Microprocessor-based Voltage Controller For Wind-driven Induction Generators

53 1

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 31, NO. 6, DECEMBER 1990

Microprocessor-Based Voltage Controller for Wind-Driven Induction Generators N. AMMASAIGOUNDEN AND M. SUBBIAH

Abstract- A microprocessor-based closed-loop system has been developed for wind-driven self-excited induction generators using a controlled rectifier to maintain a constant dc load voltage with varying rotor speeds. The configuration and implementation of the control scheme have been fully described. Test results on a self-excited induction generator demonstrate the satisfactory performance of both the hardware and software of the control scheme and the utility O f the setup as a whole. The steady-state analysis of the generator has been extended to include the controlled rectifier, and the performance characteristics have been predicted.

NOMENCLATURE p.u. frequency excitation capacitance per phase (in microfarads) generated frequency (in hertz) rated frequency (in hertz) generator load current (in amperes) dc load current (in amperes) smoothing reactor (in millihenrys) actual rotor speed (in revolutions per minute) synchronous speed corresponding to rated frequency (in revolutions per minute) synchronous speed corresponding to generated frequency (in revolutions per minute) per phase stator and rotor (referred to stator) resistance, respectively (in ohms) load resistance at the output of the controlled rectifier (in ohms) equivalent load resistance and reactance per phase, respectively, across the generator terminals (in ohms) operating slip sampling interval for dc voltage measurement sampling interval for generator frequency measurement terminal voltage per phase of the generator (in volts) dc load voltage (in volts) per phase stator and rotor (referred to stator) reactance, respectively (in ohms) capacitive reactance per phase of the excitation capacitor (in ohms) magnetizing reactance per phase (in ohms) Manuscript received December 21, 1988; revised August 14, 1989. The authors are with the of and Electronics Engineering, Regional Engineering College, Tamil Nadu, India. IEEE Log Number 9040007.

ze a

equivalent impedance per phase across the generator terminals (in ohms) firing angle

I. INTRODUCTION T IS WELL KNOWN that self-excited induction generaare being increasingly used for isolated power supplies

Itors

in wind energy systems. For a given excitation capacitance, the output voltage and frequency of these generators vary with wind speed and load impedance. A new approach for the steady-state analysis of these generators has been developed, and to sustain the self-excitation over an extended speed range, pole-changing windings have been proposed and successfully implemented by the present authors [l]. Recently, the utility of chopper circuits, in obtaining a controllable dc supply from wind-driven self-excited induction generators has been discussed [2]. As a further development, the present paper considers the application of three-phase fully controlled thyristor converter and control circuits to these generators driven by varying wind speeds. Earlier, Watson et al. [3] have described a scheme to obtain a controllable dc power supply from wind-driven machines using firing angle control. In this scheme, different values of excitation capacitances have been used to increase the operating speed range of the generator. Watanabe and Barreto [4] have proposed a force-commutated rectifier system with negative firing angles to obtain a capacitive effect on the generator terminals to keep the voltage constant on the load side of the rectifier. It has also been shown that the system can operate well even without a smoothing reactor. The present paper describes a scheme using a line-commutated thyristor bridge to obtain a desired dc voltage from the wind-driven generators. In order to have flexibility and reliability, the scheme has been developed employing a microprocessor pP-based closed-loop controller. This attempt is also in keeping with the present trend in the control of electrical drives using pP-based closed-loop systems [5], [6]. The steady-state performance of the generator is analyzed using the equivalent circuit of the induction machine with the converter load resistance suitably represented in terms of the firing angle in the equivalent circuit. As suggested in [I] and [2], pole-changing windings could be applied to extend the speed-range of the generators in the proposed system as well. Experiments were conducted on an induction generator with four/six-pole combination with a closed-loop controller fabricated in the laboratory. The test results fully bring out the usefulness and elegance of the proposed system as a whole.

0278-0046/90/12OO-0531$01.OO O 1990 IEEE

532

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 31, NO. 6 , DECEMBER 1990

lood

Fig. 1. Block schematic of @-based voltage controller for self-excited induction generator: IG-induction generator; C-excitation capacitor bank; L-smoothing reactor; P.D-potential divider network.

