Voltage-Sag Tolerance of DFIG Wind Turbine With a ... - IEEE Xplore

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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 25, NO. 4, DECEMBER 2010

Voltage-Sag Tolerance of DFIG Wind Turbine With a Series Grid Side Passive-Impedance Network Xiangwu Yan, Member, IEEE, Giri Venkataramanan, Senior Member, IEEE, Patrick S. Flannery, Member, IEEE, Yang Wang, Student Member, IEEE, Qing Dong, and Bo Zhang

Abstract—Due to the increase of the number of wind turbines connected directly to the electric utility grid, new regulator codes have been issued that require low-voltage ride-through capability for wind turbines so that they can remain online and support the electric grid during voltage sags. Conventional ride-through techniques for the doubly fed induction generator (DFIG) architecture result in compromised control of the turbine shaft and grid current during fault events. In this paper, a series passive-impedance network at the stator side of a DFIG wind turbine is presented. It is easy to control, capable of off-line operation for high efficiency, and low cost for manufacturing and maintenance. The balanced and unbalanced fault responses of a DFIG wind turbine with a series grid side passive-impedance network are examined using computer simulations and hardware experiments. Index Terms—Doubly fed induction generator (DFIG), low voltage ride-through (LVRT), voltage sag ride through, wind turbine.

NOMENCLATURE I Current space vector. Voltage space vector. V Z Impedance. Flux space vector. λ ω Speed. All parameters and quantities are considered to be transformed to the stator side. Subscripts a, b, c Three-phase stationary reference frame. dc Direct current. PCC Point of common coupling between wind farm and grid. q,d Real axis and negative imaginary axis of synchronous reference frame. s,r,g Stator, rotor, grid side. w Wind.

Manuscript received January 5, 2010; revised April 8, 2010, and May 19, 2010; accepted May 22, 2010. Date of publication August 12, 2010; date of current version November 19, 2010. This work was supported in part by the China Scholarship Council and Natural Science Foundation of Hebei under Grant E2009001400 and in part by the National Natural Science Foundation of China under Grant 50977027. Paper no. TEC-00004-2010. X. Yan, Q. Dong, and B. Zhang are with the Department of Electrical Engineering, North China Electric Power University, Baoding 071003, China (e-mail: [email protected]; [email protected]; [email protected]). G. Venkataramanan and Y. Wang are with the Department of Electrical Machine and Computer Engineering, University of Wisconsin–Madison, Madison, WI 53706 USA (e-mail: [email protected]; [email protected]). P. S. Flannery is with the American Superconductor in Middleton, Middleton, WI 53562 USA (e-mail: [email protected]). Color version of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TEC.2010.2054097

I. INTRODUCTION S a result of the doubly fed induction generator (DFIG) wind turbine’s large but lightweight mechanical structure and power electronics interface, during extreme point of common coupling (PCC) voltage sags, very high currents are induced in the rotor circuit which can damage the rotor-side converter and cause undue fatigue on the gear box [1].Older utility-connection codes allowed wind-turbine disconnection in the event of grid voltage sag below 0.8 p.u. (per unit) [2], [3]. In the recent years, due to the increase of the number of wind turbines connected directly to the electric-utility grid, new regulator codes have been issued that require low-voltage ride-through (LVRT) capability for the wind turbine. Instead of disconnection, the wind turbines have to support the electric grid during voltage sags [3], [4]. From the aspect of preventing voltage collapse, the requirement typically emphasizes provision of reactive current as a function of the sag depth within the turbines capability. In order to manage these problems, several ridethrough options for the conventional DFIG architecture have been proposed. Two modifications to the rotor circuit including the addition of either a silicon-controlled rectifier (SCR) rotor-crowbar circuit [1] or a three-phase rectifier and modulated resistive load have been demonstrated to improve in the DFIG ride-through capability [5], [6]. As an alternative, brief disconnection of the stator windings during voltage sag via an SCR static switch [7] has also been shown to reduce torque and current spikes for sags down to 15% of the nominal voltage. A modified rotor-current control method has been shown to protect the machine-side converter (MSC) for wind-turbine terminal voltage down to about 30% of the nominal voltage, with residual torque spikes and oscillations [8]. An exploration of the series grid-side converter (GSC) DFIG architecture [9]–[11] revealed not only an excellent potential for voltage sag ride through but also the short comings in power processing capability. A unified DFIG architecture in which the series GSC is partnered with a parallel grid-side rectifier is presented as an alternative for both DFIG wind-turbine power processing and robust voltage sag ride through in [12]–[14]. Further an approach for LVRT for a DFIG wind turbine using the passive-impedance networks was presented in [15]. The presence of the passiveimpedance network in series with the stator side during the grid fault allows a direct mechanism to prevent uncontrollable stator flux dynamics. Simulation results show its excellent capacity for a DFIG wind-turbine LVRT in balanced grid fault. Based on the practicality and cost, an optimized passiveimpedance network in series with the stator side for the purposes of damping synchronous frame stator-flux oscillations is

