Variable-Speed Wind Power Generation Using

414 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 17, NO. 3, SEPTEMBER 2002

Variable-Speed Wind Power Generation UsingDoubly Fed Wound Rotor Induction Machine—A

Comparison With Alternative SchemesRajib Datta and V. T. Ranganathan, Senior Member, IEEE

Abstract—In this paper, a wind energy conversion system(WECS) using grid-connected wound rotor induction machinecontrolled from the rotor side is compared with both fixed speedand variable speed systems using cage rotor induction machine.The comparison is done on the basis of (I) major hardwarecomponents required, (II) operating region, and (III) energyoutput due to a defined wind function using the characteristics ofa practical wind turbine. Although a fixed speed system is moresimple and reliable, it severely limits the energy output of a windturbine. In case of variable speed systems, comparison showsthat using a wound rotor induction machine of similar rating cansignificantly enhance energy capture. This comes about due tothe ability to operate with rated torque even at supersynchronousspeeds; power is then generated out of the rotor as well as thestator. Moreover, with rotor side control, the voltage rating of thepower devices and dc bus capacitor bank is reduced. The size ofthe line side inductor also decreasesd. Results are presented toshow the substantial advantages of the doubly fed system.

Index Terms—Rotor side control, subsynchronous and super-synchronous generation, variable speed constant frequency oper-ation, wind energy conversion system, wind turbine, wound rotorinduction machine.

I. INTRODUCTION

EVEN THOUGH harnessing power from wind can betraced back approximately 4000 years, there has been a

renewed interest in the subject in recent years. Wind power hasproven to be a potential source for generation of electricity withminimal environmental impact [1]–[3]. With the advancementof aerodynamic designs, wind turbines that can capture severalmegawatts of power are available. When suchwind energyconversion systems(WECSs) are integrated to the grid, theyproduce a substantial amount of power, which can supplementthe base power generated by thermal, nuclear, or hydro powerplants.

A WECS can vary in size from a few hundred kilowatts toseveral megawatts. The size of the WECS largely determinesthe choice of the generator and converter system. Asynchronousgenerators are more common with systems up to 2 MW, beyondwhich direct-driven permanent magnet synchronous machinesare preferred. In this paper, we restrict our comparison to WECSwithin the power range of 100 kW to 2 MW and assume that the

Manuscript received July 25, 2000.R. Datta is with ABB Corporate Research Centre, 68526 Ladenburg, Ger-

many (e-mail: [email protected]).V. T. Ranganathan is with the Department of Electrical Engineering, Indian

Institute of Science, Bangalore 560012, India (e-mail: [email protected]).Publisher Item Identifier 10.1109/TEC.2002.801993.

Fig. 1. Three WECSs under consideration.

systems considered are gear based, use asynchronous machines,and integrated to the power grid.

It is obvious that grid-integrated WECS should generate atconstant electrical frequency, determined by the grid. How-ever, it is advantageous to vary the mechanical speed of theturbine/generator to maximize power capture with fluctuatingwind velocities. Therefore, WECS is a classic example of avariable speed constant frequency (VSCF) system. Rotor sidecontrol of a grid-connected wound rotor induction machine isvery attractive for such VSCF application, particularly whenthe speed range is limited. With a suitable integrated approachtoward design of a WECS, use of a slip-ring induction generatoris economically competitive, when compared to a cage rotorinduction machine. The higher cost of the machine due to theslip rings is compensated by a reduction in the sizing of thepower converters. The generator rating can also be reducedcompared to other singly fed machines.

An idealized Vs. curve, taken from [11], is shown inFig. 2. It is observed that the power coefficient is maximum fora particular tip-speed ratio. The prime motivation for variable

0885-8969/02$17.00 © 2002 IEEE

DATTA AND RANGANATHAN: VARIABLE SPEED WIND POWER GENERATION 415

Fig. 2. C versus. � characteristics.

speed control of WECS is to track the rotor speed with changingwind velocity so that is always maintained at its maximumvalue. Using the Vs. curve of Fig. 2, the power-speedcharacteristics are plotted for a commercially available turbine(Vestas V27) [12] by using aMathCAD program. In the fol-lowing sections, these power curves, as shown in Figs. 3 and 4,are used for comparison between the different control schemesand designs.

