined Clock)
Figure 1  Measurement system used for synchronization.

As presented in Figure 1 there are two synchronization possibilities: (1) with reference clock and (2) by means of phase-locked loop (PLL) synchronization.

(1) With the reference clock , the PXI 4472 device locks their frequency timebases – the inputs of their direct digital synthesis (DDS) chips, to the PXI_Clk10 (10 MHz) clock supplied by the PXI unit backplane. This is accomplished by using PLL. After a sync pulse is sent, which aligns the sample clock timebase on each device, the oversample clocks, and the analog-to-digital converters (ADCs). Finally, a shared start trigger is sent, which starts the acquisition and generation events on each device at the same instant.

(2) Another synchronization is just done by means of PLL which provides sufficient accuracy for harmonic measurements. Having an appropriate synchronization the GPS disciplined oscillator (GPSDO) is used to combine the good short term stability of the crystal oscillator with the excellent long term stability of the GPS signal. It assures that each acquired sample by all dispersed measurement unit will be synchronized together as presented in Figure 2.

In order to achieve that GPS synchronized triggering and GPS disciplined timebase were used. As an exemplary configuration PXI-6682, PXI-6653, PXI-4495 and PXI-4472 can be used in each of measurement locations in order to assure precise synchronization and high quality (i.e. aliasing free, high resolution equal to 24 bits, suitable sample rate equal to 44.1kS/s/ch) data acquisition [2].

Synchronization between PXI-4472 and PXI-4495
Figure 2  Synchronization between PXI-4472 and PXI-4495 with filter delay compensation.

[1] Ł. H. Kocewiak, I. Arana, J. Hjerrild, T. Sørensen, C. L. Bak, and J. Holbøll, "GPS Synchronization and EMC of Harmonic and Transient Measurement Equipment in Offshore Wind Farms," Energy Procedia, vol. 24, pp. 212-228, 2012.
[2] Ł. H. Kocewiak, A. Baloi, “Evaluation of Power Quality Monitoring Systems in Offshore Wind Farms,” in Proc. The 13th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as Transmission Networks for Offshore Wind Farms, Energynautics GmbH, 11-13 November 2014, Berlin, Germany.

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.

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

Oversampling (resampling) data in harmonic data processing

When using a rectangular window, it is important to synchronize the measurement window accurately with the power system frequency to achieve integer multiple of periods in analysed time series (i.e. window period is and integer multiple of analysed frequency component natural period). For example, if the power system frequency is 50.2 Hz whereas the window size is 200 ms, the fundamental frequency spectral line of the discrete Fourier transform is no longer projected (represented) by one complex vector in the orthogonal basis but spanned over the whole basis. One can even say that the power system frequency in the estimated spectrum becomes an interharmonic with spectral leakage as a consequence. Therefore appropriate data should be prepared before the projection into frequency domain.

In order to limit possible data processing errors the signal should be adjusted before spectral analysis. Power system frequency variation can be limited by application of various interpolation methods. It is strongly recommended to always oversample the analysed signal. In this particular case acquired waveforms with sample rate of 44.1 kS/s/ch are oversampled up to 51.2 kS/s/ch. According to this procedure 1024 samples per cycle is obtained, which is an integer power of 2 and can be used to apply fast Fourier transform. Please note that for 10-cycle data blocks discrete Fourier transform should be applied.

Frequency estimation
Figure 1 Single tone frequency. (a) Measured power system frequency variation. (b) Frequency variation after resampling (spline interpolation).

Oversampling of each 10-cycle data blocks before spectrum estimation significantly limits fundamental frequency variation over analysed window and therefore the stationarity assumption is strengthened for frequency component of interest (i.e. linked with the fundamental frequency). As it can be seen in Figure 1 resampling with high precision definitely improves signal quality. The figure presents analysed one minute of acquired voltage waveform and each point is an estimated fundamental frequency over 10-cycle rectangular window. The discrepancy as a difference between two measured values, given in Figure 1(b), is acceptably small and the sample standard deviation is 4.3548•10-6. Please note that scale 0.2s used on abscissa (i.e. horizontal axis) in Figure 1 denotes the step size between neighbouring samples. If there are 300 samples in presented results (as in Figure 1) the total duration is 1 minute (i.e. 0.2s∗300=60s).