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0885-8969/$26.00 © 2010 IEEE

YAN et al.: VOLTAGE-SAG TOLERANCE OF DFIG WIND TURBINE WITH A SERIES GRID SIDE PASSIVE-IMPEDANCE NETWORK

TABLE I POSITIVE AND NEGATIVE SEQUENCE PHASORS (PER UNIT) FOR EACH SAG TYPE AS A FUNCTION OF PER UNIT CHARACTERISTIC VOLTAGE N V

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with positive and negative sequence components expressed in the stationary frame as V˙ PCC = N˙ p V˙ nom ej ω e t + N˙ n V˙ nom e−j ω e t .

(2)

B. New Requirements of LVRT Capability for DFIG Wind Turbine

presented in this paper. The effects of unbalanced grid faults on a DFIG wind turbine with series passive-impedance network and without any countermeasure are discussed in detail. Further simulation and experimental results show this passive-impedance network in series with the DFIG stator side has excellent capacity for a DFIG ride through PCC voltage sag to 15% of the nominal voltage. II. VOLTAGE SAG AND ITS EFFECT AND NEW REQUIREMENTS OF LVRT CAPABILITY FOR THE WIND TURBINE A. Voltage Sag and its Effect In a three-phase grid, a number of different fault types can occur such as single-phase-to-neutral, phase-to-phase, two-phaseto-neutral, and three-phase faults. Different fault types lead to different voltage sags. A classification commonly used is that the voltage sags are classified into seven types as discussed in [16] and [17], namely, A, B, C, D, E, F, and G. The voltage sag at the terminals of the wind turbine depends on several factors including the equivalent-network model, fault location, fault type, and properties of the interface transformer [16]. For unbalanced faults, the method of symmetrical components must be used to determine the remaining phase voltages [18], [19]. The four different types of faults result in different voltage phasor responses at the wind-turbine terminals, depending on the nature of the transformer connections between the fault point and the wind turbine [16], [17]. As the presence of Δ/Y transformers between the PCC and stator terminals prevents the occurrence of a zero-sequence component voltage at the DFIG wind turbine terminals during all types of sag events, voltage sag types B and E are omitted from discussion since they are not seen at the wind-turbine terminals, leaving only four types of possible voltage sags, namely, A, C, D, and F. Sag type G is also considered in the case of a wind turbine with only one interface transformer. The p.u. characteristic voltage N˙ v , the p.u. ratios N˙ p and N˙ n of positive and negative sequence voltage sag is defined as N˙ v =

V˙ V˙ nom

, N˙ p =

V˙ p V˙ n , and N˙ n = . V˙ nom V˙ nom

(1)

Positive and negative sequences sag p.u. phasors N˙ p and N˙ n for each sag type seen at the wind-turbine terminals are presented in Table I [14]. The voltage seen at stator terminals of a DFIG wind turbine in a typical wind farm during voltage sags can be represented