Constant speed constant frequency (CSCF) WECS usingcage rotor induction machines [Fig. 1(a)] are most widelyused because of their design simplicity and low cost. Variablespeed systems with cage rotor induction generators [Fig. 1(b)]are also commercially available, where energy output canbe substantially improved and unity power factor operationis possible [4], [5]. A grid-connected doubly fed inductiongenerator [Fig. 1(c)] is an interesting option with a growingmarket demand and presently, research is active on the variouscontrol aspects for VSCF application. Vector control [6],[7], [10] or direct torque control [8]–[10] methods have beensuccessfully employed to control the active and reactive powershandled by the generator. Although the individual technologiesare well reported, so far a comparative study of these systems isnot available. In this paper, a VSCF system using wound rotorinduction machine is compared against the existing fixed speedand variable speed systems using cage rotor machines on thebasis of

i) major hardware components required,ii) operating region,iii) energy output due to a defined wind function.

This exercise clearly brings out the superiority of the doublyfed induction machine in terms of generator sizing, generatedenergy and the cost of power electronics involved.

II. COMPARATIVE STUDY

A. Wind Turbine Characteristics

A wind turbine is characterized by its power-speed character-istics. For a horizontal axis wind turbine, the amount of power

that a turbine is capable of producing is given by

(1)

where is the air density, is the swept area (cross-sectionalarea) of the turbine and is the wind velocity. is called thepower coefficient and is dependent on the ratio between thelinear velocity of the blade tip ( ) and the wind velocity( ). This ratio, known as the tip-speed ratio, is defined as

(2)

where is the radius of the turbine.

Fig. 3. Power curves of the wind turbine against turbine shaft r/min (v1 =

5 m/s,v2 = 6 m/s. . . v10 = 14 m/s).

Fig. 4. Power curves of the wind turbine against generator shaft r/min withgear ratio of 1:23.4.

B. Conventional Fixed Speed System

A commercially available system is considered where aVestas V27 turbine is coupled to a 225 kW, 50-Hz inductiongenerator [Appendix]. The machine has two stator windings;one with six poles with a rated shaft speed of 1008 r/min andthe second with eight poles corresponding to a shaft speed of750 r/min. The maximum speed of the turbine shaft is 43 r/min.This requires a gearbox with a ratio of 43:1008 (i.e., 1:23.4).Once the rated power of the generator is reached, the turbinegoes intopitch controlmode (where the pitch angle of the bladeis mechanically adjusted to limit the turbine power transfer).

In Fig. 3, power curves of the turbine are plotted for wind ve-locities from 5 m/s to 14 m/s against the turbine shaft r/min andFig. 4 shows the same curves against the generator shaft r/minwith the gear ratio of 1:23.4. The operating locus for the con-stant speed system is given by the line A-B. The salient featuresof the fixed speed system are as follows.

i) It is designed for lower shaft speeds where the turbinepower curves (for different wind velocities) are close toeach other. Thus, natural protections against over-slipand overload is provided for the generator, but utiliza-tion of the turbine capability is poor.


Fig. 5. Operating region of WECS with cage rotor induction machine in theP � ! plane.

Fig. 6. Operating region of WECS with wound rotor induction machine in theP � ! plane.

ii) The lagging reactive power is compensated by the addi-tion of a large capacitor bank across the machine termi-nals. The switching of the capacitor banks gives rise toundesirable transients in the line currents and voltages.

iii) In order to boost the generated power under low windconditions, another winding of reduced power capacity(50 kW) is added to the motor with four pole pairs, with asynchronous speed of 750 r/min. The controller switchesover from one winding to the other depending upon thewind condition. This feature makes the machine nonstan-dard and expensive. Design compromises are also asso-ciated with this addition of a separate winding. It is seenthat the no-load current for the main generator windingis approximately 235 A (0.59 PU) (see the Appendix),which is very high for a machine of 225-kW rating.

