an Adaptive Sherbius Induction Machine - IEEE Xplore

1. INTRODUCTION

Wind Power System Using an Adaptive Sherbius Induction Machine G. RAINA Harris Corporation O.P. MALIK The University of Calgary

A wind turbine driven variable speed power generation system using a static Sherbius scheme in the generation mode is described. A control methodology to adapt the power output characteristics of double output induction generator to the output characteristics of the driving turbine is proposed. The control scheme has been implemented and experimental test results show that the induction machine can be made to adapt to the desired characteristics.

Manuscript received September 26, 1985. This work was supported by the Natural Sciences and Engineering Research Council of Canada. Authors' addresses: O.P. Malik, Department of Electrical Engineering, University of Calgary, Calgary, Alta., T2N 1N4, Canada; G. Raina, Controls and Composition Division, Harris Corporation, P.O. Box 430, Melbourne, FL 32901.

0018-9251/86/0300-0204 $1.00 ©0 1986 IEEE

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A number of utility connected induction generator schemes have been proposed based on the concepts of variable speed induction motor drives. Some of the techniques investigated are [1]: (1) frequency conversion between the utility and the induction generator (2) machine designed with higher rotor resistance (3) wound rotor induction machine with facilities for slip power recovery. The first arrangement, using a cycloconverter, allows the machine to operate in a continuously variable speed fashion. However, these converters need to have the full rating of the induction machine and are not available off the shelf. The second technique causes a reduction in the slope of the torque-speed characteristics and thereby a higher pull out speed. A larger speed range seems apparent. Even though the currents tend to be lower for a given speed and torque than a conventionally designed machine, the i2r losses still remain the same. The machine efficiency usually tends to fall off rapidly at speeds 50 percent above the synchronous speed. A derated machine would be an expensive option to avoid thermal stresses. In category (3), the induction machine is operated above synchronous speed in a wider than normal speed range, as is the case with category (1). With a wound rotor machine, the slip power (less the rotor i2r loss) is transferred from the rotor windings to an external circuit via slip rings. This slip power, however, is at a slip dependent voltage and frequency and must be converted into power at line voltage and line frequency using an external circuit. Slip power recovery has generated considerable interest and many developments have taken place in this area since the advent of power electronic converters [25]. The development effort has been directed mainly to the speed control of induction machines as motors rather than generators. Thyristor converter based slip power recovery can be competitive and even more flexible than scheme (1) discussed above. A variable speed constant frequency wind power system using a double output induction machine and feeding into an electric utility system is investigated in this paper. Given the peculiarity of the wind turbine power characteristics, the induction machine characteristics need to be adapted to match the former. A method of calculation to achieve this adaptability has been presented. The proposed control has been implemented using standard integrated components and its effectiveness has been verified by experimental tests.

11. PROPOSED GENERATION SCHEME The proposed scheme incorporates a wound rotor induction generator with means for rotor power

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-22, NO. 3 MARCH 1986

machine and the system changes from that of a semiconstant to a variable speed WECS. The firing angle of the inverter determines the power transferred from the rotor to the mains. Thus the apparent rotor circuit impedance can be varied by the variation of the firing angle of the inverter, and the induction machine characteristics can be adapted to the turbine characteristics.

