01183728

8/4/2019 01183728

1/6

IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 18, NO. 1, MARCH 2003 163

A Method of Tracking the Peak Power Points for aVariable Speed Wind Energy Conversion System

Rajib Datta and V. T. Ranganathan, Senior Member, IEEE

AbstractIn this paper, a method of tracking the peak powerin a wind energy conversion system (WECS) is proposed, whichis independent of the turbine parameters and air density. Thealgorithm searches for the peak power by varying the speed in thedesired direction. The generator is operated in the speed controlmode with the speed reference being dynamically modified inaccordance with the magnitude and direction of change of activepower. The peak power points in the curve correspond to

= 0

. This fact is made use of in the optimum pointsearch algorithm. The generator considered is a wound rotorinduction machine whose stator is connected directly to the gridand the rotor is fed through back-to-back pulse-width-modulation(PWM) converters. Stator flux-oriented vector control is appliedto control the active and reactive current loops independently.

The turbine characteristics are generated by a dc motor fed froma commercial dc drive. All of the control loops are executed by asingle-chip digital signal processor (DSP) controller TMS320F240.Experimental results show that the performance of the controlalgorithm compares well with the conventional torque controlmethod.

Index TermsPeak power point tracking, rotor side control,speed control mode, turbine characteristics, wind energy conver-sion system, wind turbine, wound rotor induction machine.

I. INTRODUCTION

I

N RECENT YEARS, there has been a growing interest in

wind energy as it is a potential source for electricity genera-

tion with minimal environmental impact. With the advancement

of aerodynamic designs, wind turbines, which can capture hun-

dreds of kilowatts of power, are readily available. When such

wind energy conversion systems (WECS) are integrated to the

grid, they produce a substantial amount of power, which can

supplement the base power generated by thermal, nuclear, or

hydropower plants.

The cage rotor induction machine is the most frequently

used generator for grid-connected WECS. When connected to

the constant frequency network, the induction generator runs

at near-synchronous speed, drawing the magnetizing current

from the mains, thereby resulting in constant speed constant

frequency (CSCF) operation. However, if there is flexibility invarying the shaft speed, the energy capture due to fluctuating

wind velocities can be substantially improved [1][3]. The

requirement for variable-speed constant frequency (VSCF)

operation led to several developments in the generator control

Manuscript received July 25, 2000; revised December 4, 2001.R. Datta is with ABB Corporate Research Centre, Ladenburg 68526, 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]).Digital Object Identifier 10.1109/TEC.2002.808346

of WECS. By using back-to-back PWM inverters between

the grid and the machine and employing vector control or

direct torque control (DTC) techniques, the active and reactive

powers handled by the machine can be controlled indepen-

dently [4][8]. Even though the mechanical time-constant of

a WECS is very high (due to the high inertia of the turbine

blades) and fast change of shaft speed is neither desirable nor

possible, use of vector control or DTC algorithms allow direct

control over the generator torque and flux, and therefore, more

optimized utilization of the machine.

Rotor side control of grid-connected wound rotor induction

machine is an attractive option for VSCF operation with lim-

ited speed range [4][8]. By suitable integrated approach to-

ward design of a WECS, use of a slip-ring induction gener-

ator is found economically competitive, when compared to a

cage rotor induction machine [8]. The power rating of the con-

verters can be considerably reduced (about 0.3 to 0.5 p.u.) when

used in the rotor circuit. Since the stator is directly connected to

the grid, the stator flux is constant over the entire operating re-

gion. Therefore, the torque can be maintained at its rated value

even above the synchronous speed. This results in higher power

output above the synchronous speed (i.e., at high wind veloci-

ties) when compared to a cage rotor induction generator of the

same frame size. Thus, the machine utilization is substantially

improved.Irrespective of the generator used for a variable-speed WECS,

the output energy depends on the method of tracking the peak

power points on the turbine characteristics due to fluctuating

wind conditions. The conventional method is to generate a con-

trol law for the target generator torque as a square

function of the angular velocity of the turbine shaft.

(1)

The generator torque is controlled accordingly through

field-oriented or direct torque control methods. The parameter

is given by the following equation:

(2)

where is the air density; is the swept area (cross-sectional

area) of the turbine; and is the radius of the turbine (blade

length). is called the power coefficient of the turbine and is

dependent on the ratio between the linear velocity of the blade

tip ( ) and the wind velocity ( ). This ratio, known as the

tip-speed ratio, is defined as

(3)

0885-8969/03$17.00 2003 IEEE
http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-

8/4/2019 01183728

2/6

164 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 18, NO. 1, MARCH 2003

Fig. 1. Rotor side control scheme with back-to-back PWM converters withcapacitive dc link.