(a)

(b)

(c)

Fig. 7. (a) Wind velocity function. (b) Turbine shaft speed for CSCF system.(c) Generator power for CSCF system.

C. Variable-Speed System Using Cage Rotor InductionMachine

A variable speed WECS enables enhanced power capture ascompared to a constant speed constant frequency system. Thepower limit is governed by the choice of generator rating, whilethe speed limit is dictated by the mechanical design of the tur-bine and the tower. Selection of the generator can be judiciouslymade based on the average wind velocity during the peak wind


(a)

(b)

Fig. 8. (a) Turbine shaft speed for VSCF system using cage rotor inductionmachine. (b) Generator power for VSCF system using cage rotor inductionmachine.

season. To present a comparative picture, the same turbine char-acteristics (Vestas V27) are considered. It is assumed that theturbine shaft speed is allowed to vary up to 120 r/min. (This im-plies a maximum tip speed of 170 m/s, which is reasonable for asystem of few hundred kilowatts rating [14].) It is also assumedthat the average wind velocity during the peak wind season is12 m/s.

From Fig. 3, it may be noted that the maximum power thatcan be delivered by the turbine at the wind velocity of 12 m/sis approximately 290 kW corresponding to the shaft speed of75 r/min. On this basis, a 300-kW, 415-V squirrel cage inductionmachine is selected. The synchronous speed of the machine iskept at 1000 r/min by using a six-pole machine. Assuming thatthe rated power is reached at the rated frequency, the gear ratioworks out to be 1000:75 (i.e., 13.3:1).

For variable speed control, the back-to-back PWM converterconfiguration as shown in Fig. 1(b) is used on the stator side.The stator side converter supplies the required reactive powerand also handles the full active power generated by the machine.The line side converter transfers the generated active power tothe grid at unity power factor and regulates the dc bus voltage.With a provision for 25% overloading, the ratings of the con-verters can be taken as 375 kVA. Potential wind sites are usuallyremote and the transmission grid is available at a higher voltage.The line side converter is interfaced to the power grid through a

transformer. Assuming the available grid voltage to be 6.6 kV,the transformer can be rated for 375 kVA with a voltage turnsratio of 6.6 kV: 415 V.

With the designed gear ratio, the V27 power curves areplotted in Fig. 5. The operating region is also marked on thecharacteristics. Once the cut-in wind velocity (3 to 4 m/s)is reached, the system is connected to the grid and it startsgeneration. Up to the rated operating point of the gener-ator corresponding to 300 kW, 1000 r/min, the system runs inpeak-power tracking mode. Beyond this point, the power is keptconstant with increasing speed. This is achieved by reducing thetorque through field weakening. With 33% reduction, the speedcan be increased upto 1500 r/min, the corresponding shaftspeed being 1500/13.3 [i.e., 113 r/min (within the allowablelimit)]. At this speed, the system goes into pitch-control mode,which restricts further increase of speed and power.

D. Variable-Speed System Using Wound Rotor InductionMachine

The system arrangement is shown in Fig. 1(c). In this case, therotor circuit is capable of bidirectional power flow allowing sub-synchronous and supersynchronous modes of operation. Duringsubsynchronous generation, the rotor circuit absorbs a fractionof the power generated by the stator, whereas under a supersyn-chronous condition, both the stator and the rotor feed in powerto the grid. Thus, if the stator generates 1 PU at a slip ofPU.,the total generated output is ( ) PU.