Considering the relevant voltage-sag ride-through standards and safe operation, a DFIG wind turbine has to fulfill the following requirements [2], [4]. 1) DFIG wind generators are required to withstand a threephase fault with nine cycles (150 ms) at the voltage sag up to 15% of the nominal voltage and single line to ground faults with delayed clearing time. 2) Uninterrupted feeding of a defined current into the grid for grid protection and system safety during grid faults. 3) Maintaining instantaneous dc-link voltage within device rating and instantaneous device (or phase) currents within twice nominal ratings. 4) Avoiding of torque transients beyond the permitted stress level to gear and drive shafts, i.e., 2.0–2.5 p.u. torque. 5) Feeding the maximum possible active power to the grid as soon as possible after the fault is cleaned. III. PRINCIPLE OF VOLTAGE-SAG RIDE THROUGH OF A DFIG WIND TURBINE WITH PASSIVE-IMPEDANCE NETWORK A. Concept of Voltage-Sag Ride-Through of a DFIG Wind Turbine With Passive-Impedance Network [15] As we know, a DFIG wind turbine converts aero-kinetic energy to electrical energy. A DFIG wind turbine provides electrical power to the grid, which can be equivalently considered as a power source connected to an infinite-bus power system. A single-phase simplified equivalent circuit is shown in Fig. 1(a). If a three-phase network short circuit happens at PCC, the system is divided into two parts; the DFIG is shorted through transformer ZT and part of line Zl . A simulated response of 2 MW DFIG wind turbine to PCC voltage sag to 15% of the nominal voltage is presented in [15]. The stator flux λs changes completely, we can also see the transient value of stator current I s and rotor current I r reaches almost four times of the rated value, driver shaft torque Te changes to almost seven times of the rated value. Therefore, grid-voltage sags can be detrimental to the DFIG wind turbines. One idea to stabilize the stator flux λs , current I s , I r , and torque Te under grid-voltage sag is to insert an equivalent impedance between DFIG and grid as shown in Fig. 1(b). The equivalent system seen from the DFIG is illustrated in Fig. 1(b), if the connected impedance meets the expression as following: Zeq I s = V g + Zg I s Zeq =

Vg Is

(3) + Zg

where V g is prefault equivalent electromotive force of grid, Zg is prefault equivalent impedance of grid, and Is is prefault equivalent stator current.

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Fig. 2. Single-phase equivalent circuit model of the passive-impedance network.

recovery is also a step change, we need to avoid large changes in the stator flux λs , the current I s and I r , and the torque Te ; therefore, it is necessary to hold the topology on line until full grid voltage recovery as shown in Fig. 1(c). After the full recovery, switch Ss is closed and switch Sp is opened through voltage cross-point or other methods as shown in Fig. 1(d). This description presents the concept of the passive-impedance network for DFIG wind-turbine voltage sag ride through. B. Model of Passive-Impedance Network and its Control Strategy A single-phase simplified equivalent circuit of the proposed system is presented in Fig. 2. As can be observed from the figure, the passive-impedance network consists of series element with a solid-state bypass switch and a shunt element with a solid-state isolating switch. The series impedance is used for modifying the stator flux, limiting short-circuit current, maintaining grid connection, and uninterruptedly feeding current into grid during the grid fault. The shunt impedance is used to balance the energy of the wind turbine during the grid fault. The shunt element and series elements are in inactive mode during steady-state operation, i.e., the series element is bypassed and the shunt element is isolated on normal operation. Here, Zp is a three-phase shunt impedance, Zsc is a threephase series impedance, Sp is a three-phase solid-state isolating switch, and Ss is a three-phase solid-state bypass switch. The algebraic model of the passive-impedance network can be expressed as follows: Fig. 1. Single phase simplified equivalent circuit of DFIG wind turbine with passive-impedance networks. (a) DFIG wind turbine on normal operation. (b) DFIG with series equivalent impedance during grid voltage fault. (c) DFIG with passive-impedance networks on grid voltage recovery. (d) DFIG with series passive-impedance networks on normal operation.

We can find equivalent impedance Zeq according to expression (3). In the ideal case, the equivalent impedance Zeq can be inserted into system without any delay; the stator flux λs , the current I s and I r , and the torque Te will not change at all during the grid-voltage sag. Further, in order to meet new code requirements, such as uninterrupted feeding of a defined current into the grid for grid protection and system safety, and to balance wind power, which is useful for controller regulation of DFIG wind turbine during the grid fault, a more detailed passive-impedance network topology is presented as Fig. 1(c). However, voltage

I s = Sp

(V g − V s ) Vs + Ss Zp Zsc

(4)

where Sp = 1 means isolated switch Sp is closed, Sp = 0 means isolated switch Sp is opened. Switch Ss is similar to switch Sp , Ss = 1 means the bypass switch Ss is closed. and Ss = 0 means the bypass switch Ss is opened. Control logic of the passiveimpedance network is expressed in Fig. 3 To meet the stator-current limit (2 p.u.), the series impedance Zsc value is set to Zb , andZb = UAN /IA , where UAN is the rated value of the stator-phase voltage of DFIG, IA is the rated value of the stator-phase current of DFIG. Based on the requirements of the DFIG power balance during the grid fault, the shunt impedance Zp value is set to equal to Zb . The bypass switch Ss and the isolation switch Sp of the impedance network are designed as the three-phase switch operated at the same time,


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TABLE II WIND-TURBINE SYSTEM SIMULATION PARAMETERS

Fig. 3.