Clearly, depends on the turbine characteristics and air

density. The turbine dimensions and the optimal values for

and (namely and ) are available with turbine man-

ufacturers. The term , on the other hand, depends on the cli-

matic conditions prevalent at a particular site. The air density

may vary considerably over various seasons. As a result, the

value of computed on the basis of some nominal air-den-

sity value will not result in optimal tracking of the peak power

point under all conditions. With the reduction in air-density,the turbine output itself reduces; at the same time, the tracking

trajectory being incorrect, there is considerable loss in output

energy.

In this paper, a method of tracking the peak power is proposed

which is independent of the turbine parameters and air density.

The algorithm searches for the peak power by varying the speed

in the desired direction. In [9], a fuzzy-logic-based controller

is proposed to track the optimum operating point locus. This

system has been designed with a cage rotor induction machine

and can possibly be extended to a doubly-fed machine. How-

ever, similar performance canbe obtained even without the com-

plication of implementing a fuzzy controller. In the algorithm

presented here, the generator is operated in the speed controlmode with the speed reference being dynamically modified in

accordance with the magnitude and direction of change of ac-

tive power. The peak power points in the curve correspond

to . This fact is made use of in the optimum point

search algorithm.

The algorithm is experimentally verified in a small-scale lab-

oratory setup. The generator considered is a wound rotor induc-

tion machine whose stator is connected directly to the grid and

the rotor is fed through back-to-back PWM converters (Fig. 1).

Stator flux-oriented vector control is applied to control the ac-

tive and reactive current loops independently [4][6]. The oper-

ating region of the system in the power-speed plane is indicated

in Fig. 2. In the experimental setup, the turbine characteristics,taken from a commercial model Vestas V27 (Appendix A), are

generated by a dc motor fed from a commercial dc drive. All of

the control loops are executed by a single-chip DSP controller

TMS320F240.

II. PEAK POWER TRACKING ALGORITHM

The proposed algorithm is explained with the help of Fig. 3,

where the curves corresponding to three wind velocities

are shown. Let the present wind velocity be . The generator

is run in the speed control mode with a speed reference of

(which corresponds to the optimum operating point for ).

Fig. 2. Operating region of WECS with wound rotor induction machine in theP ! plane.

Fig. 3. Shift of operating points in the proposed peak power trackingalgorithm.

The generator output power and speed are sampled at regular in-

tervals of time. If the wind velocity is steady at , the difference

between successivesamples ofactivepower (i.e., ) willbe

very small and no action is taken. Now, let there be a step jump

in wind velocity from to . Since the turbine shaft speed

cannot change instantaneously (the reference for the speed con-

troller is not yet changed and the inertia of the system is ex-

tremely high), this would result in a change of operating point

from to . Therefore, would be large and positive.

Corresponding to this change in , a positive change in speed

reference is commanded. The change in speed reference is
http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-

8/4/2019 01183728

3/6

8/4/2019 01183728

4/6


Fig. 5.P

!

characteristics in the region of operation of peak power trackingalgorithm.

overall power generated and the small power fluctuations of the

individual machines will not be directly reflected on the grid.

B. Selection of

determines the change in speed reference for a given

change in . Therefore, it depends on the slope of the

characteristics. To choose a value of , an approximate idea of

the turbine characteristics is needed. The characteristics

in the region of operation of the peak power tracking algorithm

are considered. This is shown in the V27 power curves of

Fig. 5. The wind velocities over this region vary between 6and 12 m/s. The approximate changes in for successive

changes in wind velocities, and hence, are also shown. It

is obvious that the is more for lower wind velocities

and vice-versa. If is set to the maximum value of

in the operating range then, for changes in wind velocities

during high wind conditions, the increment in speed reference

would be more than desired. This would result in overshooting

of the optimum operating point. The system would oscillateabout the peak power point before it settles down. Therefore,

the maximum value of is limited by the lowest value of

. A large value of will also result in a large transient

in generator torque which is not desirable. Hence, the value of

selected is substantially lower than the limit imposed by the

minimum value of .

From Fig. 5, it can be seen that the curvesare flat-topped

near the peak power points. Therefore, the change in for

an increment in speed would be very small in this region. The

may be set at 5% of the nominal power rating of the gen-

erator. So, the final operating point may not move exactly to the

peak power point, but may settle down close to it.

III. EXPERIMENTAL RESULTS

The proposed algorithm is verified on a small-scale labora-

tory prototype, where the principle and implementation remain

identical to that of a practical WECS. Depending on the size of

the WECS, the converter topology and the generator employed

may change. The inner current control loops need to be mod-

ified accordingly. However, the proposed algorithm deals with

the setting of the speed reference for the generator, which is the

outermost loop in the control structure and will always remain

the same irrespective of the size of the turbine.

