IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008 1205 An Improved Control Strategy of Limiting the DC-Link Voltage Fluctuation for a Doubly Fed Induction Wind Generator Jun Yao, Hui Li, Yong Liao, and Zhe Chen, Senior Member, IEEE Abstract—The paper presents to develop a new control strategy of limiting the dc-link voltage fluctuation for a back-to-back pulsewidth modulation converter in a doubly fed induction generator (DFIG) for wind turbine systems. The reasons of dc-link voltage fluctuation are analyzed. An improved control strategy with the instantaneous rotor power feedback is proposed to limit the fluctuation range of the dc-link voltage. An experimental rig is set up to valid the proposed strategy, and the dynamic performances of the DFIG are compared with the traditional control method under a constant grid voltage. Furthermore, the capabilities of keeping the dc-link voltage stable are also compared in the ride-through control of DFIG during a three-phase grid fault, by using a developed 2 MW DFIG wind power system model. Both the experimental and simulation results have shown that the proposed control strategy is more effective, and the fluctuation of the dc-link voltage may be successfully limited in a small range under a constant grid voltage and a non-serious grid voltage dip. Index Terms—Back-to-back pulsewidth modulation (PWM) converter, DC-link voltage, doubly fed induction generator (DFIG), instantaneous power feedback, ride-through control, wind power generation. I. INTRODUCTION OUBLY fed induction generators (DFIGs) are popular configurations for large variable-speed constant-frequency wind generator systems [1]–[8]. As the penetration of wind power continually increases, more wind turbines are required to stay in grid connected during a grid fault. The DFIG may successfully ride through by using appropriate control strategies, and the power electronic devices [such as insulated gate bipolar transistors (IGBTs)] connected in rotor circuits may also be effectively protected during a grid fault [9]–[16]. A back-to-back pulsewidth modulation (PWM) converter is usually used in the rotor circuit of a large-scale DFIG system [1]. Several control methods have been proposed to control the rotor-side converter in order to realize the DFIG ride-through [9]–[16]. For example, an improved control strategy has been D Manuscript received March 1, 2007; revised October 23, 2007. Recommended for publication by Associate Editor J. Guerrero. J. Yao and Y. Liao are with the State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, China (e-mail: topyj@163.com; yongliaocqu@vip. sina.com). H. Li is with the State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, China and also with the Institute of Energy Technology, Aalborg University, Aalborg East DK-9220, Denmark (e-mail: cqulh@163.com). Z. Chen is with the Institute of Energy Technology, Aalborg University, Aalborg East DK-9220, Denmark, (e-mail: zch@ iet.aau.dk). Digital Object Identifier 10.1109/TPEL.2008.921177 proposed to control the rotor current to counteract the effect of the transient components in the stator flux, and make the DFIG ride through a grid fault [14]. Whatever the control strategy of the rotor-side converter is used, the grid-side converter should be properly controlled and the dc-link voltage should be kept stable to realize the ride-through control of a DFIG, if possible. However, when a grid fault occurs, the corresponding strategies of the grid-side converter and dc-link voltage have not been discussed in detail. The rotor-side converter may be protected with “crowbar protection” technology that shorting the rotor windings through resistors. After the fault is removed, the grid-side converter can be controlled again to establish the dc-link voltage; however, the dc-link voltage is likely to be fluctuated during this period. In addition, the rotor current control might be affected by the dc-link voltage fluctuation when the DFIG is running back to the normal condition. In the paper [6], the ride-though capability of a DFIG system has been evaluated under different levels of voltage dips. It is shown that power electronic converters may be kept in operation during a non-serious voltage dip. However, it is important to keep the dc-link voltage stable and limit the fluctuation of the grid-side converter current during a grid fault, so that the dc-link capacitor could be protected and the adequate voltage on the rotor could be