Control logic of the passive-impedance network.

Fig. 4. Model block diagram of a grid connecting wind turbine with a doubly fed induction generator.

respectively, for all of the balanced and unbalanced voltage sags. Based on the analytical analysis, the timing requirements of the insertion/switching of the network are not very high, especially a half-cycle delay in the insertion of the network is acceptable; moreover, the network should be withdrawn within one to five cycles after the grid voltage recovery. IV. INFLUENCE OF BALANCED AND UNBALANCED GRID FAULTS ON A DFIG WIND TURBINE A. Simulation Model of a Grid Connecting DFIG-Based Wind Turbine With Control System A complete wind-turbine model includes the wind-speed model, the aerodynamic model of the wind-turbine rotor, the mechanical model of the transmission system, and models of the electrical components, namely, the DFIG, the pulsewidth modulation back-to-back voltage source converters, the transformer, and the wind-farm collection feeder network. Fig. 4 shows a model block diagram of a grid connecting megawatt scale wind turbine with a doubly fed induction generator, and its main parameters are shown in Table II. A d–q synchronous reference frame is chosen for modeling the DFIG. The model of the doubly fed induction machine is based on the fifth-order two axes representations that are commonly known as the “Park model” [20]. The synchronous rotating reference frame is used with the direct-axis aligned with the stator-voltage vector. In this way, the decoupled control of the electrical torque and the rotor-excitation current is obtained. When modeling the DFIG, the motor convention will be used, which means that the currents are inputs and that real power and reactive power have a negative sign when they are fed into the grid. The following simplifying assumptions are made in the development of the model.

1) The iron losses, mechanical losses, and power-converter losses are negligible. 2) The magnetic circuit of the machine can be represented by a linear model. 3) The entire mechanical system can be modeled using a lumped inertia parameter referred to the electrical angle and speed of the induction generator. 4) The power converters can be modeled using state-space averaged representation to represent their low frequency dynamics. 5) The wind-farm collection network to PCC is electrically stiff. A full feedback-control model includes control of the windturbine torque, power extraction, and grid reactive power by controlling the currents GSC and MSC, respectively. A high bandwidth PI regulator is implemented to control the rotor current via the MSC. With d–q synchronous reference frame alignment, q-axis component of the rotor current is proportional to torque and d-axis component of rotor current supplies reactive power. B. Dynamic Response of a Grid Connecting DFIG-Based Wind Turbine With and Without Series Passive-Impedance Network During Grid Balanced and Unbalanced Faults The proposed ride-through approach for DFIG using passiveimpedance network under balanced conditions was simulated

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TABLE III PARAMETERS USED IN HARDWARE SYSTEM

Fig. 5. Response of a 2 MW DFIG wind turbine with and without series passive-impedance network at Type “A” (3 ϕ fault) voltage sag down to 15% of the nominal voltage. (a) “A” type of voltage sag without any countermeasures. (b) “A” type of voltage sag with passive-impedance network.

in detail for a candidate 2 MW system as presented in [15]. Unbalanced voltage sag cases were discussed to show their detrimental effects to a DFIG wind turbine on the rotor-side converter and rotor shaft when no LVRT countermeasure is taken [21]. This paper studies the performances of a DFIG wind turbine with series passive-impedance network for LVRT under unbalanced voltage sag cases. As can be seen in Figs. 5(a)–8(a), the grid fault can lead to considerable over-currents, over-voltages, and over-torque, putting the whole facility under stress when no LVRT countermeasure is taken. In detail, it is clear that the A-type voltage sag leads to the highest stress on the whole facility, D- and F-type of voltage sags are similar, they not only lead to very high over-currents, over-voltages, and over-torque, but also lead to torque reverse. The C- and G-type of voltage sags are similar too, but they have shallower stress to the terminal connected to wind turbines at the same voltage sag depth. At the same time, the dc-link voltage during type A, D, and F voltage sags exceeds the limits, likely destroying the back-to-back converter. It should be noted that the most serious situation happens at the voltage-recovery stage in all kind of voltage sags. Due to the space limitations and that the voltage sag types D and F is similar, the characteristics of voltage sag type F are not included in this discussion. Further, Figs. 5(b)–8(b) demonstrate the dynamic responses of a DFIG wind turbine using series passive-impedance network under typical voltage sags. It is shown that a DFIG wind turbine using series passive-impedance network can fully relieve the stress of the whole facility through mitigating sag effects on the stator flux. The rotor current, dc-link voltage, and torque fulfill the requirements at the same time. In addition, the DFIG wind turbines can uninterruptedly feed current, active power, and reactive power into the grid during voltage faults. After the fault, the DFIG wind turbine can feed the maximum possible active power to the grid almost immediately after the fault is cleaned.


