STUDY AND ANALYSIS OF WIND TURBINE FOR POWER QUALITY ASSESSMENT

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1 STUDY AND ANALYSIS OF WIND TURBINE FOR POWER QUALITY ASSESSMENT Kishor Bhimrao Gawale1, Ansari Habibur Rahman2 1,2 Department of Electrical Engineering Veermata Jijabai Technological Institute, Mumbai, India. Abstract Power fluctuations play an important role in the evaluation of the impact of wind turbines on the power quality, as stated in the IEC Std which addresses the measurement and assessment of power quality of grid connected wind turbines. The assessment of an individual wind turbine contribution to power quality measured at a point is not easy, because there are usually several sources connected simultaneously at the site. Wind turbines may cause significant impacts on the power quality of their connected grid. Voltage fluctuations produced by wind turbines are usually due to wind speed variations, power and voltage fluctuations. This simulations for numerical models of two wind turbine schemes, fixed and variable speed types, by using Matlab/Simulink, where simplified analytical model of wind turbines for voltage spectral analysis are illustrated for the power quality assessment. Index Terms Wind turbines, fliker, interharmonics, power quality. I. INTRODUCTION With growing concerns about environmental pollution and a possible energy shortage, great efforts have been taken by the governments around the world to implement renewable energy programs, based mainly on wind power, solar energy, small hydro-electric power, etc. Wind turbines produce power fluctuations due to their aerodynamic behavior and wind speed variability, where the wind turbulence influence is indeed the main contribution to voltage fluctuations. Therefore, it would be interesting to know in advance how a group of wind turbines fed into the local distribution network or a large wind farm connected to a high voltage network that may affect the power quality. The reactive power consumption or generation of a wind turbine is essential for determining the impact on the steady state voltage level. For this purpose, the reactive power must be specified as ten minutes average values as a function of the output power of the wind turbine. Fixed speed wind turbines employ induction generators and consume reactive power while producing active power. Wind turbines employing frequency converters for operation at variable rotational speed may consume or produce reactive power depending on the design and operational strategy of the converter. II. WIND TURBINE SYSTEMS There are three types of commonly seen wind turbines: 1) Fixed-speed wind turbine 2) Variable-speed wind turbine with synchronous generator 3) Variable-speed wind turbine with doubly-fed induction generator The variable-speed turbines are pitch-regulated and the torque or the rotor speed regulated to the maximum power point (MPP), 81

2 Fig. 1) Fixed-speed wind turbine Fig. 2) Variable-speed wind turbine with synchronous Generator Fig. 3) ) Variable-speed wind turbine with doubly-fed Induction Generator The reactive power consumption or generation of a wind turbine is essential for determining the impact on the steadystate voltage level. For this purpose, the reactive power must be specified as ten minutes average values as a function of the output power of the wind turbine. Fixed speed wind turbines employ induction generators and consume reactive power while producing active power. In practice, the reactive power consumption is compensated by installation of capacitors. Wind turbines employing frequency converters for operation at variable rotational speed may consume or produce reactive power depending on the design and operational strategy of the converter. Typically, the converter is operated to produce near zero reactive consumption or to compensate for voltage variations from the reference value. Both types of wind turbines are considered in the study. In the following, the voltage flicker level represented by ΔV10 at PCC of the variable-speed wind farm systems shown in Fig. 3 is analyzed. A variable-speed wind turbine employs a permanent magnet synchronous generator or a doubly-fed induction generator, as shown in Fig. 2 and Fig. 3, respectively. The wind turbine is connected to the grid by a full-power back-toback converter. It is able to decouple the variable-frequency operation from the fixed-frequency electric power that is transferred to the grid. The voltage source inverter connected to the turbine generator controls the MPP of the turbine. The inverter allows transferring the proper active and reactive power at fixed grid frequency. As given in (1) and (2), the wind power, Pw (W), is proportional to the cubic of the average wind speed value, Vw 82