provided. In this paper, Section II illustrates some reasons of dc-link voltage fluctuation based on the power flow characteristics of converter. In Section III, an improved control strategy for gridside converter is proposed to limit the dc-link voltage fluctuation. In Section IV, an experimental rig is presented, and some experimental results of the dynamic performances are obtained and compared with the traditional strategy when the grid voltage is constant. In Section V, a 2 MW DFIG wind power generation system with a back-to-back PWM converter is further simulated during a grid fault, and the proposed control strategy is also demonstrated. II. ANALYSIS OF DC-LINK VOLTAGE FLUCTUATION Fig. 1 shows the main circuit topology of a DFIG system with a back-to-back PWM converter, which is composed of a grid-side converter, a rotor-side converter and a dc-link capacitor [4]. Though a few schemes of control the dc-link voltage of the back-to-back PWM converter have been studied [17], [18] , an improved control strategy of limiting the dc-link voltage fluctuation in the DFIG system is further proposed in this paper. The grid-side converter is usually controlled with a vector control strategy with the grid voltage orientation [2]. The axis 0885-8993/$25.00 © 2008 IEEE Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply. 1206 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008 Fig. 1. Main circuit topology of a back-to-back PWM converter for DFIG. Fig. 2. Vector control block diagram of grid-side PWM converter. voltage component is fixed with the orientation of grid voltage space vector, and the axis voltage component is zero. So the active power and reactive power of the grid-side converter can be described as When the bidirectional power is dynamically changed between the grid-side converter and the rotor-side converter, the instantaneous power of the dc-link capacitor can be described as (1) (3) where is the axis component of grid voltage, and are the axis and axis components of grid current, respectively. The active power and reactive power can be independently controlled with the double closed-loop strategy [2]. Fig. 2 shows the control block diagram of a traditional vector control strategy for the grid-side converter. The outer voltage control loop and inner current control loop are used to realize the stable control of the dc-link voltage. In the back-to-back PWM converter of DFIG, the bidirectional power is transferred between the grid side and the generator rotor side. Under a constant dc-link voltage, the input power from the grid side should be equal to the input power of the generator rotor when ignoring the power losses of power electronic devices, the following (2) can be derived as: Therefore, from the viewpoint of keeping the instantaneous power balance, the power, is equal to the sum of the instanand taneous input rotor power (2) , and are the instantaneous grid voltages, rewhere , and are the instantaneous grid currents, spectively, is the instantaneous input power of grid-side respectively, converter, , and are the instantaneous rotor voltages , and are the instantaneous of a DFIG, respectively, is the instantaneous input rotor currents, respectively, and rotor power of a DFIG. (4) Thus, the above equation can be also rewritten as (5) Assuming the value of the instantaneous voltage, , is constant under a normal condition, the variation of dc-link axis component capacitor voltage is determined by the , and the instantaneous power, . The of grid current, current of dc-link capacitor will suddenly change as long is varied, so that it may make the dc-link voltage as fluctuate. In addition, for the grid-side converter, the dynamic response of outer dc-link voltage control loop is much slower than that of the inner current control loop, so that the gridside converter can not transfer enough instantaneous energy to the rotor-side converter when the generator rotor current suddenly increases. On one hand, the capacitor will release some stored energy to feed the rotor-side converter and the Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply. YAO et al.: IMPROVED CONTROL STRATEGY OF LIMITING THE DC-LINK VOLTAGE FLUCTUATION 1207 Fig. 3. Control block diagram of grid-side converter when grid voltage is constant. dc-link voltage will decline. On the other hand, when the rotor-side converter runs at the energy feedback status, the grid-side converter can not feed more instantaneous power back to the grid so