Ill. ANALYSIS OF A DOIG WITH EXTERNAL ROTOR RESISTANCE Because of a comparatively large inertia, a wind turbine does not respond quickly to normal wind speed variations. Therefore, the induction machine can be adequately represented by a steady-state equivalent circuit. regeneration. Introduction of a counter emf is a form of For analysis, the equivalent circuit of an induction slip power recovery and can be exploited in the machine can be considerably simplified by the application generation mode of the induction machine. The Scherbius of Thevenin's network theorem. The equivalent circuit system is a good example of introducing a counter emf in thus assumes the form shown in Fig. 2 [6]. In this figure, the rotor circuit for slip power recovery. E' and Z' the Thevenin equivalent source represent Basic circuit configuration of the proposed dispersed and voltage impedance, respectively. The slip power wind energy conversion system (WECS) involving the recovered from the rotor and fed into the system is Scherbius system is illustrated in Fig. 1. Power generated an represented resistance R, in the rotor by equivalent from the stator is at the line frequency and is directly fed circuit. to the electric power system. The terminals of the rotor of With due care for signs in the generation mode, the the induction machine are connected to an ac/dc/ac power transferred from the stator to the electric network power converter. Rotor power at the slip dependent rotor voltage is given by and slip dependent frequency is rectified to dc by an uncontrolled rectifier. A choke in the dc link smooths out ++ 1 Rs + 12 Pmains - PM - 3(IISI2 the harmonic ripples associated with the dc voltage. The IER0Il2rI2R 1 r Rr)) (1) dc power is fed to a three-phase thyristor bridge inverter where Ro is the resistance corresponding to the core loss, whose output terminals are connected to the mains is the sum of the rotor resistance Rr and Rx, and Pm is RrT a through transformer (to reduce counter emf at low slips). The thyristor bridge operates as a line commutated the power provided by the turbine. The power Px associated with the equivalent inverter when, in theory, the firing angle ox is varied resistance Rx, given by between 900 and 1800. In practice, the upper limit of ao cannot exceed 1650. This scheme is also called a double Px = 3 IIr 12 Rx (2) output induction generator (DOIG) scheme since the is recovered and delivered from the rotor to the mains. power output is derived both from the stator as well as the rotor. For an induction machine, the slip at which the maximum power/torque occurs is directly related to the The wound rotor induction machine with slip power recovery has been used primarily as a drive in wide speed effective resistance in the rotor circuit. The maximum range motor applications. For use in the generator mode, torque, however, does not change but the slope of the it is necessary that the induction machine system be torque-speed characteristics can be controlled by a variation of the equivalent resistance Rx in the rotor symbiotic with the turbine. The speed/power output circuit. characteristics of the wind turbine are well defined and cannot be modified to suit the induction machine. It is thus necessary to develop a strategy for varying the characteristics of the induction machine continuously with the turbine speed. X Ir Rr/S The scheme shown in Fig. 1 meets the double purpose of variable speed and adaptability to turbine 9 f ~~~~~~~~~RX characteristics. Since the power is recovered from the S rotor circuit by a power converter, the recovered power is B reflected as an additional resistance in the rotor circuit. Because of this feature the operating speed range is Fig. 2. Induction machine equivalent circuit simplified by Thevenin's widened compared with a standard squirrel cage induction theorem. Fig. 1. Schematic of the proposed dispersed WECS.

Is

RAINA & MALIK: WIND POWER SHERBIUS INDUCTION MACHINE

205

2.0f

As the power available from a turbine varies as the cube of its rotor speed, there is a large variation in the power output over the full operating speed range. As a consequence, a fairly large R, would be needed to transfer turbine output at full speed through the induction machine stator and rotor. This would imply that power absorbed by the external rotor resistance would be approximately equal to that delivered to the network from the stator. As a first approximation, this fact would have the implication that an induction machine rated close to one-half of the maximum turbine output could be selected without overloading the stator or the rotor [1, 7].

1.5 _

'. 1.0 o 0.5 a-

0.0 _ -0.5 .~1500

IV. NUMERICAL CALCULATIONS BASED ON ROTOR RESISTANCE CONTROL In order to illustrate the concept of rotor resistance control for continuously varying the induction machine torque-speed characteristics, a 2.5 hp, 4 pole, 60 Hz, 208 V, 8 A, 1690 rev/min, A/Y connected wound rotor induction machine has been used. Parameters and constants for this machine as measured in the laboratory are given as follows: base voltage - rated phase voltage = 120 V base current = rated line current = 8 A base impedance = base voltage/base current = 15 fl base power = base voltage x base current = 960 W base frequency = rated frequency = 377 rad/s = 0.0576 pu stator resistance per phase = rotor resistance per phase = Rr = 0.0660 pu stator leakage reactance per phase = xl= 0.0883 pu rotor leakage reactance per phase = xlr = 0.0883 pu magnetizing reactance per phase = xm = 2.0352 pu exciting conductance per phase = IRO = 0.0937 pu.

