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International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal...
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal...
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal...
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal...
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal...
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal...
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal...
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.4, pp. 1305-1312
ISSN 2078-2365
http://www.ieejournal...
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A Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems

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As the wind power installations are increasing in number, Wind Turbine Generators (WTG) are required to have Fault Ride-Through (FRT) capabilities. Lately developed grid operating codes demand the WTGs to stay connected during fault conditions, supporting the grid to recover faster back to its normal state. In this paper, the generator side converter incorporates the maximum power point tracking algorithm to extract maximum energy from wind turbine system. A hybrid control scheme for energy storage systems (ESS) and braking choppers for fault ride-through capability and a suppression of the output power fluctuation is proposed for permanent-magnet synchronous generator (PMSG) wind turbine systems. During grid faults, the dc-link voltage is controlled by the ESS instead of the line-side converter (LSC), whereas the LSC is exploited as a STATCOM to inject reactive current into the grid for assisting in the grid voltage recovery. A simple model of the proposed system is developed and simulated in MATLAB environment. The effectiveness of the system is validated through extensive simulation results

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A Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems

  1. 1. International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.4, pp. 1305-1312 ISSN 2078-2365 http://www.ieejournal.com/ 1305 Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems A Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems S.Rajkumar1 , S.T.Suganthi2 1 Department of EEE,SNS College of Technology, Coimbatore-35 2 Department of EEE,SNS College of Technology,Coimbatore-35 1 rajkumareee89@gmail.com, 2 suganthi.sb@gmail.com Abstract— As the wind power installations are increasing in number, Wind Turbine Generators (WTG) are required to have Fault Ride-Through (FRT) capabilities. Lately developed grid operating codes demand the WTGs to stay connected during fault conditions, supporting the grid to recover faster back to its normal state. In this paper, the generator side converter incorporates the maximum power point tracking algorithm to extract maximum energy from wind turbine system. A hybrid control scheme for energy storage systems (ESS) and braking choppers for fault ride-through capability and a suppression of the output power fluctuation is proposed for permanent-magnet synchronous generator (PMSG) wind turbine systems. During grid faults, the dc-link voltage is controlled by the ESS instead of the line-side converter (LSC), whereas the LSC is exploited as a STATCOM to inject reactive current into the grid for assisting in the grid voltage recovery. A simple model of the proposed system is developed and simulated in MATLAB environment. The effectiveness of the system is validated through extensive simulation results Index Terms- Boost converter, Braking Chopper(BC),dc-link control, energy storage system(ESS),ride through, STATCOM, Permanent Magnet Synchronous Generator. I. INTRODUCTION The development of various wind turbine (WT) configurations in the last decade has been very dynamic and has resulted in larger ratings and higher operating speed ranges allowing them to be tied up to the grid more easily. Variable speed operation of wind energy conversion systems (WECS) make them more ‘grid-friendly’. Permanent magnet synchronous generators (PSMG) based WECS are emerging as strong competitors to the other variable speed technologies. The power converter, whose rating is the same as that of the generator, connected between PMSG and grid allows full controllability of the system during normal operation and fault conditions. Further, PMSG operates at higher efficiency and better power factor than its counterparts especially when it functions as a direct driven generator [1-3]. WECS based on PMSG can be connected to the grid by using a voltage source converter (VSC) on the grid side and by using either a diode converter with a buck-boost converter or a VSC on the machine side. Evidently, using VSC on both machine and grid sides offer full control of active and reactive powers resulting in the best performance [4-6] in terms of power output, quality of power and performance during faults. Different strategies have been presented by various authors, to enhance fault ride through (FRT) capabilities of PMSG based WECS. Many devices, such as static var compensator (SVC), dynamic voltage restorers (DVR), Static Synchronous Compensators (STATCOM) etc