that the dc-link voltage will increase due to the overmuch instantaneous energy. So, the dc-link voltage may fluctuate because of the imbalanced power flow between the input and output instantaneous energy of the converter during the dynamic regulation of DFIG. Furthermore, if the dc-link voltage excessively fluctuates, the DFIG might not realize the accurate power regulation even under the normal condition. III. IMPROVED CONTROL STRATEGY OF GRID-SIDE CONVERTER A. Control Strategy Under a Constant Grid Voltage As shown in Fig. 2, in the traditional control scheme, the acis usutive current setting value of the grid-side converter ally set as the output of the dc-link voltage PI controller when the grid voltage is constant. However, as the above analyzed, in order to keep the dc-link voltage stable, the setting value should be properly changed to realize the instantaneous power transfer. reflects the variAs it can be seen from (5), the term ation of the output power of the rotor-side converter. After the fault, the voltage of the grid-side converter will recover to the can be used as a steady state value. So, the item forward feed variable to describe the rotor power variation. In the proposed control strategy, the active current setting value of the grid-side converter is set to be the sum of the output of . In this the dc-link voltage PI controller and the item is named as a new variable , which case, the item represents the compensated current component of the instantaneous rotor power. Fig. 3 shows the proposed control block diagram of the grid-side converter when the grid voltage is conin active current setting value ( ) stant. The item is considered to keep the dc-link voltage stable in the proposed control strategy. B. Control Strategy During a Grid Voltage Dip From (4), the variation of the dc-link voltage can be also described as (6) Fig. 4. Control block diagram of grid-side converter during a grid voltage dip. When a grid fault occurs, the input grid voltage ( ) of the grid-side converter will drop down to a low value, and the gridside converter can not feed enough current ( ) to the rotor-side converter if the traditional double close-loop control strategy is used. In this case, the above two factors would introduce a between the instangreat instantaneous power difference taneous power ( ) of the grid-side converter and of the rotor-side converter, so that the control of dc-link voltage will be extremely restricted and the dc-link voltage would excessively fluctuate [13]. In fact, a grid fault period is rather short (100 ms 300 ms), and it may be not necessary to keep the dc-link voltage constant when a grid voltage drops. The ride-through control can be effectively realized as long as the fluctuation of the dc-link voltage is limited to a small range. As analyzed above, the fluctuation range of the dc-link voltage may be reduced by considering the effect of the instantaneous output power of the rotor-side converter. Thus, a single inner current control loop is proposed to limit the dc-link voltage fluctuation. Fig. 4 shows the control block diagram of the grid-side converter during a voltage dip. The active current setting value of the grid-side converter is ( ), which reflects the instantaneous output set to be power of the rotor-side converter, and enough instantaneous power of the rotor-side converter could be supplied with the input power of the grid-side converter. From (6), if the input current of the grid-side converter is fed enough and the instan) is nearly equal to , the instantaneous taneous power ( under such condition would be equal power difference to zero and the dc-link voltage would be kept as a constant. equals to zero However, it is difficult to realize that Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply. 1208 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008 Fig. 5. Block diagram of an experimental DFIG excitation system. because the instantaneous power ( ) is not enough equal when is low. So, as the single current regulation to control is used, the maximal input instantaneous power of the grid-side converter will depend on two factors: the grid voltage during the fault and the maximal input current . When the grid voltage drops down to a fixed value, the fluctuation range of the dc-link voltage will be ultimately determined ) of the by the current limitation (e.g., the peak value converter semiconductors. In the proposed control scheme, the should be limited by a saturator