Fig. 6. Response of a 2 MW DFIG wind turbine with and without series passive-impedance network at type “C” (2 ϕ fault) voltage sag down to 15% of the nominal voltage. (a) “C” type of voltage sag without any countermeasures. (b) “C” type of voltage sag with passive-impedance network.

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Fig. 7. Response of a 2 MW DFIG wind turbine with and without series passive-impedance network at type “D” (1 ϕ fault to ground) voltage sag down to 15% of the nominal voltage. (a) “D” type of voltage sag without any countermeasures. (b) “D” type of voltage sag with passive-impedance network.

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Fig. 9.

Illustration of the experimental hardware setup.

Fig. 10. Experimental results, voltage sag type “A” (3 ϕ fault), Nv = 0.15, ω r = 1.20 p.u. From top to bottom: (a) vfa b c (25 V/div); (b) ira b c (10 A/div); (c)λq d s (0.2 Wb/div); (d)vd c (25 V/div), isa b (10 A/div); 50 ms/div.

Fig. 11. Experimental results, voltage sag type “C” (2 ϕ fault), Nv = 0.15, ω r = 1.20 p.u. From top to bottom: (a) vfa b c (25 V/div); (b) ira b c (10 A/div); (c)λq d s (0.2 Wb/div); (d)vd c (25 V/div), isa b (10 A/div); 50 ms/div.

V. LABORATORY SCALE TEST DEMONSTRATION Fig. 8. Response of a 2 MW DFIG wind turbine with and without series passive-impedance network at type “G” (2 ϕ fault to ground) voltage sag down to 15% of the nominal voltage. (a) “G” type of voltage sag without any countermeasures. (b) “G” type of voltage sag with passive-impedance network.

The simulation results in this section indicated excellent performance of the 2 MW DFIG wind-turbine system with series passive-impedance network under a variety of sag conditions. Hardware demonstrations were carried out on a 2-kW DFIG setup as described in this section.


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B. Hardware Results Scope captures from balanced and unbalanced voltage sag events on the 2 kW laboratory scale hardware test-bed are presented in Figs. 10–13. In each case, the DFIG speed is 1.20 p.u., running near rated power at nominal voltage and frequency. The scope image captures show response of the DFIG to type A, C, D, and G sags representative of three-phase, phase-to-phase, onephase-to-ground, and two-phase-to-ground faults, each with a characteristic p.u. voltage, Nv of 0.15. The rotor currents (see frame (b) of figures) have some second-harmonic component, but they are within the bounds of the MSC current rating. VI. CONCLUSION

Fig. 12. Experimental results, voltage sag type “D” (1 ϕ fault to ground), Nv = 0.15, ω r = 1.20 p.u. From top to bottom: (a) vfa b c (25 V/div); (b) ira b c (10 A/div); (c)λq d s (0.2 Wb/div); (d)vd c (25 V/div), isa b (10 A/div); 50 ms/div.