11. PROPOSED SYSTEM The block schematic of the proposed pP-based controller with rectifier is shown in Fig. 1. The rectifier is a three-phase fully controlled thyristor bridge. The controller portion mainly consists of an Intel 8085 pP, pulse amplifier circuits, and an eight-channel, 8-b A/D converter ADC 0809. Digital input data for the pP are the reference voltage V, corresponding to the desired constant dc voltage at the output of the rectifier, the lowest and highest frequency limits f, and fh (within which the wind turbine-generator system has been designed to work), the initial firing angle ai(with which the rectifier starts working), and the sampling intervals T,, and T,, for dc voltage and frequency measurements, respectively. Depending on the load requirement, a small deviation in the reference dc voltage V, can be permitted, where the lower and upper limits of V, are specified as V, and Vh, respectively. Based on the reference and measured dc voltages, the pP adjusts the firing angle until the two voltages become equal; it also generates the gate trigger pulses for the thyristors at the appropriate instants. A. Implementation of the Control Scheme

The Intel 8085 pP is interfaced with other peripheral chips, and the details are shown in Fig. 2. The 1/0 device 8255A is programmed so that ports A and C are configured as input ports and port B as the output port. One of the line voltages of the induction generator is stepped down, rectified, and converted to a square pulse, which will be in synchronization with the generator output voltage. This square pulse is fed to line 1 of port C, and the pP calculates the number of counts for one complete cycle of pulse. This count represents the frequency fg of the induction generator output voltage. The program first checks whether fg lies within the limits f,5 fg 5 fh. If this condition is not met, the program contin-

. I

1 IO

8085

Fig. 2. Circuit diagram for peripheral chips interfaced with Intel 8085: CH-channel section bits in ADC.

ues the measurement and checking of the frequency fg. Once this condition is met, counts for 60" and for the firing angle aiare computed and stored in memory. Then, zero crossing of the generator line voltage is sensed using the same square pulse, and gate trigger pulses are produced after a delay of ai.Three lines of port B (B,-B,) are used to output the gate pulses. Fig. 3 shows the power circuit of the controlled rectifier and the gate pulses. Each pulse is fed to a current amplifier circuit and then to the primary of a pulse transformer that has two secondaries. The secondaries of each pulse transformer are connected to the gate terminals of one set of thyristors in the same limb of the rectifier. For example, the pulse output at line 2 of port B (PB,) drives the gates of thyristors THl and TH4. Of course, of these two thyristors, only the one whose anode is positive at the instant

533

AMMASAIGOUNDEN AND SUBBIAH: MICROPROCESSOR-BASEDVOLTAGE

I

-y

msosurc frequency of

generator output M I t q e

FI E F E EI l find count

for initial

OC

load timer with count for interrupt

1

1/ .cs

I

I I

I I I

0

2A 3

A

4 2

5 2

2lI

I

wt

I

(b) Fig. 3. Three-phase fully controlled rectifier (PB denotes port B of 8255A, and suffixes 2, 3, and 4 refer to bit numbers of that port: (a) Power circuit; (b) gate trigger pulses.

I I

1. I I

I I

of receiving the gate pulse will conduct. The pulse program continues for a time interval T', . The rectifier gives an output dc load voltage corresponding to the initial a, and this voltage is also given to the ADC through a calibrated potential divider network. Two lines of port B (Bo and B,) are used for sending the address latch enable (ALE) and the start of conversion (SOC) signals to ADC and another three lines (B,-B,) for input channel selection. As soon as the conversion is over, the ADC sends the end of conversion (EOC) signal to the pP through line 0 of port C. All eight lines of port A are used to read the 8-b output data from the ADC. The pP compares the ADC output with the reference voltage and then increases or decreases the firing angle, depending on whether the ADC output voltage is more than v h or less than VI.