and then set as item the active current setting value of the grid-side converter. The maximal active current setting value should be set according to the peak current of the semiconductors to fully utilize the current capacity of the semiconductors. Then the inner current to the control loop can rapidly regulate the grid current setting value , the power error between the instanta) and with the proposed neous input power ( scheme is much smaller than that of the traditional scheme, i.e., , when the grid voltage drops down a same value. So, a smaller fluctuation range of dc-link voltage would be achieved by the proposed control strategy. After the grid fault is cleared, the control strategy of the gridside converter should be switched back to the control strategy described in Fig. 3. As the active current setting value ( ) is the sum of the output of the dc-link voltage PI controller and the , the grid current can be regulated smoothly item when changing the conwith respect to the reference current trol strategy, and the maximal current of the grid-side converter can be controlled in a safe range. IV. EXPERIMENTAL VALIDATION An experiment rig is set up to validate the proposed control strategy of the grid-side converter when the grid voltage is under the normal condition. Fig. 5 shows the block diagram of the DFIG experimental system. A digital signal processor TMS320F2812 made by TI Inc. is used to realize the high performance control algorithm. The two intelligent power modules (PM100RLA060) made by Mitsubishi Electric Corporation are used to construct the main power circuits including grid-side converter and rotor-side converter. A direct current motor is used to simulate the prime mover -wind turbine. The DFIG system parameters are given in the Appendix A. The active power and reactive power are controlled independently with the vector control strategy described in [2]. Figs. 6 and 7 show the experimental results including stator and rotor currents, active and reactive power and rotor speed of the generator when the grid voltage is constant. The active power, reactive power and rotor speed are calculated via TMS320F2812 and the waveforms are recorded. The stator and rotor currents are the actual waveforms recorded by the oscillograph. In this case, it is assumed to output active power to the grid when the active power is negative in the following figures, while the positive reactive power means the outputting lagging reactive power, and vice versa. Figs. 6 and 7 show the results when the generator changes operation mode suddenly from delivering reactive power to absorbing reactive power, respectively. Fig. 6 shows the results when the grid-side converter is controlled by the traditional double closed-loop strategy, and Fig. 7 shows the results by using the proposed control strategy in this paper. From Figs. 6(c) and 7(c), it can be seen that the rotor currents will rapidly decline when the generator absorbs reactive power, and the instantaneous input power of the converter should rapidly decline. Figs. 6(d) and 7(d) show the wavevariable ( ), grid active current setting forms of ) and grid active current ( ). As it can be seen value ( reference value drops from Figs. 6(d) and 7(d), the down rapidly, but in Fig. 6(d), the active current setting value declines slowly without instantaneous power feedback control. This has been also clarified in Fig. 6(b), the input grid , declines slowly. So, the dc-link voltage converter current, will rise up in Fig. 6(a) because of the overmuch instantaneous input power of the grid-side converter, and it will be harmful to the dc-link capacitor and converter. However, as from Fig. 7(d), the declines rapidly with the change of Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply. YAO et al.: IMPROVED CONTROL STRATEGY OF LIMITING THE DC-LINK VOLTAGE FLUCTUATION Fig. 6. Waveform diagram without instantaneous power feedback control (P with instantaneous power feedback control, and from Fig. 7(b), the current of the grid-side converter rapidly declines. The dc-link voltage can be kept constant in Fig. 7(a) while the generator absorbs reactive power and the generator operation status changes smoothly. 1209 = 01 kW, Q = 1 kvar to 0 1 kvar). Therefore, the experimental results demonstrated that the dynamic performance of the DFIG can be effectively improved by the proposed method when the reactive power of the DFIG is changed suddenly. It is helpful to keep the dc-link voltage stable and improve the DFIG system stability. Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply. 1210 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008 Fig. 7. Waveform diagram with instantaneous power feedback control (P = 01 kW, Q = 1 kvar to 0 1 kvar). V. SIMULATION COMPARISONS DURING A GRID FAULT In this section, a 2 MW DFIG wind power generation simulation system with a back-to-back PWM converter is set up to demonstrate the proposed control strategy for the grid-side con- verter under a grid fault. The simulated DFIG system parameters are given in the Appendix B. Fig. 8 shows the typical configuration of the DFIG wind generation system. Some assumptions for the simulation are as follows: 1) The generator rotor speed keeps as a constant during a grid fault. 2) Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply. YAO et al.: IMPROVED CONTROL STRATEGY OF LIMITING THE DC-LINK VOLTAGE FLUCTUATION Fig. 8. The configuration of DFIG system. The over-modulation PWM technology for the rotor-side converter is used to improve the ride-through control effect, and the maximal rotor voltage is assumed to be a high value (1.35 p.u.). 3) The rated maximal current of the grid-side converter is set to be 800 A, and the current limitation of the grid-side converter is 1700 A during a grid fault. 4) The over-modulation PWM technology for the grid-side converter is also used and the maximal control voltage is set to be 1.5 p.u. 5) A three-phase symmetrical grid fault occurs at 3.5 s at the wind farm busbar, and it is removed at 3.61 s. The fault causes the busbar voltage drops down to 0.6 pu. The fault location is at the high voltage side of the step-up transformer at the wind farm busbar. Before the fault, the generator is controlled with the vector control strategy described in paper [2]. The output reactive power of DFIG is zero and the generator outputs the rated active power (2 MW) at a speed of 1950 r/min (the maximal slip 0.3). When the fault occurs at 3.5 s, the DFIG is controlled to ride through the fault. In this case, the grid-side converter is controlled by the proposed control strategy described in Fig. 4, and the rotor-side converter is controlled by the control strategy described in [14] and the back-to-back converter is still connected to the generator. After the grid fault is cleared at 3.61 s, the control strategy of the grid-side converter is switched back to the proposed control strategy described in Fig. 3. In addition, when the grid voltage recovers at the time of 4 s, the rotor-side converter is controlled again under the normal condition by the vector control strategy described in paper [2]. Figs. 9 and 10 show the simulation results of the DFIG ridethrough control under a three-phase symmetrical grid fault. is the three-phase modulation indices of rotor voltage. is the grid voltage, and the is grid current of the grid-side converter. Fig. 9(a) and (b) show the simulation results of the DFIG system with the traditional control strategy of the grid-side converter during the fault and Fig. 10(a) and (b) show the simulation results with the proposed control strategy of the grid-side converter. 1211 As it can be seen from Figs. 9 and 10, the DFIG can successfully ride through the grid fault by using either of two different control schemes when the stator voltage drops down to 0.6 p.u. However, it can be observed from Fig. 9(b), the dc-link voltage excessively fluctuates in a large range with the change of grid voltage by using the traditional double closed-loop control. variable changes rapidly with the From Fig. 9(b), the change of the grid voltage, but the variable changes slowly. So, the grid-side converter can not feed the overmuch active current back to the grid or provide the enough active current to supply the dc-link capacitor and rotor-side converter, and the input peak current is only 1000 A. From Fig. 9(b), the maximal value of the dc-link voltage has arrived at 1520 V (the rated value is 1200 V) during the fault, and the capacitor would be under excessive voltage stress and possibly destroyed. The minimum value of the dc-link voltage is 960 V, and it would drop down to the much lower value if the input voltage of the grid-side converter declines more deeply. As it can be seen in Fig. 10(b), compared with the traditional control scheme, the fluctuation range of the dc-link voltage can be limited to 50 V by using the proposed control strategy. From Fig. 10(b), changes rapidly with the change of , so