With the same assumptions regarding wind conditions andspeed range, a six-pole slip-ring induction machine of 300 kWis selected. One important design criterion for slip ring inductionmachines is the choice of rotor and stator turns ratio. It is advan-tageous to put lesser number of turns on the rotor side. However,this increases the current rating of the rotor winding. A compro-mise can be achieved by using a delta-connected stator windingand a star-connected rotor winding. The rotor turns can be made1/ times the stator turns to make the effective turns ratio 1:1;the current rating for the rotor winding is also not largely en-hanced. The synchronous speed being 1000 r/min and assumingthat rated stator power is reached at the rated frequency, the se-lected gear ratio remains same (i.e., 13.3:1).

The converter rating in this case depends on the range of op-erating speed. Assuming 0.5 PU slip on either side of the syn-chronous speed, the converter rating can be half the power ratingof the stator. Allowing the same amount of overloading as ear-lier, the converter can be rated for 3750.5 (i.e., 187.5 kVA).

The transformer connecting the system to a 6.6-kV gridshould have two secondaries; one winding connecting the statorbeing rated at 415 V and the second winding, connecting thefront end converter being rated at 415/2 (i.e., 208 V). Withoutthis voltage reduction on the rotor side, it is not possible tooperate at a lower dc bus voltage. Consequently, the voltageratings for the devices and the capacitor bank cannot be opti-mized. The rating of the transformer also has to be increasedby 50% due to the extra power being generated from the rotorside during supersynchronous operation. Therefore, a 3751.5(i.e., 560–kVA transformer is to be used with two windingshaving turns ratio of 6.6 kV:415 V/ 208 V).


(a)

(b)

(c)

(d)

Fig. 9. (a) Turbine shaft speed for VSCF system using wound rotor inductionmachine. (b) Stator power of wound rotor induction machine for VSCF system.(c) Rotor power of wound rotor induction machine for VSCF system. (d) Totalgenerated power for VSCF system using wound rotor induction machine.

The operating region of the WECS with rotor side controlis shown in Fig. 6. The speed of operation is limited to therange of 500 to 1500 r/min. When the wind velocity exceeds thecut-in value, the system is allowed to accelerate until the gener-ator shaft speed reaches 500 r/min. The system is connected tothe grid at this point and rotor side control is activated. Whilein operation, if the generator power falls below 40 kW (corre-sponding to 6 m/s of wind velocity), the rotor speed is main-tained at 500 r/min by operating in the speed control mode. Oncethe power exceeds 40 kW, the system goes into peak-powertracking mode upto the synchronous speed of 1000 r/min. Atthis operating point, the stator power has reached its limit andthe rotor power is zero (zero slip). This also corresponds to the

rated torque condition. From 1000 to 1500 r/min, the machineoperates at constant rated torque with power being recoveredfrom the rotor circuit as well. The total generated power fol-lows a straight-line locus above the synchronous speed with anadditional 150 kW being regenerated from the rotor side at 1500r/min. Therefore, operation upto a higher wind velocity can beachieved before the system goes into pitch control mode. Theloci for the stator and rotor powers are also shown in Fig. 6.

Table I summarizes the design results for the three systems.On this basis, the following conclusions can be drawn regardingvariable speed WECS with wound rotor induction machine.

The ratings of the converters are significantly reduced. This ismanifested in the lower voltage ratings required for the devices.

The stator flux is constant over the entire operating region.Therefore, the torque can be maintained at its rated value abovethe synchronous speed. This results in higher power above thesynchronous speed (i.e., at high wind velocities) when com-pared to a cage rotor induction generator of the same frame size.Thus, the machine utilization is substantially improved.

A lower dc bus voltage is required. This reduces the voltagerating of the capacitor bank resulting in significant savings inthe cost of the capacitor.

The line side inductance value is also reduced (with reducedinput voltage, for the same PU reactance, the actual value of theinductance is lower).