B. Interrupt Service Routine As the generator frequency varies with wind speed, new values of count for (Y and 60" are to be computed at frequent intervals. This feature is incorporated in the system by using RST 7.5 hardware interrupt that is available in Intel 8085. Occurrence of the interrupt is controlled by an 8253 programmable timer. The 8253 has three independent 16-b counters with separate gate, clock, and output pins, of which counter 1 is used in mode 0 configuration, i.e., 'interrupt on terminal count' mode. An external clock of low frequency of about 500 Hz is generated using a 555 oscillator and given as input to clock 1 of 8253. A count corresponding to the required interrupting time Ts2 is loaded into the counter. After this specified time, the interrupt occurs, and the interrupt service routine directs the program to the start. The time Ts2can be much longer than T,, if the wind speed is fairly steady. From the instant the interrupt occurs to the instant the pulse program is restarted, the thyristors are off, and this off time of the converter is only of the order of 50 ms. Normally, storage batteries are present in the system. These

I L_t Yes L _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - - - - - - - - --I- - - - - t I

Fig. 4. Flowchart for the working of voltage controller for induction generator.

Fig. 5. Equivalent circuit of induction generator with resistance load. All reactances correspond to rated frequency.

batteries also ensure continuity of supply at times when the wind velocity is below or above the normal range (i.e., when The entire sequence of working of the program is presented as a flowchart in Fig. 4. Wind driven self-excited induction generators are mostly used in isolated locations where wind potential is high. In such places, it is desirable to install the associated control systems that are basically maintenance free. Therefore, in such remote locations, simplicity and reliability are the most important criteria. Only with these objectives, a basic strategy using an 8-b pP and an A/D converter has been proposed for the control of these wind-driven generators.

III. STEADY-STATE ANALYSIS The steady-state analysis of the induction generator is made using the conventional equivalent circuit of the induction machine. However, as the operating frequency of the generator varies with the driving speed, the equivalent circuit

I IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 31, NO. 6, DECEMBER 1990

534

is modified, as is shown in Fig. 5 , where all the parameters are referred to the rated frequency [ 11, [7]. In Fig. 5 , fg a = p.u. frequency = -

(1)

4 0

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and

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--

(3)

a

where N; and Ns are synchronous speeds corresponding to generated frequency and rated frequency, respectively and N = actual rotor speed. Using this equivalent circuit, expressions for the various performance quantities of the self-excited induction generator have been derived; a computer program was developed earlier [ l]. However, when a controlled rectifier is used between the generator and load resistance, the load resistance has to be suitably represented in the equivalent circuit discussed as follows: Neglecting harmonics, the input voltage and current to the rectifier can be written as [3], [4] fivsinwt

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2.5

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The rectifier average output voltage is given by

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1100

1200

1300

1400

1500

1600

1700

b

rotor speed, rev I min

From (4) and (3, the equivalent impedance per phase at the generator terminals can be expressed as (7)

(b) Fig. 6 . Generator load current/speed characteristics for four-pole operation: (a) R,j = 125 a; @) R,j = 230 n.

formance prediction of induction generators are now extended to the generator-rectifier combination. A. Prediction of Performance

Using (6) and (7), 2, can be written as Ze =

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where

(9) and

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(10)

The load resistance R is represented as R l a in the equivalent circuit shown in Fig. 5. Therefore, the parameters R e and X e representing the equivalent resistance and reactance of R should also be represented as R e / a and X , / a , respectively. With these modifications, the computer algorithm and the expressions developed earlier [l] for the per-

A self-excited induction generator with four/six-pole combination is chosen. The equivalent circuit parameters and magnetization characteristics of this generator are given in the Appendix. Let it be assumed that this generator is connected to a three-phase fully controlled thyristor bridge shown in Fig. 3. For this system, the variation of generator load current with rotor speed has been predicted for various values of a for both four-pole and six-pole settings, each with two different load resistances; these characteristics are shown in Figs. 6 and 7. In all these cases, an excitation capacitance C = 100 pF is assumed. From such curves, the dc load current can be calculated for any rotor speed and a using the expression ?r

Id

=

-1,.