The experimental setup (Fig. 6) consists of a 3-kW woundrotor induction machine (Appendix B) with its stator connected

to the 415-V, 50Hz, 3- power grid, and the rotor being fed

by two back-to-back IGBT-based PWM converters. The setup

is organized for generation operation where the torque-speed

characteristics of the wind turbine is generated by a 5-hp dc

motor driven by a commercial four-quadrant thyristor drive. A

TMS320F240 DSP-based digital control platform is designed

and employed for implementing the direct power algorithm. The

processor runs at a clock frequency of 36 MHz and the sampling

frequency used is 56 s. The software is assembly coded for fast

real-time execution.

The V27, characteristics corresponding to four wind

velocities , , , and (10, 11, 12, and 13 m/s) are ex-pressed in per unit and are shown in Fig. 7. These characteris-

tics are then stored in the form of lookup tables in the external

memory of theDSP as 32 word arrays. The generatorshaft speed

is computed, scaled to the resolution of the table, and the corre-

sponding turbine power is then read from memory. The turbine

torque is subsequently calculated. This is given as a reference

to the dc drive. The dc drive is a stand-alone unit with an inde-

pendent analog controller. It can be operated in either current

control mode or speed control mode with external analog refer-

ences. In the present case, the torque reference for the dc drive,

suitably scaled, is output via the DAC in the processor board.

It is then routed to the reference input of the torque controller.

The dc motor is thus made to emulate the characteristics of the

chosen wind turbine.

The system is started in the following manner. Initially,

a small constant torque reference is given to the dc drive.

Since the rotor side control is not yet released, the generator

torque is zero. The dc motor speed ramps up. When the

speed crosses a threshold (1200 r/min in the present case) the

software switches in the turbine characteristics. This further

accelerates the motor. In the absence of any generating torque,

the torque controller for the dc drive saturates and the machine

speed settles at the maximum value depending on the input

voltage and the field current (nominally at 1875 r/min). The

reference for the generator torque at this speed saturates at therated value (since the rated speed is exceeded). So when the

rotor side current control is enabled, the system decelerates and

eventually settles down to a steady-state operating point where

the generator torque equals the prime mover torque.

The speed loop time constant is designed to be 250 ms, and

the sampling period for the active power is taken to be 1 s. From

Fig. 6, it is observed that the minimum value of (in

this case between and ) is approximately 0.1/0.2 (i.e., 0.5).

(Beyond , the system operates in the constant torque mode, so

this region is not taken into consideration for deciding the value

of .) Hence, according to the design procedure, has to be

much lower than 0.5 for the generator speed to settle close to

8/4/2019 01183728

5/6

DATTA AND RANGANATHAN: TRACKING PEAK POWER POINTS FOR A WIND ENERGY CONVERSION SYSTEM 167

Fig. 6. Schematic block diagram of the experimental setup.

Fig. 7. Turbine characteristics for experimental verification and operatingpoints.

the maximum power point without any overshoot. The selected

value for is 0.25. With these parameters, the algorithm is run

for the different wind velocities. The resulting operating points

for the generator are plotted in Fig. 7 along with the optimum

power curve of the turbine. Due to the flat-topped nature of the curves of the turbine, the error in the settling speed does

not result in appreciable reduction in the generated power.

The transient response of speed and (which is a direct

measure of the generator torque in per unit) for transitions be-

tween and are shown in Fig. 8(a). At instant A, there is

a step change in wind velocity from to . The torque in-

stantaneously falls with a small drop in speed. This is because

of the time constant associated with the speed controller. At the

subsequent sample (at instant B), this change in active power is

detected and a decrement in speed reference is commanded. The

transient in (in the positive direction) is due to the action of

the speed controller. The subsequent samples show insignificant

(a)

(b)

Fig. 8. (a) Experimental result for transients in ! and i due to change inwind velocity between v 6 and v 8 (5 V represents 1-p.u. speed and 1.5-p.u. rotorcurrent). (b) Experimental result for transients in ! and i due to change inwind velocity between v 7 and v 9 (5 V represents 1-p.u. speed and 1.5-p.u. rotorcurrent).

change in active power, and therefore, almost constant operating

speed. The reverse operation is observed when the wind velocity

changes from to . In Fig. 8(b), similar waveforms of speed

and are presented for changes in wind velocity between

and . The saturation of the torque beyond the rated speed is

clearly observed from the plot of .

8/4/2019 01183728

6/6


IV. CONCLUSION

An algorithm for searching the optimum operating point for a

WECS in speed control mode is proposed. This technique makes

peak power tracking independent of the turbine characteristics

and the air density. The criteria for selecting the critical control

parameters are described. The algorithm is implemented on a

laboratory setup using a grid-connected wound rotor induction

generator controlled from the rotor side. Experimental results

show that the performance of the control algorithm compares

well with the conventional torque control method.