III. ENERGY CAPTURE

The basic objective of this exercise is to compare the energycapture for the three systems under consideration. A simpli-fied model of the electromechanical system is taken for simula-tion of the different WECS. The simulation is done on Matlab-Simulink platform. The turbine is modeled with the Vs.characteristics and (1) and (2). The electrical and mechanicallosses of the system are neglected. The limits of power andspeed are imposed. In case of the converter-fed systems, it isassumed that the dynamic response of the torque controller ismuch faster compared to the mechanical time constant of thesystem [7]–[9]. Therefore, in the simulation, the dynamics ofthe current-control loops are neglected and the generated torqueis assumed equal to the reference torque. A wind function is de-fined and all the three WECS are subjected to this function todetermine the energy capture in each case.

Wind is a randomly fluctuating variable. Therefore, it is diffi-cult to model and evaluate the performance of a WECS theoret-ically without implementing it and subjecting it to actual envi-ronmental conditions. Some theoretical predictions are possiblewith the statistical data of wind variations at a particular location[14]. However, these are more appropriate for optimal planningof WECS in terms of cost, overall energy output per unit landarea, etc. [15], rather than evaluating the relative performancesof the different generating schemes. For effective performanceevaluation, the system has to be operated over the entire rangeof wind variations so that all of the design limits are reached.This is simulated by defining a wind function as follows:

(3)


TABLE ISUMMARY OF THE DESIGN RESULTS FOR THETHREE WECSS

Fig. 7(a) shows the wind profile; it is observed that the windvaries between maxima and mimima with a periodicity of 20 sand these peaks and troughs are modulated over a longer periodwith a periodicity of 10 mins. The minimum touches the cut-inspeed of 3 m/s, whereas the global maximum reaches 25 m/s.All three systems are subjected to this wind function and theshaft speed, generator power, and generated energy are plotted.

A. Fixed Speed System

In this case, the generator shaft speed is kept constant at 1000r/min (since the allowable slip is only 8 r/min, which can beneglected). When the turbine input power falls below 50 kW,the second winding with eight poles is brought into operation,in which case the speed is fixed at 750 r/min. Since the tur-bine shaft speed is constant, there is no change in the storedenergy of the system and the blade inertia does not come intopicture. Neglecting losses, the turbine and generator powers arethe same. The total energy output due to the defined wind func-tion over a period of 10 min. is 21.5 kWh (Fig. 10). The sim-ulated waveforms for the shaft speed and generated power aregiven in Figs. 7(b) and 7(c).

B. Variable Speed System Using Cage Rotor InductionMachine

The system is designed to track the peak power by varyingthe rotor speed up to the rated operating point corresponding to300 kW, 1000 r/min. From 1000 to 1500 r/min, the power is keptconstant by reducing the generator torque through field weak-ening. Beyond this point, the speed and power are held con-stant through pitch control. Optimum operating point trackingin torque control mode is employed in the present simulation forenergy calculation.

The target power is set according to the following equation:

(4)

Hence, the torque corresponding to the peak-power locus variesas the square of the rotor r/min

(5)

From Fig. 5, it can be seen that beyond the rated speed, thegenerator torque is varied as

(6)

The system is simulated with the same turbine data (seethe Appendix). It is observed that due to constant variation inthe wind velocity, the system is always in the transient statesearching for the optimum operating point. The large inertia ofthe rotating blades tends to reduce the fluctuations in torqueand power to a substantial extent. The relevant plots are givenin Fig. 8(a) and (b). The system starts with an initial speed of10 r/min. (Before this point, the turbine torque is approximatelyzero and the system fails to accelerate in the simulation model.)The generator power is limited to 300 kW and the generatorshaft speed is limited to 1500 r/min (corresponding to a turbineshaft speed of 113 r/min). It is seen that the energy output forthe defined wind function (3) over 10 minutes is 28.5 kWH(Fig. 10), an increment of 32.5% with respect to the previouscase.


Fig. 10. Generated energy for the three systems.