that the rapid response of the input grid current can be realized whatever the fault occurs or cleared. VI. CONCLUSION In this paper, an improved control strategy of limiting the dc-link voltage fluctuation is proposed for the grid-side converter based on the instantaneous power feedback scheme. Furthermore, a single inner current control loop is also proposed for the control during a grid voltage dip. An experimental rig is set up to validate the improved control strategy when the grid voltage is constant, and a 2 MW DFIG wind power generation system is simulated to demonstrate the proposed control strategy during the grid fault. The proposed control strategy has been validated. Both the experimental and simulation results have shown that the fluctuation of the dc-link voltage can be effectively controlled by using the proposed control strategy. Therefore, it may be helpful to improve the stability of the doubly fed induction wind power generation system during the grid faults. APPENDIX A EXPERIMENTAL SYSTEM PARAMETERS Machine parameters rated power: 7.5 kW Frequency: 50 Hz; pole pairs: 3 Connection: Y/Y; stator rated voltage: 380 V Stator rated current: 18 A; rotor rated voltage: 185 V Rotor rated current: 28 A; stator resistance: 0.8285 ; stator leakage inductance: 3.579 mH; rotor resistance: 0.7027 ; rotor leakage inductance: 3.579 mH; magnetizing inductance: 62.64 mH; inertia: 0.15 kg m . During the course of experiment, the line to line voltage of generator stator is 210 V, and the generator is connected with the grid via a step-up transformer. Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply. 1212 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008 Fig. 9. Simulation results of DFIG ride-through control (with the traditional control strategy of the grid-side converter). Fig. 10. Simulation results of DFIG ride-through control (with the proposed control strategy of the grid-side converter). Grid-side converter parameters 5 mH, 0.1 ; reactor: 6800 F 450 V; DC-link capacitor: DC-link resistance: 960 ; DC-link voltage reference value: 80 V; SPWM and unity power factor control. Base value in p.u. system: : 100 V, : 50 A, : 5000 VA APPENDIX B SIMULATION SYSTEM PARAMETERS Machine parameters: rated power: 2 mW Frequency: 50 Hz; pole pairs: 2 Connection: Y/Y; stator rated voltage: 690 V Ns/Nr: 0.45; stator resistance: 0.00488 p.u.; stator leakage inductance: 0.1386 p.u.; rotor resistance: 0.00549 p.u.; rotor leakage inductance: 0.1493 p.u.; magnetizing inductance: 3.9527 p.u.; H: 3.5 s. Step-up transformer parameters: rated power: 2.5 MW Frequency: 50 Hz; primary winding: 20 kV-Delta Secondary winding: 690 V-Yg; 0.0098 pu short circuit impedance: Grid-side converter parameters: 0.6 mH, 6m ; reactor: 38000 F; DC-link capacitor: DC-link voltage reference value: 1200 V; SPWM and unity power factor control. REFERENCES [1] S. Muller, M. Deicke, and R. W. De Doncker, “Doubly fed induction generator systems for wind turbines,” IEEE Ind. Appl. Mag., vol. 8, no. 3, pp. 26–33, May/Jun. 2002. [2] R. Pena, J. C. Clare, and G. M. Asher, “A doubly fed induction generator using back-to-back PWM converters and its application to variable-speed wind-energy generation,” Proc. Inst. Elect. Eng., vol. 143, no. 5, pp. 231–241, May 1996. Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply. YAO et al.: IMPROVED CONTROL STRATEGY OF LIMITING THE DC-LINK VOLTAGE FLUCTUATION [3] G. Ramtharan, J. B. Ekanayake, and N. Jenkins, “Frequency support from doubly fed induction generator wind turbines,” IET Renewable Power Generation, vol. 1, no. 1, pp. 3–9, 2007. [4] Y. Liao, L. Ran, G. A. Putrus, and K. S. Smith, “Evaluation of the effects of rotor harmonics in a doubly-fed induction generator with harmonic induced speed ripple,” IEEE Trans. Energy Conversion, vol. 18, no. 4, pp. 508–515, Dec. 2003. [5] F. Blaabjerg and Z. Chen, “Power electronics as an enabling technology for renewable energy integration,” J. Power Electron., vol. 3, no. 2, pp. 81–89, Apr. 2003. [6] T. Sun, Z. Chen, and F. Blaabjerg, “Transient stability of DFIG wind turbines at an external short-circuit fault,” Wind Energy, vol. 8, pp. 345–360, 2005. [7] B. Hopfensperger, “Stator-flux oriented control of a doubly-fed induction machine with and without position encoder,” Proc. Inst. Elect., vol. 147, no. 4, pp. 241–250, 2000. [8] A. Petersson, L. Harnefors, and T. Thiringer, “Evaluation of current control methods for wind turbines using doubly-fed induction machines,” IEEE