It can be seen that both factors such as main tone detection algorithm as well as interpolation algorithm are crucial in appropriate harmonic magnitude and phase estimation. Various interpolation techniques can give different results. The most sophisticated unfortunately are usually the most time consuming. Therefore taking into consideration enormous amount of data to process from few months of multipoint measurements also the interpolation should be optimized and agreement between acceptable accuracy and calculation speed should be found.

Roughly speaking, during interpolation process new samples between existing samples are computed. Different methods were used in data processing such as nearest (coerce) method, linear method, spline method, cubic method, and finite impulse response filter method [1]. The nearest method finds the point nearest to xi  in an X array and then assigns the corresponding y value in an interpolated Y to yi. The linear method interpolates yi on the line which connects the two points (xj,xj+1) when the corresponding location xi of interpolated value yi which is located between the two points in an X array. The spline method known also as cubic spline method is almost always the most suitable solution. Within the method the third-order polynomial for each interval between two adjacent points in X (xj,xj+1) is derived. The polynomials have to meet the following conditions: the first and second derivatives at xj and xj+1 are continuous, the polynomials pass all the specified data points, the second derivatives at the beginning and end are zero. The cubic Hermitian spline method is the piecewise cubic Hermitian interpolation. This method is similar to cubic spline interpolation and derives a third-order polynomial but in Hermitian form for each interval (xj,xj+1) and ensures only the first derivatives of the interpolation polynomials are continuous. Compared to the cubic spline method, the cubic Hermitian method is characterized by better local property. The cubic Hermite interpolation method guarantees that the first derivative of the interpolant is continuous and sets the derivative at the endpoints in order to preserve the original shape and monotonicity of the data [2]. Also interpolation based on finite impulse response filter is popular [3], [4]. This method provides excellent results also for frequency analysis, although it is more intensive computationally [2].

Order Interpolation method Computation time [ms] Marker
1 Linear 110 x
2 Coerce 70 *
3 Cubic spline 130
4 Hermitian spline 430 +
5 FIR filter 290

Table above shows the computation time of data interpolation with different algorithms. In each of the cases the presented time is measured for resampling of 10 cycles of measured data. It can be seen that the simplest algorithms exhibit less computation burden. On the other hand in many cases simple algorithms cannot give satisfactory interpolation results and thus affect the harmonic calculation results. Figure 2 shows how different interpolation techniques affect harmonic magnitude estimation from measurements carried out at the LV side of the wind turbine transformer. It can be seen that the coerce method is affected the worst while other algorithms give comparable results.

Oversampled measurements
Figure 2 Different interpolation techniques used in oversampling of waveforms measured at LV side of the wind turbine transformer.

It was observed that in case of more distorted waveforms (e.g. gird-side converter output voltage) interpolation method choice plays a crucial role. In Figure 3 one can see that also linear interpolation and cubic Hermitian spline interpolation do not give satisfactory results. Therefore the most suitable for harmonic components estimation appear to be cubic spline interpolation as well as finite impulse response filter interpolation. Since the spline method (as presented in table above) is less time consuming, this method was used in further data processing. The selected measurement period was selected when the power system frequency were varying below 50 Hz or above 50 Hz.

Oversampled measurements
Figure 3 Different interpolation techniques used in oversampling of waveforms measured at the grid-side converter AC terminals.

[1] S. C. Chapra and R. Canale, Numerical Methods for Engineers: With Software and Programming Applications, 6th ed. McGraw-Hill Science, 2009.
[2] National Instruments, "LabVIEW 2011 Help," National Instruments Manual, 2011.
[3] R. A. Losada, "Digital Filters with MATLAB," The Mathworks, 2008.
[4] R. W. Schafer and L. R. Rabiner, "A digital signal processing approach to interpolation," Proceedings of the IEEE, vol. 61, no. 6, pp. 692-702, Jun. 1973.