C. Variable-Speed System Using Wound Rotor InductionMachine

The same method of control to track peak power is used inthis case. The generator speed is restricted between 500 and1500 r/min (0.5-PU slip). Therefore, for wind velocities lowerthan 6 m/s, the system operates in constant speed mode at500 r/min. The peak power is tracked from 500 to 1500 r/mincorresponding to wind velocities of 6 to 12 m/s, respectively.Below 50 kW, the system is run in speed control mode at aconstant speed of 500 r/min. When the generator power exceedsthis threshold value, it switches over to peak-power trackingby torque control as discussed in the previous section. From1000 until 1500 r/min, the torque is kept constant at the ratedvalue, beyond which pitch control becomes effective. Thus,the stator power is limited to 300 kW beyond 1000 r/min,whereas the rotor generates an additional amount dependingon the slip. The question of flux weakening does not arise inthis case because the stator flux is dictated by the grid voltageand frequency. The relevant simulated waveforms are shownin Fig. 9(a) through Fig. 9(d). The energy output in this casefor the same wind cycle for 10 minutes is found to be 35 kWh(Fig. 10), an increase of 22.8% with respect to the variablespeed system using cage rotor machine and 62.7% with respectto the conventional fixed-speed system. The improvementin energy capture is due to the rated torque capability of themachine up to the maximum speed. Above the synchronousspeed, although thestator power is saturated to 300 kW, therotor, in addition, generates a substantial amount of power, sothat the net power captured is largely enhanced. The advantageof this scheme lies in the fact that this excess power is obtainedfrom the same frame size of the generator.

IV. CONCLUSION

A fixed speed system, even though more simple and reliable,severely limits the energy output of a wind turbine. Since thereis no torque control loop, fluctuations in generated power aremore. In case of variable speed systems, comparison shows, that

for a machine of similar rating, energy capture can be signif-icantly enhanced by using a wound rotor induction machine.In this case, the rated torque is maintained even at supersyn-chronous speeds whereas, in a system using cage rotor machine,field weakening has to be employed beyond synchronous speed,leading to reduction of torque. It is therefore possible to operatethe proposed system up to higher wind velocities. The voltagerating of the power devices and dc bus capacitor is substantiallyreduced. The size of the line side inductor also decreases. It can,therefore, be concluded from this comparative study that a vari-able speed system using wound rotor induction machine is supe-rior because of higher energy output, lower rating (hence, lowercost) of converters, and better utilization of a generator whencompared to the existing systems using a cage rotor inductionmachine.

APPENDIX

A. Rotor

Diameter: 27 mSwept Area: 573 mRotational speed, generator 1: 43 r/minRotational speed, generator 1: 33 r/minNumber of blades: 3Cut-in speed: 3.5 m/sRated wind speed (225 kW): 14 m/sCut-off wind speed: 25 m/sSurvival wind speed: 56 m/s

B. Gearbox

Nominal power: 433 kWRatio: 1:23.4

C. Generator1

225 kW, 400 V, 396 A, 50 Hz, 1008 r/min, 163 kVAR

D. Generator2

50 kW, 400 V, 101 A, 50 Hz, 760 r/min, 48 kVAR

REFERENCES

[1] D. F. Warne, “Generation of electricity from the wind,”Proc. Inst. Elect.Eng., vol. 124, pp. 963–985, Nov. 1977.

[2] C. V. Nayar, “Wind power—The near term commercial renewable en-ergy source,”Australi. Sci., Summer Issue, vol. 16, no. 4, pp. 25–26,1995.

[3] “Getting Connected—Integrating Wind Power With Electric Utility Sys-tems,” Rep. Ameri. Wind Energy Assoc., 1997.

[4] A. Miller, E. Muljadi, and D. S. Zinger, “A variable speed wind turbinepower control,”IEEE Trans. Energy Conversion, vol. 12, pp. 181–187,June 1997.