APPENDIX A

WIND TURBINE AND GENERATOR DATA FOR VESTAS V27

TURBINE

1) Rotor

Diameter: 27 m;

Swept area: 573 m ;

Rotational speed, generator 1: 43 r/min;

Rotational speed, generator 1: 33 r/min;

Number of blades: 3;Cut-in speed: 3.5 m/s;

Rated wind speed (225 kW): 14 m/s;

Cut-off wind speed: 25 m/s;

Survival wind speed: 56 m/s;

2) Gearbox

Nominal power: 433 kW;

Ratio: 1:23.4;

3) Generator main winding225 kW, 400 V, 396 A, 50 Hz, 1008 r/min, 163 kVAR;

4) Generator low-power winding

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

APPENDIX B

WOUND ROTOR INDUCTION MACHINE USED IN LABORATORY

PROTOTYPE

3 kW, 415 V, 50 Hz, four pole, three phase;

Stator : 415 V, connected, 7.2 A;

Rotor : 415 V, Y connected, 6.6 A.

REFERENCES

[1] A. Miller, E. Muljadi, and D. S. Zinger, A variable speed wind turbinepower control, IEEE Trans. Energy Conversion, vol. 12, pp. 181187,June 1997.

[2] D. S. Zinger and E. Muljadi, Annualized energy improvement usingvariable speeds, IEEE Trans. Ind. Applicat., vol. 33, pp. 14441447,Nov./Dec. 1997.

[3] L. J. Fingersh and P. W. Carlin, Results from the NREL variable-speedtest bed, Proc. Conf. Rec. AIAAWind Energy Symp., pp.233237, 1998.

[4] Y. Tangand L. Xu,A flexible active andreactive powercontrol strategyfor a variable speed constant frequency generating system, Proc. Conf.

Rec. IEEE/IAS Annu. Meeting, pp. 568573, 1993.[5] R. Pena, J. C. Clare, and G. M. Asher, Doubly fed induction gener-

ator using back-to-back PWM converters and its application to vari-able-speed wind-energy generation, Proc. Inst. Elect. Eng., pt. B, vol.143, no. 3, pp. 231241, May 1996.

[6] R. Datta andV. T. Ranganathan,A simplepositionsensorless algorithmfor rotor side field oriented control of wound rotor induction machine,IEEE Trans. Ind. Electron. Soc., vol. 48, pp. 786793, Aug. 2001.

[7] , Direct power control of grid-connected wound rotor inductionmachine without rotor position sensors, IEEE Trans. Power Electron.Soc., vol. 16, pp. 390399, May 2001.

[8] R. Datta, Rotor Side Control of Grid-Connected Wound Rotor Induc-tion Machine and its Application to Wind Power Generation, Ph.D.,Indian Inst. of Science, Dept. of Elect. Eng., Bangalore, India, 2000.

[9] M. G. Simoes, B. K. Bose, and R. J. Spiegel, Design and performanceevaluation of a fuzzy-logic-based variable-speed wind generationsystem, IEEE Trans. Ind. Applicat., vol. 33, pp. 956965, July/Aug.1997.

Rajib Datta received the B.E. and M.Tech degrees

in electrical engineering from Jadavpur University,Calcutta, India, and fromthe Indian Institute of Tech-nology, Kharagpur, India, in 1992 and 1994, respec-tively. He received the Ph.D. degree from the IndianInstitute of Science (I.I.Sc.), Bangalore, India.

Currently, he is with GE Global Research Centerin the Electronic Power Conversion Lab, Schenec-tady, NY. From 2000 to 2001,he workedon convertertopologies for large-scale wind parks at ABB Cor-porate Research Center, Ladenburg, Germany. From

1995 to 2000, he was a Research Scholar in the Department of Electrical Engi-neering at the I.I.Sc. His research interests include design, modeling,and controlof power-electronic systems, particularly related to alternative energy and dis-tributed energy resources.

V. T. Ranganathan (SM92) received the B.E. andM.E. degrees in electrical engineering from the In-dianInstituteof Science (I.I.Sc.)and the Ph.D. degreefrom Concordia University, Montreal, QC, Canada.

Currently, he is a Professor in the Electrical En-gineering Department at I.I.Sc., where he has beensince 1984. He is also a consultant to industry in theareas mentioned before and has participated in manyresearch-and-development projects. His research in-terests are in the area of power electronics and motordrives.

He has published several papers in the areas of vector control of ac drives,pwm techniques, split-phase induction motor drives, and rotor side control ofslip ring induction motors. He has been a recipient of the prize paper award ofthe IEEE-IAS Static Power Converter Committee; the Tata Rao Prize of the In-stitution of Engineers, India; the VASVIK Awardin electrical sciences and tech-nology; and the Bimal BoseAward of the Institutionof Electronicsand Telecom-munication Engineers, India. He is a fellow of the Indian National Academy of

Engineering and fellow of the Institution of Engineers of India.

Documents

01183728