&

(11)

Then, for the particular value of load resistance considered, the dc load voltage and the generator line voltage V, can also

AMMASAIGOUNDEN AND SUBBIAH: MICROPROCESSOR-BASED VOLTA(>E

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700

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Fig. 7. Generator load current/speed characteristics for six-pole operation: (a) R , = 125 n; (b) R , = 230 0 .

535

the prediction of performance in Section 111-A, were used, and load tests were conducted by varying the rotor speed of the generator. In all these tests, the required dc voltages were obtained, which establishes the satisfactory working of the generator-rectifier combination and the associated controls. Both four-pole and six-pole settings of the generator were used in the appropriate speed ranges. Variation in firing angle with rotor speed obtained experimentally is indicated in Fig. 8 along with predicted values, and the agreement between them is satisfactory. Oscillographic waveforms of the output voltage and current of the rectifier and input voltage and current to the rectifier at steady state, when supplying 340 V to a load resistance of 230 62, are shown in Fig. 9 for two different speeds in the four-pole setting of the generator feeding the rectifier. Ideally, the output power of the wind-driven generator should be proportional to the cube of the wind speed. Therefore, an attempt was made to achieve this and to maintain a constant dc voltage using firing angle control and suitable load resistance variation. Two typical operating points obtained in this test with a constant dc voltage of 320 V are as follows: generator delivering 2400 W at 1430 r/min with a = 22.3" at a four-pole setting and 504 W at 850 r/min with a = 49.5" at a six-pole setting; both settings using a single excitation capacitor of 100 CIF. A dc motor has been used as a prime mover, and the speed of the motor is varied over the required speed range to simulate the expected speed variation of the wind turbine and the connected gear. In earlier studies, a dc motor has been used as the variable-speed drive for the induction generator [4], [7]- [lo]. The results of such studies have been found to be of relevance when the generator is installed along with the windmill. In the operation of self-excited induction generators, for a given capacitance value and load, there is a minimum speed below which the excitation cannot be sustained, i.e., the operating speed range is limited. To extend the speed range, pole-changing winding has been employed in the generator. This design consideration along with other practical points such as variation of frequency and output power with wind speed have been taken into account in the laboratory model. Of course, after the proposed system is installed at site along with the windmill, any required modification appropriate to a particular location can be easily made either in the hardware or in the software of the system.

be obtained using (6). For example, referring the Fig. 6(a) drawn for Rd = 125 62, the value of I, = 2.0 A for a = 20' at a rotor speed of 1295 r/min. Then, using ( l l ) , Id = 2.57 A and from (6), Vd and V, are calculated as 321.3 V and 253.2 (= 146.2 x 6 ) V, respectively. Since the main requirement is to maintain a specified dc load voltage over the working speed range of the wind-driven generator, it is of interest to deduce the variation of firing angle with rotor speed. Such curves are shown in Fig. 8 for various values of constant dc voltages and load resistances. V . CONCLUSIONS These curves show that the firing angle has to be raised steeply with increase in speed to hold the dc voltage at the 1) A closed-loop system using a microprocessor-based prescribed level. controlled rectifier has been developed for the voltage control of wind-driven self-excited induction generators. The details INVESTIGATIONS IV . EXPERIMENTAL of the configuration and working of the system have been The controlled rectifier was actuated by the pP-based fully described. Experimental results on the system amply controller described in Section II. This setup was connected demonstrate the successful implementation of both the hardto the self-excited induction generator with the four/six-pole ware and software of the control scheme and the ease with combination mentioned in Section 111-A. The generator was which the firing angle is automatically adjusted to maintain driven by a dc motor. The same excitation capacitance, load the desired load voltage with varying rotor speeds. resistances, and constant dc load voltage levels, assumed for 2) The steady-state analysis developed earlier for induc-

536

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 37, NO. 6, DECEMBER 1990

4 - pole

6 - pole

6 - pole

60

6C

50

50

40

40

0) 0)

(Y

m

9

4-po1e

0 0"

30

30

#

Y

20

20

IO

IO

0

0

700

900

1100 rotor

1300

speed

, rev/

1500

7

I

1100

900

min.