Results of a computer simulation study showing the desired variation of external rotor resistance Rx with speed are illustrated in Fig. 3. The variation is obviously nonlinear. The reason why Rx is constant after about 2950 rev/min is apparent in Section V.

3000

3500

3000 2500 RPM

3500

2500

2000

j i-A

4000

RPM

1.0o

0.5 CL 3 0

0.0

-n 1v E%

1500

,

2000

4000

Fig. 4. Power relationships in a double output induction machine. a, maximum power available from turbine; b, total power output from induction machine; c, rotor output; d, stator output.

Based on the principles expounded above, the results from the power relationships are illustrated in Fig. 4 which indicate a close matching of turbine characteristics with the total power converted by the stator and the rotor of the induction machine over a fairly wide speed range. V. ANALYSIS OF THE PROPOSED WECS USING

SCHERBIUS DOIG

31

Referring to Fig. 1, if the leakage reactances are assumed negligible and the filtering of the dc voltage is ideal [8], the rotor current has a waveform consisting of alternating square pulses of 2I/3 rad duration. The rms values of the rotor current Ir and its fundamental component I22 are related as

cr

irr

=

2000

2500

3000 RPM

3500

4000

Fig. 3. Desired variation of external rotor resistance Rx. (RPM = rev/min.) 206

(3)

emf due to inverter equals 1.35 VT line voltage on the low voltage is the cos (X, where VT In principle, the inverter extracts transformer. side of the the inverter operates with the when rotor from the energy firing angle between the limits of 900 and 1800.

The FI00

22.

average counter

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-22, NO. 3 MARCH 1986

RI

Following the method developed in [8], the power balance equation of each rotor phase can be written as E22I22= r22I2 +

Rb

Rf I2C

1

,Rb s

1

-(1.35 VTcos a- W)Id,

Pm

Fig. 5. Equivalent circuit corresponding to (7).

(4)

+-

3

3

where E22 is the rms value of the rotor emf at fundamental frequency per phase, Rf is the resistance of the smoothing reactor, and W is the voltage drop in the thyristors. The dc current Idc is related to the rotor current Ir by the equation [9]

practical upper limit of a is 1650, the values of a after about 2950 rev/min are not within the range of interest. Thus, the value of a corresponding to 2950 rev/min is held constant for the remaining speed range. As a consequence of invariability of ao after a particular speed, the variation of external resistance in the resistance control calculations is terminated as shown in Idc Ir. (5) Fig. 3. Minutiae of continuously adapting the characteristics of the induction machine to that of a wind Power dissipation in Rf is, therefore, given by 1/2 Rf2I. turbine by providing control means to modify the A further assumption can be made that the mechanical apparent rotor resistance according to the turbine speed torque is produced as a result of the fundamental are discussed in Section VI. component '22 of the rotor current. The equation for Pm at a slip s can be then written as VI. IMPLEMENTATION OF THE CONTROL Rf SCHEME 3 + [ (r22 PM -) I22 =

=

(6)

2

The control technique described above was tested by laboratory experimentation. A block diagram representation of the various elements involved in the implementation of the proposed control is shown in Fig. 7. Most of these elements have been developed in the power electronics laboratory of the University of Calgary.

Idc (1.35 VTcosa O-W) 3 1 S-S 1-

S

Therefore, =2(I22

(9

)(r22 +

-

)(

2)

+

[(r22

+

-

(1.35

VTcosoa -

'22

A. Speed Measurement

22

W)

d I22]

1

(7)

An equivalent circuit simulated on the basis of (7) is shown in Fig. 5 where the effect of the stator resistance is included. In the equivalent circuit, R, = (Tr2/9 1) (r22 + Rf/2) represents the losses due to harmonics in the -

