449

DOUBLE FED INDUCTION GENERATOR WITH TWO PAIR O F POLES

DjuroviC M.*, JoksimoviC G.*, SaveljiC R.,* MariEiC I.**

* Elektrotehnitlu fakultet Podgorica," *Sever Subotica, Yugoslavia

INTRODUCTION

The performance of the induction machine when it is double supplied presents certain features which are of practical and theoretical interest (1). By using appropriate control of the machine it is possible to obtain that the induction machine works as the generator in the regime above the synchronous speed as well as in the domain under the synchronous speed. This is specially convenient when a variable speed - constant frequency generator is required (2). When the induction machine is constructed in this way it is very suitable for use as the generator in windmills.

When the rotor of the induction machine is supplied through the six pulse, three phase, cycloconverter, and the stator is connected to the mains the machine operation is determined by the relative phase sequence of the stator and rotor supplies and the magnitude of rotor's voltage. When the rotor of such machine (double fed induction machine -DFIM) is supplied through the cycloconverter, the machine can operate as the generator in the wide range of the speed, i.e. (0.67 - 1.33) of the synchronous speed. If the machine is made such that it is possible to connect stator's and rotor's coil such to change the number of pair of poles, the range of the speed, for which that machine can work as the generator, is extended significantly.

THEORETICAL CONSIDERATION

The double fed induction machine generates the power under the condition that :

where or is electrical angu!ar velocity of the rotor, p number of pair of poles, f l frequency of the stator's current, and f2 frequency of the rotor's current. The operating condition of the double fed induction machine is characterized by inherent instability, which results as the consequence of the two components of the torque : induction, which is related to the slip, and synchronous, which is

dependent on the torque angle between the rotor's m f and the resultant mmf.

To satisfy the condition /1/ one has to take care of the speed of the rotor and, iin relation to it, to adjust the frequency of the: rotor's supply . Therefore , it is required to m'onitor the speed of the rotor, when the generalm is concerned, or to specify the speed of the rotor when the motor is concerned, and then to determine the proper frequency of the rotor's supply. This is done by specifying appropriate angles for firing the thyristors in the cycloconverter.

The analysis of such machine is possible by means of "d-q" imodel and it has bee,n done for the case when such machine is considered to work as the machine (3) .

The performance of such machine when it works as the generator can be analyzed by the rather simple model of an equivalent cmcuit, too. By taking in account only fundamental (component , the circuit can be derived in the s,hape shown of Fig.1, where RI, R21, XI, X21, Rm, and Xm are the parameters defined in the cclrnnion manner (4).

11

&

d Fig.1: The simpliOed equivalent circuit of DFIG

This double fed induction generator (DFIG) represents the combination of the synchronous and induction machine what is taken in account in the equivalent circuit , The voltage U2 is represented by the magnitude and the angle yr. The angle yr, between U1 and U21 can be tireated trs the sum of the angles : p - which represents the phase shift of the rotor's (cycloconverter's,) voltage in relation to

'Electrical Machines and Drives', 11-13 September 1995, Conference Publication No. 4 12, 0 /€E, 7995

450

the induced ems in the rotor, and the angle 8 - which defines the phase shift between phase A on the stator and phase a on the rotor at the instant t=O, and they are given in electrical radians. The model , the equivalent circuit , is applied in the situation when the load is constant.

The model, based on the equivalent circuit, have been used to analyze the performances of the generator in the wide range of speed with the aim to investigate the effect of characteristic parameters on the maximum generated power on the stator. The parameters : the phase angle p between the rotor's voltage and emf of the rotor, the magnitude of the rotor's voltage, and the speed of the rotor n, were varied in the range which was allowed by the nature of naturally commutated cycloconverter.

connecting the coils in double star or delta connection. The typical results are show in Fig.3 for tested machine ,

The numerical results which are based on the equivalent circuit model , as well as those calculated by "d-q" model are pven , too.

1800 I 1

Pl(W *

0 x* 3 x

o * x O *

X * x o 1400

ox *

1200 i x o x * o

IOoo I - .

EXPERIMENTAL RESULTS 800 ' I I

The specially constructed induction generator has been tested in the laboratory. The laboratory rig has been designed such that the rotor of the generator is supplied through the six pulse, three phase cycloconverter and its layout is shown on Fig.2.

3 phase indumao ma

2u phase a pllase b Phase c

cgdomnverter

Fig. 2: The layout of experimental rig.