rotor

1300

1500

s p e e d , r e v / min

(b)

(a)

6 - pole

4 -pole

6C

5c

c

40

? a

30

20

IO

0 700

900

1100 rotor

1300

1500

1700

speed, r e v / min

(c)

Fig. 8. Firing angle/speed characteristics of generator-rectifier system for constant direct voltage ( A four-pole calculated; A four-pole experimental, 0 six-pole calculated, 0 six-pole experimental): (a) v d = 260 V, Rd = 125 8;(b) V d = 300 V, R d = 125 8;(c) V d = 340 V, R d = 230 8.

tion generators has been extended for the generator feeding a tor has been well demonstrated in the context of using a controlled rectifier, and the performance characteristics of controlled rectifier -scheme. The configuration of the entire the generator-rectifier combination have been predicted. control scheme can be easily adopted for generators of any 3) The advantage of applying pole-changing windings in other rating and pole combination suitable for wind-driven extending the operating speed range of the induction genera- applications.

537

AMMASAIGOUNDEN A N D SUBBIAH: MICROPROCESSOR-BASED VOLTAGE

1

for the four-pole setting and

E =

263 .O - 0 .05X,, 0 < X , I39 .O 343.4 - 2.16Xm, 39.0 < X, 5 52.5 410.8 - 3.48Xm, 52.5 < X, 5 67.5 850.0 - lO.OX,, 67.5 < X, 5 75.0

for the six-pole setting, where R I ,R , = stator and rotor (referred to stator) resistance, respectively; X , , X, = stator and rotor (referred to stator) reactance, respectively; X,,, =

(ii)

REFERENCES

~~

141 C. H. Watanabe and A. N. Barreto, “Self-excited induction genera-

(b) Fig. 9. Oscillographic waveforms at the input and output of the controlled rectifier fed from induction generator (a) N = 1340 r/min; (b) N = 1190 r/min. For both (a) and (b): (i) input voltage 200 V/div; 5 ms/div; (ii) input current 1.1 A/div, 5 ms/div; (iii) output voltage 200 V/div, 5 ms/div; (iv) output current 2.2 A/div, 5 ms/div. Smoothing reactor used is 50 mH.

APPENDIX

The parameters of the equivalent circuit of the induction generator, with four/six-pole combination used for the prediction of performance and for the experimental investigation, are as follows: four-pole six-pole

RI 1.o 4.0

R2 1.66 4.25

x,= x2 2.40 6.05.

tor/force-commutated rectifier system operating as a DC power supply,” Proc. Inst. Elec. Eng., vol. 134, no. 5 , pp. 255-260, 1987. K. P. Gokhale and G. N. Revankar, “Microprocessor-controlled separately excited DC-motor drive system,” IEE Proc. B , Electr. Power Appl., vol. 129, no. 6, pp. 344-352, 1982. B. K. Bose, “Technology trends in microcomputer control of electrical machines,” IEEE Trans. Ind. Electron., vol. 35, pp. 160- 177, Feb. 1988. L. Quazene and G. McPherson, Jr., “Analysis of the isolated induction generator,” IEEE Trans. Power A p p . Syst., vol. PAS-102, pp. 2793 -2798, 1983. S. S. Murthy, 0. P. Malik, and A. K. Tandon, “Analysis of self-excited induction generators,” IEE Proc. C, Gen., Trans. Distrib., vol. 129, no. 6, pp. 260-265, 1982. r91 A. K. Tandon, S. S. Murthy, and G . J. Berg, “Steady-state analysis of capacitor self-excited induction generators,” IEEE Trans. Power A p p . S y ~ t .vol. , PAS-103, pp. 612-618, 1984. r101 M. B. Brennen and A. Abbondanti, “Static exciters for induction generators,” IEEE Trans. Industry Appl., vol. IA-13, pp. 422-428, 1977. S. B. Dewan and A. Straughen, Power Semiconductor Circuits. New York: Wiley, 1975. Peripheral Design Handbook, Intel Corp., 1981.