[5] A. K. Unnikrishnan, R. Sudeepkumar, P. S. Chempakavally, G. Poddar,A. Joseph, and S. Bindu, “A variable speed power controller for windturbines,” inProc. Workshop Wind Power Generation Power QualityIssues, Thiruvananthupuram, India, 1999.

[6] BR. Pena, J. C. Clare, and G. M. Asher, “Doubly fed induction gen-erator using back-to-back PWM converters and its application to vari-able-speed wind-energy generation,”Proc. Inst. Elect. Eng., vol. 143,pp. 231–241, May 1996.


[7] R. Datta and V. T. Ranganathan, “A simple position sensorless algorithmfor rotor side field oriented control of wound rotor induction machine,”IEEE Trans. Ind. Electron., vol. 48, pp. 786–793, Aug. 2001.

[8] , “Method for direct control of active and reactive power from therotor side for a grid connected doubly-fed slip-ring induction machinewithout position encoder,” Indian Patent applied (application number797/MAS/99, dated 6/8/99), 1999.

[9] , “Direct power control of grid-connected wound rotor inductionmachine without rotor position sensors,”IEEE Trans. Power Electron.,vol. 16, pp. 390–399, May 2001.

[10] R. Datta, “Rotor side control of grid-connected wound rotor inductionmachine and its application to wind power generation,” Ph.D. disserta-tion, Dept. Electr. Eng., Indian Inst. Sci., Bangalore, India, 2000.

[11] L. J. Fingersh and P. W. Carlin, “Results from the NREL variable-speedtest bed,” inConf. Rec. AIAA Wind Energy Sym., 1998, pp. 233–237.

[12] “Vestas, Danish Wind Technology V27-225 kW, 50 Hz WindturbineProduct Brochure,” Vestas Wind Systems, Denmark, 1994.

[13] S. Heier,Grid Integration of Wind Energy Conversion Systems. NewYork: Wiley, 1998.

[14] D. S. Zinger and E. Muljadi, “Annualized energy improvement usingvariable speeds,”IEEE Trans. Ind. Applicat., vol. 33, pp. 1444–1447,Nov./Dec. 1997.

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Rajib Datta received the B.E. and M.Sc. degrees in electrical engineering fromJadavpur University, Calcutta, India, and the Indian Institute of Technology,Kharagpur, in 1992 and 1994, respectively. From January 1995 to July 2000,he was a research scholar in the Department of Electrical Engineering, IndianInstitute of Science, Bangalore, from where he received the Ph.D. degree.

Currently, Dr. Datta is employed with GE Global Research Center, Sch-enectady, NY, in the Electronic Power Conversion Lab. From 2000 to 2001,he worked on converter topogies for large-scale wind parks at ABB CorporateResearch Center, Ladenburg, Germany. His research interests include thedesign, modeling, and control of power-electronic systems, particularly relatedto alternative energy and distributed energy resources.

V. T. Ranganathan(SM’92) received the B.E. and M.E. degrees in electrical en-gineering from the Indian Institute of Science, (IIS), Bangladore, and the Ph.D.degree from Concordia University, Montreal, QC, Canada.

He joined the Electrical Engineering Department at IIS in 1984. Currentlya Professor, he has published several papers in the areas of vector control ofac drives, pwm techniques, split-phase induction motor drives, and rotor sidecontrol of slip ring induction motors. He is also a consultant to industry in theareas mentioned and has participated in a number of research-and-developmentprojects. His research interests are in the area of power electronics and motordrives.

Dr. Ranganathan received the Prize Paper Award of the IEEE-IAS StaticPower Converter Committee, the Tata Rao Prize of the Institution of Engineers,India, the VASVIK award in Electrical Sciences and Technology, and the BimalBose Award of the Institution of Electronics and Telecommunication Engineers,India. He is a Fellow of the Indian National Academy of Engineering and aFellow of the Institution of Engineers, India.

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Variable-Speed Wind Power Generation Using