3 (m/s), but the extractable part of the wind power is related to the air density, ρ (kg/m3 ), the area swept out by the turbine blades, A(m2 ), and a dimensionless power coefficient, Cp that depends on the type and operating condition of the wind turbine, where Cp is a function of λ (tip speed ratio, the ratio of blade tip speed Ωr, R, to the wind speed) and β (pitch angle). Pw = 0,5 Cp (λ,β) AVw3 (1) λ= (ΩrRb/VW) (2) with Ωr is the rotor instantaneous angular velocity and Rb is the blades radius. According to the p-q theory [4], the reference currents that the converter has to track have to meet the following relationships: P*DG= VPCC,d I dg,d +VPCC,q I dg,q (3) Q*DG=VPCC,d I dg,q +VPCC,q I dg,d (4) where d and q are the coordinates in a rotating system with grid frequency. P*DG,Q*DG,Vdg and Idg are the captured active wind power, reactive power, PCC voltage, and the injected current by DG, respectively. Neglecting aerodynamic and electrical conversion losses, the reference active power is the captured one from the wind. The reactive power value is zero, if a unitary power factor between Vdg and Idg is requested at PCC; otherwise, the PCC voltage amplitude regulation is required. III. SIMULATION RESULTS In this section, the wind turbine system is simulated by MATLAB/SIMULINK to evaluate the power quality associated with wind turbine operations. The rated wind speed of 11m/s is applied to wind turbines in case 1, and the Gaussian distributed random wind speed (mean wind speed equals to 11m/s) is applied to wind turbines in case 2. The total simulation time is 50 seconds. A. Voltage Envelope Estimation with Squaring Demodulation Method To evaluate the envelope AEn of the amplitudemodulated (AM) signal, many methods have been proposed in the literature. Although it is possible to analyze the AM signal directly with the spectral estimation methods, the flicker components will spread aside the fundamental spectral line. Moreover, once the number of flickers is increased, it is difficult to identify the true components. Therefore, the mechanism of demodulation is necessary. In general, the architecture of estimating the components of flicker can be divided into two parts shown in Fig. 4. With the demodulation block, the voltage envelope which is composed of all flicker components can be separated from the fluctuated waveform. Then, the obtained voltage envelope will be fed into the spectral analysis block to extract each flicker component. Among many available spectral analysis methods, the fast Fourier transform (FFT) has attracted much attention and is widely used due to its simple structure and computational efficiency. Therefore, FFT is applied to simplify the computation and comparisons of demodulation methods for the voltage flicker envelope estimation. The squaring demodulation method used to demodulate in the simulation consists of a square demodulator and two filters. As shown in Fig. 5, it is the basic block proposed by IEC in the design of flickermeter. The voltage fluctuation can be extracted firstly by squaring the input signal in (5) as V2(t)=(AEncos(ω0t+φ0))2 (5) 83

4 Then, a first-order high-pass filter with a cutoff frequency of 3 db at 0.05 Hz is used to eliminate the DC component and a 6th-order Butterworth low-pass filter with a 3 db cutoff frequency of 42 Hz for 60 Hz system (35 Hz for 50 Hz system) to reject the double main frequency components caused by the squaring demodulator. B. Simulations There are two cases in the study: a rated wind speed of 11m/s is applied to wind turbines in Case I and the Gaussian distributed random wind speed (mean wind speed equals to 11m/s) is applied to wind turbines in Case II. The total simulation time is 50 seconds. Figure 6 depicts the one-line diagram of the actual wind turbines system under study. Figure 7 illustrates the block diagram of each variable-speed wind turbine (doubly-fed IG) of Fig. 6 modeled by Matlab/Simulink. Major parameters of the wind turbine are listed in Table I. 84

5 B1. CASE I The rated wind speed of 11m/s is applied to wind turbines in this case. Figure 8 shows line-to-ground rms voltage of wind turbine 1 and the collector and the voltage is about 0.88 pu. Figure 9 shows the harmonic spectrum of Fig. 8. It is observed that the larger components are at 60h ± 20 and 60h ± 40 Hz (h>0). The voltage flicker can be observed in the simulation. After the voltage envelope estimation by using the squaring demodulation method, results obtained are shown in Fig. 10. The larger components of line-to-neutral voltage spectrum after voltage envelope estimation extraction are shown in the Table II. It is obvious to see that the magnitudes of 20 Hz and of 40 Hz are dominant components and ΔV10 can be calculated from the spectrum of Fig. 10, which is less than the recommended voltage flicker limit of 0.45% in the study. 85