The speed measurement consists of counting clock pulses for a period equal to one revolution of the shaft. A disc with 16 slots is mounted on the shaft. A light source and detector arrangement produces a pulse every time a slot passes a fixed point. The pulses are divided in frequency by 16 to produce one pulse per shaft revolution

rotor current. Further,

Rb = b

r22

+

--(

(1.35 VTcosoX

~~23

-

W)

IT

6I2

190

(8)

and it takes into account the effect of the inverter. Neglecting the filter resistance and semiconductor voltage drop, a comparison of Rb with RrT of Section III (where an equivalent external rotor resistance is used) yields the expression for inverter firing angle ao as

U) W

a 140 I

a. _J :

)] (C2 Based on the values of I, and RrT calculated with

165

W

115

1.35 VT n 2

resistance control, the variation of ao over the speed range under consideration is shown in Fig. 6. Because the RAINA & MALIK: WIND POWER SHERBIUS INDUCTION MACHINE

RPM

Fig. 6. Variation of inverter firing angle

o

with speed.

207

ISOLATED LOW LEVEL SIGNAL

REFERENCE

SIGNALS

T2

,T3

,T4 ..

T6

GE H2IAI .5

AMP\ GATED CARRIER OUT PU T

PULSE AMPLIFIER

IISOLATOR

GATE r CATHODE

DUTY CYCLE

1/3

Fig. 7. Block diagram representation of control elements for thyristor triggering. (a) Timing signal generation for thyristors. (b) Gate triggering circuit.

Since a ROM is programmed in hexadecimal values only, the values of C,, calculated from (11) and converted into hexadecimal equivalent pose a peculiar problem. The hexadecimal values calculated are each 2 bytes in size. Each memory location, however, is only 1 byte in size. As an example, the hexadecimal value of C0, corresponding to a certain C (8 bit address) of 250 is 17E1-the lowest significant byte (LSB) being El and the most significant byte (MSB) 17. Thus each value of C0, has to be accommodated in two consecutive memory locations. The upper one is allocated to the LSB and the lower one to the MSB. This can be accomplished by using an EPROM having at least one more address line than the minimum of 8 needed. An Intel 2732 (32K) EPROM [11] with 12 address B. Firing Angle Data Storage lines serves the purpose. The address generated is fed to lines A, to A8 leaving Ao as zero. When Ao is zero (with the half of 60 Hz, C = 250), the output of the EPROM is the LSB El. One Based on the supply frequency a into converted is of the functions of the microsequencer shown in Fig. 7 is period of 8.333 ms (covering 180°) clock or to change the address bit Ao from 0 to 1. The resulting count, CO, using 1 MHz as a reference change in address, C = 251, causes the EPROM to spill frequency. The count is given by the MSB 17. It may be noted that C = 251 is only an 8.333 ox a*8.333 x 10-3 * 1 X 106 (11) artificial address created to facilitate output from the . =cotr_ 180 180 EPROM in 2 bytes. The output from the EPROM is at a very fast rate (including changing Ao from 0 to 1) Thus the desired values of a over the given speed range compared with the change in Al to A8 address values have been boosted into numbers C. suitable for further updated every half a second. It is apparent that the processing. capacity of the 2732 EPROM used is memory is a particular Given that for each value of speed there but the use of this chip is worth the underutilized the be is to digital fired, value of a at which the inverter in manipulating addresses compared it offers flexibility and the speed count C corresponding to C,, corresponding EPROM with only 8 address lines. 2716 (16K) with Intel C count The useful a purpose. to the firing angle serve of a three phase controlled In the usual applications ROM in a location to a memory serves as an address laboratory, the address to in electronics converter power the to are corresponding contents equal whose the EPROM is derived from a reference voltage to value of CO.

which in turn starts and stops the counting of the clock pulses [10]. The count after one revolution is given by 60 XfcLk (10) C N where fclk is the clock frequency in hertz and N is the induction machine speed in revolutions per minute. Using a 2 Hz clock, the count is updated every half a second. For the speed range under consideration (1800 rev/min to 3600 rev/min) and a clock frequency of 5 kHz, the count is detected as an 8 bit output from the speed measurement system.