This paper has demonstrated the capability of LVRT of a DFIG wind turbine using a stator-side series passive-impedance network at balanced and unbalanced short-circuit grid faults. The presence of the series passive-impedance network allows a mechanism to mitigate the effects of sags on the stator flux. Simulation and experimental results of the designed system show excellent performance of LVRT in the balanced and unbalanced grid short-circuit fault. It also indicates many advantages by using stator-side series passive-impedance network for ride through at a variety of sag events, e.g., the simple control logics, avoiding the phenomenon of the impact of PCC voltage sag on orientation vector, and more practical and reliable implementations. Furthermore, the assistant equipment used for DFIG wind turbine for the LVRT is fully independent, thus, it is convenient to install, operate, and maintain. Finally, this assistant equipment also indicates lower cost for application for MW scale DFIG wind turbines. APPENDIX See Table II and Table III. ACKNOWLEDGMENT

Fig. 13. Experimental results, voltage sag type “G” (2 ϕ fault to ground), Nv = 0.15, ω r = 1.20 p.u. From top to bottom: (a) vfa b c (25 V/div); (b) ira b c (10 A/div); (c)λq d s (0.2 Wb/div); (d)vd c (25 V/div), isa b (10 A/div); 50 ms/div.

The authors would like to thank for the support and motivation provided by the Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC) of the University of Wisconsin– Madison.

A. Experimental Hardware Setup An illustrationof the experimental hardware setup is presented in Fig. 9. The electrical parameters are shown in Table III. The shaft of the DFIG is driven by a permanent magnet ac motor. A grid emulator generates desired ac voltage sags at the point of interconnection. The rotor windings of the DFIG are accessed via slip rings and connected to the MSC. The dc bus of the MSC is shared by the GSC. Each of the inverters is controlled by one of the two DSP/FPGA control boards, and is sine-triangle modulated with third-harmonic injection. Switching and sample frequencies are both set to 5 kHz. The rotor position is determined from an encoder, and is also used for rotor-speed estimation in state feedback decoupling terms. The angle of the voltage vf is estimated from a phase locked loop for use in synchronous frame controllers for the MSC and GSC.

REFERENCES [1] J. Morren and S. W. H. de Haan, “Ride-through of wind turbines with doubly-fed induction generator during a voltage dip,” IEEE Trans. Energy Convers., vol. 20, no. 2, pp. 707–710, Jun. 2005. [2] P. Fairley, “Steady as she blows,” IEEE Spectr. Mag., vol. 40, no. 8, pp. 35–39, Aug. 2003. [3] I. Erlich and U. Bachmann, “Grid code requirements concerning connection and operation of wind turbines in Germany,” in Proc. IEEE Power Eng. Soc. Gen. Meet., Jun. 2005, pp. 1253–1257. [4] Federal Energy Regulator Commission, Regulatory Order 661A: Interconnection for Wind Energy, 2005 [Online]. Available: http://www.ferc.gov/industries/electric/indus-act/gi/wind.asp.. [5] J. Niiranen, “Voltage dip ride through of a doubly fed generator equipped with active crowbar,” presented at the Nordic Wind Power Conf., Gothenburg, Sweden, 2004. [6] I. Erlich, H. Wrede, and C. Feltes, “Dynamic behavior of DFIG-based wind turbines during grid faults,” in Proc. 4th Power Convers. Conf., 2007, pp. 1195–1200.