have been shown to improve FRT of WECS, but they will also increase the overall cost of the system [7-8]. In [9], a nonlinear controller design for power converter based WT system is presented which ensures that current levels remain within design limits, even at greatly reduced voltage levels. A back to back connected voltage source- converter (VSC) configuration is discussed in [10] where the machine side converter (MSC) controls the speed of the generator by using a flux vector control technique and the grid side converter (GSC) controls the power flow by PWM technique Transient analysis of a grid connected wind driven PMSG is presented in [11] and a comparison is presented with the other generators at fixed and variable speeds. An electromagnetic braking resistor controlled using power electronic switch is used to dissipate
  2. 2. International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.4, pp. 1305-1312 ISSN 2078-2365 http://www.ieejournal.com/ 1306 Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems the excess energy in the DC link Circuit, preventing DC link over voltage in [12]. Flexible active power control of distributed generation systems during Grid Faults is discussed in [13]. Reactive power as well as real power manipulation using current control is described in [14]. Control of grid converter in synchronously rotating reference frame is described in [15-16]. In this paper, the generator side converter incorporates the maximum power point tracking algorithm to extract maximum energy from wind turbine system. In addition, FRT technique of the PMSG wind turbine system is proposed during the grid fault. By switching the control mode, the ESS is operated to control the dc-link voltage of the back- to-back converters during the grid voltage sags. Meanwhile, the LSC is utilized to supply the reactive current to the grid for satisfying the reactive current requirements of the grid code. By this, the grid voltage can be recovered rapidly without an external STATCOM after fault clearance. Also, the generator active power can be absorbed fully by the ESS and the BC during the voltage sags. In addition, the output power fluctuation of wind turbine systems operating in steady state is smoothened by the ESS. With this control scheme, the system can still operate well even though the grid voltage is fully interrupted. The validity of the proposed control algorithm is verified by simulation and experimental results. II. SYSTEM DESCRIPTION Fig. 1 presents a block diagram of the simulation model used for the FRT and Maximum Power point tracking. In this paper, the wind turbine converts the power of the wind to mechanical power in the rotor shaft. This is then converted to electricity using a permanent magnet synchronous generator (PMSG). The output voltage is rectified using a three-phase diode bridge rectifier. The dc-to-dc converter is used to control the dc voltage Vdc . The MPPT controller delivers a voltage reference that is compared to the actual value of Vdc. The result is fed into a PI controller whose output is compared to a triangular waveform to determine when to turn the dc -dc boost converter switch ON or OFF.The ESS consists of a Electric Double Layer capacitor bank and a bidirectional DC/DC converter and is also connected to the dc-link. Super capacitors are suitable for wind power applications, as they present the features of high efficiency, high power density, long cycle life and easy maintenance . A BC is connected in parallel with the dc-link. The BC will be activated to dissipate the excessive power beyond the capacity of the ESS in cases of deep voltage sags or high wind speed variations. The control objective of the DC/DC converter is to maintain the dc-link voltage magnitude at a constant level, by absorbing any mismatch between the generated power and the power transferred to the grid. In normal conditions, the LSC controls the dc-link voltage , and the ESS is able to smoothen the power ripples. In grid fault conditions, on the other hand, the LSC functions as a STATCOM, and the ESS controls the dc- link voltage. The FRT control scheme is designed according to the grid code requirements on FRT. III. MODELLING OF PROPOSED CONTROL SCHEME A. Maximum Power Extraction Algorithm Due to its monotonic characteristics, wind turbines can be controlled to yield maximum power using search control methods. Before explaining the maximum power tracking controller, it is important to understand the basic physics of the system. The generated mechanical power is given by [17- 19] Pmech=Tmech(t)ωR(t) (1) Where, Tmech is the mechanical torque. For simplification, the generated electric power of a one-phase generator is given by Pe(t)=Va(t)Ia(t) (2) Fig 1:Block diagram of the proposed MPPT and FRT Method Va and Ia are the generator voltage and current respectively. Assuming no losses in the system, then Tmech(t).ωR(t)= Va(t)Ia(t) (3) The basic electrical and motion equations are Te = kIaIf (4) Ia=(Va-Ea)/Ra (5) Ea=kIaωe (6) Where, ωe = (p/2) ωR and p is the number of poles of the generator. Maximum power is at =0 (7)