Trans. Power Electron., vol. 20, no. 1, pp. 227–235, Jan. 2005. [9] J. Morren and S. W. H. de Haan, “Ridethrough of wind turbines with doubly-fed induction generator during a voltage dip,” IEEE Trans. Energy Conversion, vol. 20, no. 1, pp. 435–441, Jun. 2005. [10] J. B. Ekanayake, L. Holdsworth, X. G. Wu, and N. Jenkins, “Dynamic modeling of doubly fed induction generator wind turbines,” IEEE Trans. Power Syst., vol. 18, no. 2, pp. 803–809, May 2003. [11] T. Sun, Z. Chen, and F. Blaabjerg, “Voltage recovery of grid-connected wind turbines with DFIG after a short-circuit fault,” in Proc. 35th Annu. IEEE Power Electron. Spec. Conf., Aachen, Germany, 2004, pp. 1991–1997. [12] L. Holdsworth, X. G. Wu, J. B. Ekanayake, and N. Jenkins, “Comparison of fixed speed and doubly fed induction wind turbines during power system disturbances,” Proc. Inst. Elect. Eng. GTD, vol. 150, no. 3, pp. 343–352, 2003. [13] Y. He, J. Hu, and R. Zhao, “Modeling and control of wind-turbine used DFIG under network fault conditions,” Inst. Ciencia Eng. Mater., vol. 2, pp. 986–991, Sep. 2005. [14] D. Xiang, L. Ran, P. J. Tavner, and S. Yang, “Control of a doubly fed induction generator in a wind turbine during grid fault ride-through,” IEEE Trans. Energy Conversion, vol. 21, no. 3, pp. 652–662, Sep. 2006. [15] S. Seman, J. Niiranen, and A. Arkkio, “Ride-through analysis of doubly fed induction wind-power generator under unsymmetrical network disturbance,” IEEE Trans. Power Syst., vol. 21, no. 4, pp. 1782–1789, Nov. 2006. [16] L. Xu and Y. Wang, “Dynamic modeling and control of DFIG-based wind turbines under unbalanced network conditions,” IEEE Trans. Power Syst., vol. 22, no. 1, pp. 314–323, Feb. 2007. [17] L. Malesani, L. Rossetto, P. Tenti, and P. Tomasin, “AC/DC/AC PWM converter with reduced energy storage in the DC link,” IEEE Trans. Ind. Appl., vol. 31, no. 2, pp. 287–292, Mar./Apr. 1995. [18] N. Hur, J. Jung, and K. Nam, “A fast dynamic dc-link power-balancing scheme for a PWM converter-inverter system ,” IEEE Trans. Ind. Electron., vol. 48, no. 4, pp. 794–803, Aug. 2001. 1213 Jun Yao received the M.Eng. and Ph.D. degrees in electrical engineering from Chongqing University, Chongqing, China, in 2004 and 2007, respectively. He is currently a Lecturer of electrical machinery and apparatus at Chongqing University. His research interests include renewable energy and distributed generation. Hui Li received the M.Eng and Ph.D. degrees in electrical engineering from Chongqing University, Chongqing, China, in 2000 and 2004, respectively. In 2000, he joined the Electrical Engineering Department, Chongqing University, where he is currently an Associate Professor. Since 2005, he has been a Visiting Researcher and then a Postdoctoral Researcher at the the Institute of Energy Technology, Aalborg University, Denmark. His main research areas are renewable energy systems and distributed generation. Yong Liao received the M.Eng. degree in electrical machinery and the Ph.D. degree in power system control from Chongqing University, Chongqing, China, in 1988 and 1997, respectively. He is currently a Professor of electrical machinery and apparatus at Chongqing University. His research interests include the control of doubly fed electrical machines as used in renewable energy systems, including wind and micro-hydro generators. In 1998, he participated in the Global Development Programme of Rockwell Automation, Milwaukee, WI. From 2001 to 2002, he was a Visiting Professor at Norhtumbria University, Newcastle, U.K. Zhe Chen (M’95–SM’98) received the B.Eng. and M.Sc. degrees from Northeast China Institute of Electric Power Engineering, Jilin City, China, and the Ph.D. degree from University of Durham, Durham, U.K. He was a Lecturer and then a Senior Lecturer with De Montfort University, Leicester, U.K. In 2002, he became a Research Professor and is now a Professor with the Institute of Energy Technology, Aalborg University, Denmark. He has more than 140 publications in his technical field. His background areas are power systems, power electronic, s and electric machines; and his main current research areas are renewable energy and modern power systems. Dr. Chen is an Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS, a Member of the Institution of Engineering and Technology (London, U.K.), and a Chartered Engineer in the U.K. Authorized licensed use limited to: CHONGQING UNIVERSITY. Downloaded on December 8, 2008 at 20:35 from IEEE Xplore. Restrictions apply.