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[7] A. Dittrich and A. Stoev, “Comparison of fault ride-through for wind turbines with DFIM generators,” in Proc. 11th Eur. Conf. Power Electron. Appl., Sep. 2005, pp. 1–8. [8] D. Xiang, L. Ran, P. Tavner, and S. Yang, “Control of a doubly fed induction generator in a wind turbine during grid fault ride-through,” IEEE Trans. Energy Convers., vol. 21, no. 3, pp. 652–662, Sep. 2006. [9] A. Petersson, “Analysis modeling and control of doubly-fed induction generators for wind turbines,” Ph.D. dissertation, Chalmers Univ. Technol., Goteborg, Sweden, 2005. [10] P. S. Flannery, G. Venkataramanan, “A grid fault tolerant doubly fed induction generator wind turbine via series connected grid side converter,” presented at WINDPOWER Conf., Pittsburg, PA, Jun. 2006. [11] P. S. Flannery and G. Venkataramanan, “Evaluation of voltage sag ridethrough of a doubly fed induction generator wind turbine with series grid side converter,” in Proc. 38th Annu. IEEE Power Electron. Spec. Conf., Jun. 2007, pp. 1839–1845. [12] P. S. Flannery and G. Venkataramanan, “A unified architecture for doubly fed induction generator wind turbines using a parallel grid side rectifier and series grid side converter,” in Proc. 4th Power Convers. Conf., Apr. 2007, pp. 1442–1449. [13] P. S. Flannery and G. Venkataramanan, “A fault tolerant doubly fed induction generator wind turbine using a parallel grid side rectifier and series grid side converter,” IEEE Trans. Power Electron., vol. 23, no. 3, pp. 1126–1135, May 2008. [14] P. S. Flannery, “Doubly fed induction generator wind turbines with series grid side converter for robust voltage sag ride-through,” Ph. D. dissertation, Dep. Electr. Mach. Comput. Eng., Univ. Wisconsin–Madison, Madison, WI, 2008. [15] X. Yan, G. Venkataramanan, P. S. Flannery, and Y. Wang, “Low voltage ride-through for DFIG wind turbines using passive impedance networks,” in Proc.1st Int. Conf. Sustainable Power Generator Supply (SUPERGEN), 2009. [16] M. H. J. Bollen, G. Olguin, and M. Martins, “Voltage dips at the terminals of wind power installations,” Wind Energy, vol. 8, pp. 307–318, Jul. 2005. [17] M. H. J. Bollen, Understanding Power Quality Problems, New York: IEEE Press, 2000. [18] J. L. Blackburn, Symmertical Components for Power Systems Engineering. New York: Marcel Dekker, Inc., 1993. [19] A. Bergen, V. Vittal, Power System Analysis, Upper Saddle River, NJ: Prentice-Hall, 2000. [20] D. Novotny and T. Lipo, Vector Control and Dynamics of AC Drives. Oxford, U.K.: Oxford Univ. Press, 2000. [21] X. Yan, G. Venkataramanan, P. S. Flannery, and Y. Wang, “Evaluation the effect of voltage sags due to grid balance and unbalance faults on dfig wind turbines,” in Proc.1st Int. Conf. Sustainable Power Generator Supply (SUPERGEN), 2009, pp. 1–10.

Xiangwu Yan (M’09) received the B.E. degree in electrical engineering from the Hunan University, Changsha, China, in 1986, the M.S. degree from the North China Electric Power University, Baoding, China, in 1990, and the Ph.D. degree from the Harbin Institute of Technology, Harbin, China, in 1997. He was an Honorary Fellow of the Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC), University of Wisconsin– Madison, Madison. He joined the North China Electric Power University as a Faculty Member, where he is currently involved in the research in electronic power conversion, power quality, and renewable energy generation as a Professor.

Giri Venkataramanan (M’92–SM’06) received the B.E. degree in electrical engineering from the Government College of Technology, Coimbatore, India, in 1986, the M.S. degree from the California Institute of Technology, Pasadena, in 1987, and the Ph.D. degree from the University of Wisconsin– Madison, Madison, in 1992. After teaching electrical engineering at Montana State University, Bozeman, he returned to the University of Wisconsin as a Faculty Member in 1999, where he is currently involved in the research in various areas of electronic power conversion as an Associate Director of the Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC), Madison. He holds several U.S. patents and is the coauthor of more than 100 technical publications.

Patrick S. Flannery (M’99) received the B.S. degree in mechanical engineering from the Pennsylvania State University, University Park, in 1998, the M.S. and Ph.D. degrees in electrical engineering from the University of Wisconsin–Madison, Madison, in 2003 and 2008, respectively. From 1998 to 2001, he was an Electromechanical Engineer at CSA Engineering in Mountain View, CA. He is currently a Principal Engineer at American Superconductor in Middleton, Middleton, WI. His research interests include the application power electronics, electric machines and control to renewable energy generation. Dr. Flannery is a member of the American Society of Mechanical Engineers.

Yang Wang (S’09) received the B.S. degree in electrical engineering from the Zhejiang University, Hangzhou, China, in 2007. He is currently working toward the M.S. and Ph.D. degrees in electrical engineering from the University of Wisconsin–Madison, Madison. His current research interests include power electronics, drives, and control.

Qing Dong received the B.S., M.S., and Ph.D. degrees from the North China Electric Power University, Baoding, China, in 1990, 1994, and 2003, respectively. He is currently an Associate Professor at the North China Electric Power University. His research interest includes power system robust control.

Bo Zhang was born in Hebei, China, in 1981. He received the B.S. and M.S. degrees in electrical engineering from the North China Electric Power University, Baoding, China, in 2005 and 2008, respectively. He is currently with the Department of Electrical Engineering, North China Electric Power University. His research interests include the application of power electronics in power system and PWM converter.