The speed was controlled by the DC motor which was mechanically coupled with the induction generator. The firing angles of the thyristors were determined in the cycloconverter by the microprocessor control. It was possible to hold the load constant for variety of speeds. This was enabled by appropriate control of the current of the induction generator. The results were obtained for two different pair of poles: 2 an 4, which were possible to obtain, on the tested machine, by

0 5 10 15

Legend: x - experimental results * - equivalent circuit o - d-q model

Fig.3: Comparison of the numerical and the experimental results( YY sub synchronous).

The agreement between the numerical and experimental results has shown to be satisfactory and its value was of few percents. The extreme value was registered for the case when p=4 . This enables rather simple model to be used to optimize the generated power P1 on the stator in relation to the investigated parameters. The numerical model has enabled further calculations and the analysis of the effect of the crucial parameters: the phase shift p , magnitude of the cycloconverter's voltage U2, and the mechanical speed of the rotor arm. These results are shown on Fig.4 and Fig.5.

It is obvious that the machine is working as the generator in the wide range of the speed, above and under the synchronous speed in both connections. The generated power is obtained continuously in the range from 500 rpm to 2000 rpm. The power which is presented on the Fig.4 and Fig.5 does not include the portion of the power which is exchanged through the rotor to the grid (slip power).

It is show that the variation of the generated power is the most sensitive on the angle which defines the phase shift between the rotor's voltage and

45 1

'O x - 0 ,

1600 ~

x 1400 - 0 -

__ rotor's induced emf. This effect is demonstrated in the case when the generator is working with two pair of poles (double star connection) on Fig. 6. The angle p was varied in the range of k 180 degrees. It x o

40

x o o *

-20 -10 0 10 f2(Hz) 20 2100 1800 1500 1200 n(rpm) 900

a/ U2 = f(n)

2400 I I I

800 I I I I I -20 -10 0 10 f Z ( W 2 0 2100 1800 1500 1200 n(rpmp00

Legend: * - experimentally results x - equivalent circuit o - dqmodel

Fig.4: The experimental and numerical (optimized) results for p=2 (star connected) DFIG.

n, x iox X

0 __ 0 I

-20 -10 0 10 f w z ) 20 1050 900 750 600 n(rpm) 450

l0L I

a i U2=f(n)

1200

1000

900

800

* T

* O xc,

i 500 1 * X

I 400 I -_ I X

-20 -10 0 i o f 2 W ) 20 1050 900 750 600 n(rpm) 450

b/ Pl=f(n)

Legend: * - experimc:iatally results x - equivalent, circuit o - dq moCld

Fig.5. The experimental and nunierical(optimized) results for p=4 (delta connected ) DFIG.

is obvious, by inspection of re!iults, that the angle should not be varied more than k 30 degrees, since generated power is rapidlly decreasing beyond these values of the angle. This range of the slip was easy to icontrol by the naturally commutated cycloconverter.

452

180\

p(degrees)

Fig.6 Effect of the phase shift p , and the speed to generated power P1 = f(p, s) , p=2 at U2= 10 V (YY connection)

CONCLUSIONS

It has been demonstrated that the double fed induction generator for two pair of poles can work successful in the wide range of the speed. For the examined case it was possible to generate continuously the power from 500 rpm to 2000 rpm. This is very suitable for the case when the generator is used in windmills where the speed of the rotor of the generator varies significantly, and where regulation of the speed when it is done mechanically on the propeller is very costly and complicated.

It has been shown that the analysis of DFIG can be done satisfactory by simple model of equivalent circuit which takes in account only the fundamental harmonics.

By increasing the number of pair of poles to three in the machine it would be possible further to increase the range of the speed.

Such machine is easy to control , too. It has been demonstrated that it was possible to obtain the stable working conditions by the real time control.

REFERENCES

1.Ouchi K, 1981, "A stabilized double fed motor with the separately controlled current source converter", Trans IEEE, IECI-28, 336-341

3.Holmes P G and Elsonbaty N A, 1984, "'Cycloconvertor -excited divided -winding doubly- fed machine as a wind-power convertor", Proc IEE , Vol 131, Pt B, NO 2,61-69

4. Bird B M and Burbidge R F, 1996,"'Analyisis of doubly fed slip-ring machines"', Proc IEE, Vol 113, NO 6, 1016-1020

APENDIX

The specially constructed induction generator which was used in this work had the following data: connection: DNY, f=SOHz, Un =380V, P1= 307514250 W, P2= 220013500 W, In = 10.3517.9 A , Ir = 12/16 A, n = 72211447 rpm , cos (p=0.45/0.815, S, = 3.73 / 3.5

2.DjuroviC M and Saveljie R, 1989, "Double fed induction generator for variable speed constant frequency, Proc on UK/Yugoslav seminar on Wind Energy, NEL Glasgow,