  3. 3. International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.4, pp. 1305-1312 ISSN 2078-2365 http://www.ieejournal.com/ 1307 Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems Fig. 2. Typical power versus speed characteristics of a wind turbine. The power extracted from the wind can be controlled by varying the dc bus voltage, which is a function of If and ωe. Considering the wind turbine characteristics given in Figure 2, we know that the maximum power point is obtained whe ω = 0 (8) This equation can be written as: = =0 (9) According to equation (9),maximum power point is when: =0 (10) The function Pmech (Vdc) has a single point where maximum power extraction is achieved. It also means that the maximum power can be tracked by searching the rectified dc power, rather than environmental conditions, such as wind speed and direction. The MPPT algorithm is as follows. One initiates the maximum power searching process by setting an arbitrary dc side voltage reference Vref. The controller then measures both the dc side current and voltage, and calculates the initial electric power Po = VdcIdc. Next, the reference voltage Vref is increased by ΔVdc so that. Vref(k)= Vref(k-1)+ ΔVdc (11) Then the dc power is calculated with P(k) = Vdc(k)Idc(k). If P(k) is bigger than P(k - 1), the maximum power point has not been reached therefore, the voltage reference needs to be increased by ΔVdc and the dc power needs to be compared. This process will repeat until maximum power is reached. And if P(k) is less than P(k - 1), the dc voltage reference is then decreased by ΔVdc. In order to search for maximum power at any wind speed four conditions must be met. 1. If P(k)≥ P(k-1) and Vdc(k)≥ Vdc(k-1), the dc side voltage reference need to be increased by ΔVdc. This condition is met when the turbine operates on the low speed side of the power curve, shown on Fig 3. 2. If P(k)≥ P(k-1) and Vdc(k)< Vdc(k-1), the wind turbine is being operated in the high speed side and the dc reference voltage needs to be decreased by ΔVdc. 3. When P(k)< P(k-1) and Vdc(k)≥ Vdc(k-1), the maximum power point is passed and a step back must be taken, decreasing the reference voltage by ΔVdc. This condition is met when the turbine is operated in the high speed side of the dome and the power is decreasing. 4. When P(k)< P(k-1) and Vdc(k)< Vdc(k-1), the power is decreasing on the low speed side, therefore the voltage reference is to be increased by ΔVdc. Fig 3 Maximum Power Tracking Process In Figure 3, the power-speed plot is shown for three different wind speeds, where υ1 < υ 2 < υ 3. The arrows show the trajectory in which the turbine will be operated using the maximum power tracking algorithm explained above. If the wind speed is υ1, the controller will search for the maximum power. If the wind changes to υ3 the turbine is no longer being operated at the maximum power point so the controller will search for the new maximum power point. Fig 4. Flowchart for MPPT Algorithm
  4. 4. International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.4, pp. 1305-1312 ISSN 2078-2365 http://www.ieejournal.com/ 1308 Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems After reaching the maximum point it will operate the wind turbine at the optimal point until wind changes, thus searching for maximum power at any wind speed. In order to optimize the maximum power search algorithm presented above, a step that combines speed of convergence and accuracy of results was developed. The variable step method is based on the Newton Raphson method. The value of the root can be calculated as, Xn+1=Xn - (12) Where Xn is the current known value of X, ƒ(Xn) represents the value of the function at Xn, and ƒ’(Xn) is the derivative at Xn. The function ƒ (Xn) can be expressed as: ƒ(Xn)= ƒ(Vdc(k)) = = = Slope(k) (13) And ƒ’(Xn) as ƒ’(Xn)=ƒ’(Vdc(k)= = (14) Using (12,)(13) and (14), ΔVdc can be express as follows: ΔVdc = = (15) This variable step will allow the maximum power tracker to converge faster to the maximum power point and will decrease power oscillations due to large values of ΔVdc when maximum power is achieved. For protection the value of ΔVdc is limited. The ΔVdc limit can be changed based on the generator size and design parameters. B. DC/DC Converter Controller Fig 5 Typical DC-to-DC Converter Controller The maximum power tracker will generate a reference voltage that will be used to control the dc voltage at the rectifier dc side terminals. The dc-to-dc converter uses a simple feedback controller. The dc voltage reference is compared to the actual dc voltage, and the error signal is fed to a PI controller. The output signal is compared with a fixed frequency repetitive triangular waveform to deliver a signal that will turn ON or OFF the switch. This is shown in Fig 5. C. Control of ESS and BC The ESS and the BC are used to suppress the generator output power fluctuation in normal conditions by absorbing or releasing the pulsated power component from or to the grid, in which the power command, P*ESS, is obtained through a high-pass filter to the generator power [20]. The ESS power is regulated by an outer PI regulator, whereas the EDLC current is controlled by an inner PI regulator. Fig. 6. Control block diagram of the ESS and BC.