6 b) Figure 8. Line to ground rms voltage of wind turbines and the collector versus time: ( turbine 1 and (b) collector. b) Figure 9. Line-to-neutral voltage spectrum of ( turbine 1, (b) the collector 86

7 Figure 10. Line to neutral voltage spectrum after voltage envelope estimation extraction: ( turbine 1, (b) collector. B2. CASE 2 A Gaussian distributed random wind speed (variance of 2 and mean wind speed of 11m/s) is applied to wind turbines in this case. The wind speed to the turbines is shown in Fig.11. Figure 12 is line-to-ground rms voltage of wind turbine 1 and the collector, which is about 0.88 pu. Figure 13 shows harmonic spectrum of Fig. 12. The larger components are still at 60h ± 20 and 60h ± 40 Hz (h>0). The voltage flicker also can be observed in the simulation. After implementing voltage envelope estimation with the squaring demodulation method, results are shown in Fig. 14. The dominant components after the voltage envelope estimation extraction are shown in the Table III. Figure 11. Wind speed of turbines 87

8 b) Figure 12. Line to ground rms voltage of wind turbines and the collector versus time: ( turbine 1 and (b) collector. b) Figure 13. Line-to-neutral voltage spectrum: ( turbine 1, (b) the collector. 88

9 b) Figure 14. Line to neutral voltage spectrum after voltage envelope estimation extraction: ( turbine 1, (b) collector IV. CONCLUSIONS By observing simulated results in both cases I and II, it is found that ΔV10 of the collector with the rated wind speed is lower than the one with Gaussian distributed random wind speed. The voltage flicker level produced by each wind turbine in the system with Gaussian distributed random wind speed may be offset by each other. Such cancellation could lower the value of ΔV10. Even when the values of ΔV10 shown in the simulations are within the recommended limit, however, the maximum value can get close to the allowable limit under some operating conditions. When facing different wind speed variations or several turbines are integrated, severe flicker problem may still occur. In this paper simulations of wind turbine system to assess power quality that will affect the power system before actual installation have been performed. Because of the complexity of the model under study, the computational burden is high. Future work will focus on the reduction of simulation time and increase the solution accuracy. REFERENCES [1] Z. Chen, F. Blaabjerg, and T. Sun., Voltage Quality of Grid Connected Wind Turbines, TEQREP Workshop, [2] C. Vilar, H. Amaris, and J. Usaola, Assessment of Flicker Limits Compliance for Wind Energy Conversion System in the Frequency Domain, Renewable Energy 31 (2006), pp [3] R. Langella, F. Liccardo, P. Marino, A. Testa, and M. Triggiance, On the Assessment of Light Flicker due to the Interharmonic Distortion Produced by Wind Turbine, Proceedings of the 2007 International Conference on Clean Electrical Power, May 2007, pp

10 [4] H. Hyasung and H. Akagi, The instantaneous Power Theory on the Rotating p-q-r Reference Frames, Proceedings of PEDS'99, Vol. 1, July 1999, pp [5] G. W. Chang, C. I Chen, and Y. L. Huang, Evaluation of Voltage Flicker Estimation Methods, Proceedings of the 28th Symposium on Electric Power Engineering, Kaohsiung, Taiwan, Dec [6] Electromagnetic Compatibility (EMC). Part4: Testing and Measurement Techniques. Section 15: Flickermeter. Functional and Design Specifications, IEC Std , [7] J. Ruiz, A. Lazkano, E. Aramendi, L. A. Leturiondo, Analysis of Sensitivity to the Main Parameters Involved in the Digital Implementation of the UIE Flickermeter. IEEE Electrotechnical Conference, Vol. 2, MEleCon 2000, pp

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