  5. 5. International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.4, pp. 1305-1312 ISSN 2078-2365 http://www.ieejournal.com/ 1309 Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems Before you begin to format your paper, first write and save the content as a separate text file. Keep your text and graphic files separate until after the text has been formatted and styled. Do not use hard tabs, and limit use of hard returns to only one return at the end of a paragraph. Do not add any kind of pagination anywhere in the paper. Do not number text heads- the template will do that for you. D. DC-Link Voltage Control During grid sags, the dc-link voltage of the back-to- back converters is controlled by the ESS instead of the LSC. Hence, an outer PI voltage controller is employed, which produces a current reference for an inner PI current controller. Fig. 6 shows the overview control block diagram of the ESS and the BC in both normal and grid sag conditions. Neglecting the power losses of the converters and considering the active power negligible flowing into the grid, the dynamic equation of the dc-link voltage is expressed as Pgen-PBC-PESS=0.5C(dV2 dc/dt) (16) where C is the dc-link capacitance, Pgen is the generator power, PBC is the power dissipated by the BC, and PESS is the power of the ESS computed from the ESS voltage, VESS, and the EDLC current, IESS, as PESS=VESS*IESS (17) From (16 ) in order to keep the dc-link voltage constant, the ESS and the BC should be able to absorb the generator powerfully. From the control block diagram shown in Fig. 6, the output of the dc-link voltage controller, I∗ ESS, is given as I*ESS=Kp2(Vdc*-Vdc)+ (Vdc*-Vdc) + (18) where Kp2 and KI2 are PI controller gains of the dc-link voltage control. In Fig. 6, IESS_max represents the maximum current of the ESS. By expanding a Taylor series of the dc-link voltage at operating point Vdc0, the following can be obtained: V2 dc=V2 dc0+2Vdc0(Vdc-Vdc0) (19) From (16)–(19), the dc-link voltage equation can be rewritten in the “s” domain as CVdc0sVdc=-VESSKp2(Vdc*-Vdc)-VESS (Vdc*-Vdc) (20) The transfer function of the voltage controller is derived as [21] ∗ = (21) where ξ is the damping ratio, and ωn is the natural frequency. It is indicated in (21) that the transfer function has a zero and two poles, which are always located in the left-half plane. Hence, control stability is achieved. E. EDLC Current Control To establish the current control law for the dc/dc converter, a voltage across the inductance, VLf , is investigated. The dynamic equation of the inductance voltage is expressed as [22], [23] VLf=Lf =DESSVdc-VESS (22) VLf=Lf =DESSVdc-VESS (22) where Lf is the boost inductance, and DESS is the duty cycle. As shown in Fig. 6, the output of the current controller, V ∗Lf , is given as V*Lf=Kpc(I*ESS-IESS)+ (I*ESS-IESS) (23) where Kpc and KIc are PI controller gains of the current control. The duty cycle is calculated by DESS=(VESS+V*Lf)/Vdc (24) Then, the gating signals for switches S1 and S2 are generated by comparing the duty cycle with the carrier wave of 2 kHz as shown in Fig. 6. F.BC Control During the grid disturbance, the ESS may not absorb the full generator power, and then the BC will be activated to dissipate the rest of power, PBC as PBC=Pgen-PESS (25) The BC is controlled by the switch S3 shown in Fig. 6.The duty ratio DS3 for the switch depends on PBC, which is expressed as DS3= PBC (26) where Rbc is the braking resistance.
  6. 6. International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.4, pp. 1305-1312 ISSN 2078-2365 http://www.ieejournal.com/ 1310 Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems G.Control of LSC The LSC controls the dc-link voltage, Vdc, to be constant under normal conditions. Cascaded control structure with an inner current control loop and the outer dc-link voltage control loop is applied . During grid voltage sags, however, the dc-link voltage is controlled by the ESS. Hence, the LSC is exploited as a STATCOM to supply the reactive current to the grid according to the requirements of the grid code. For this, the control strategy of the LSC is a voltage- controlled current source, in which the LSC is operated as a current source . In the case of unbalanced grid sags, the dual-current controllers for positive- and negative-sequence components are adopted for the LSC. The control block diagram of the LSC is shown in Fig. 7. In normal operation, the current references for positive- and negative-sequence current controllers are calculated from the output of the dc-link voltage controller as shown in Fig. 7 . During grid sags, dc- link voltage control by the LSC is deactivated, and the LSC injects the reactive current component. Hence, the reference of the active current component, Ip* qe is set to zero Ip* qe =0 (27) Fig. 7. Control block diagram of LSC The dq-axis current references In* dqe, of negative-sequence components are set to zero to eliminate the unbalanced current components flowing into the grid, which are expressed as In* qe =0 (28) In* de =0 (29) A proportional-integral (PI) controller is usually used for dc-link voltage control. Hence, an error accumulation in the integral regulator should be considered for fast transition when the dc-link voltage controller is reactivated after fault clearance. The PI controller is implemented in a discrete time domain. The actual dc-link voltage is able to track its reference by ESS control during the sag. Hence, an initial accumulated error, ΔVdc_I_int, of the integral regulator is set to zero as ΔVdc_I_int = 0. (30) IV SIMULATION RESULTS To perform the feasibility of the proposed scheme, Matlab simulations have been performed for a PMSG wind turbine system. The system parameters for the simulation are listed in Appendix . Fig 8 shows the variation of the wind speed and its corresponding output voltage of the PMSG. With the increase in wind speed the power fed to the grid also increases which is indicated by an increase in magnitude of PMSG phase voltage. At t =2.4 s, wind speed is changed from 8 to 12 m/s in step, whereas tip-speed ratio is maintained at Cp maximum in steady state conditions. Fig 8 (a) wind Speed (b) Phase voltage
  7. 7. International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.4, pp. 1305-1312 ISSN 2078-2365 http://www.ieejournal.com/ 1311 Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems Fig 9 (a) Grid voltage (b) Grid Current Fig 9 shows the performance of the LSC at voltage sags. Fig. 9(a) shows the grid voltage, where three grid-phase voltages drop to 20%, 20%, and 50%, respectively, during 0.5 s. Fig 9(b) shows the grid current which increased during the fault condition Fig 10(a) Grid Voltage (b) DC-Link Voltage The two-phase grid voltage interruption is considered as shown in Fig. 10(a) .With the proposed system, the dc-link voltage is controlled well by the ESS, which is shown in Fig.10(b). The increase in the dc-link voltage is less than 1.5%. Fig. 11 shows the performance of the ESS and the BC. The dc link voltage is controlled well as shown in Fig. 11(a), in which its transient value is less than 2.5%. Fig. 11(b) shows the ESS powers, in which the control performance is good for normal conditions. When the grid fault occurs, the power controller is deactivated, and the power is absorbed by the ESS as seen in Fig. 11(b) to maintain the dc-link voltage. The current control performance is shown in Fig. 11(c). Since the ESS is not able to absorb the full generator power, the rest of the power is dissipated by the BC. The BC current is shown in Fig. 11(d). When the EDLC absorbs the power from the wind system, the EDLC voltage is increased as shown in Fig. 11(e). Fig. 11. Performance of ESS and BC under unbalanced sag (a) DC-link voltage. (b) ESS power. (c) EDLC current. (d) Braking chopper current. (e) EDLC voltage. CONCLUSION This paper has proposed the maximum power point tracking algorithm to extract maximum energy from wind turbine system and combines the ESS and the BC for the LVRT in PMSG wind turbine systems which is an cost effective solution. The maximum power was tracked by searching the rectified dc power, rather than environmental conditions, such as wind speed and direction. Controlling the dc-link voltage by the ESS, the LSC is able to comply with the reactive current requirements of the grid code. By this, the grid voltage can be recovered rapidly without an external STATCOM after fault clearance. Also, the output power fluctuation of the wind turbine system operating in steady state is smoothened by the ESS. This control scheme offers an FRT
  8. 8. International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.4, pp. 1305-1312 ISSN 2078-2365 http://www.ieejournal.com/ 1312 Rajkumar and Suganthi Hybrid Control Scheme for Fault Ride-Through Capability using Line-Side Converter and an Energy Storage System for PMSG Wind Turbine Systems capability for the wind turbines even though the grid voltage is fully interrupted. Appendix: PMSG parameters: Stator resistance Rs=2.1Ω, Stator Inductance: 0.00083mH, inertia J= 0.01197 kgm2 , magnetic flux Φ=0.118 Wb and number of poles=4. Converter parameters: Low voltage side capacitor C1=500μF, High voltage side capacitor C0=3600μF, Inductor L=200mH, Switching frequency fd=20kHz, system frequency f=50Hz. ESS and BC parameters: ESS power ratingPESS-rated -0.6 KW, Capacitance of EDLCCEDLC-100 F, Operating VoltageVESS- 440 V,Power rating of BCPbc-rated-1.13 KW,Resistance Rbc-1.5 Ω REFERENCES [1] Rahul Nema, Anurag Trivedi “Application of Super Capacitor Based Energy Storage System for Power Quality Improvement of a Decentralized Power Plant” IEEJ Trans vol 04 no 1 pp 846- 855 [2] H. Li and Z. Chen, “Overview of different wind generator systems and their comparisons”, in IET Renewable Power Generation, Vol. 2, No. 2, pp. 123-138, 2008. [3] Rajveer Mittal, K.S. Sandhu and D.K. Jain., “An Overview of Some Important Issues Related to Wind Energy Conversion System (WECS)”, International Journal of Environmental Science and Development, Vol. 1, No. 4, pp. 351-362, October 2010. [4] Z. Chen and E. 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