_

208

_

_

_

_

_

_

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-22, NO. 3 MARCH 1986

convert ac voltage into the desired dc value. Its function was modified with the address serving as speed input to the EPROM and its contents loaded with values corresponding to ao to modify induction machine characteristics continuously. The function of other elements in the control circuit is described in the next section.

C. Generation of Timing Delays and Triggering Pulses Three single phase transformers in each phase of the line voltage generate low level isolated signals. Any notches in these signals are eliminated using filters which cause a phase shift in the process as well. The resulting signals are used as an input to generate square waves with phase relationship restored back to the prefilter condition. The square waves generated serve as reference signals and form inputs to the gates of an Intel 8253 interval timer/counter [12] (having 3 counters) and the microsequencer. The primary function of this chip is to generate accurate timing delays for thyristor triggering. The status of AO, A,, and WR determined by a microsequencer decides the loading of a counter and the number of the counter. Output from the EPROM is directed to the address bus of this chip and accepts 16 bit data in 2 consecutive bytes as explained earlier. Outputs from the microsequencer are rectangular pulses with appropriate time delays for triggering the thyristors. The amplitude of these outputs is not sufficient to trigger a thyristor and therefore needs to be amplified. The outputs are modulated by a carrier of duty cycle 1/3 to produce a pulse train (to avoid saturation of pulse amplifier) and further amplified to produce trigger pulse of 1.5 A amplitude. Only the first pulse of the gated carrier output is useful. Once it turns on the thyristor, the rest of the pulses do not serve any purpose. The thyristors are line commutated.

VIl. TEST RESULTS

Using (9), the values of ot were recalculated for the driving power shown in Fig. 8(a). The corresponding values of C and Ca were also calculated. Using ROMANAID [13] software (computer aided design of memory based circuits) available on the VAX 11/780 computer, the Ca data was stored and transferred to Intel microcomputer for blasting of the EPROM. Using the experimental setup shown in Fig. 7, measurements were made and are shown in Fig. 8. The measured values can be seen to concur with the calculated values.

VIll. CONCLUSIONS Static Scherbius double output induction machine has mainly been restricted to speed control in the motoring mode. Its application in the generation mode has been discussed particularly in the realm of wind energy conversion. Given the peculiarity of the wind turbine power characteristics, the induction machine characteristics need to be adapted to match the former. A method of calculation to achieve this adaptability has been presented. The control principle has been verified by laboratory implementation of a different driving torque characteristic. The results show that, in principle, the induction machine can be made to adapt to any desired characteristic.

1.95 1.85

1.75 ° 1.65

0

a

1.55 ,

Ir

U

b a . .,,

1

, .,' .

1

... ..

.,

Since the turbine characteristics shown in Fig. 4 could 1950 2050 2150 2250 2350 RPM not be duplicated in the laboratory, an alternative procedure was formulated to verify the principle of Fig. 8. Power relationship in a dc machine driven DOIG. a, power adapting the induction machine characteristics. A dc available from the dc machine; b, total power output from the induction motor was used to drive the induction machine and machine. x x x represent experimental values. controlled to provide the output torque/power equal to the rated power of the induction machine over a limited speed range (2000 to 2200 rev/min). The dc machine speed could not be increased beyond 2200 rev/min as it ACKNOWLEDGMENTS became unstable with the reduction in field current. Power relationships of the dc and induction machine are The authors wish to acknowledge the help of P. shown in Fig. 8. Walsh with the experimental work.

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[7]

REFERENCES

[1]

[2]

[3]

[4]

[5]

[6]

Bolton, H.R., Lam, W.C., and Freris, L.L. (1980) Double output induction generator scheme for wind energy conversion. In Proceedings of the 2nd BWEA Conference, Cranfield, England, 1980. Smith, G.A., and Nigim, K.A. (1981) Wind energy recovery by a static Scherbius induction generator. Proceedings of the IEE, 128, 6 (Nov. 1981), 317-324. Rao, N.N., Dubey, G.K., and Prabhu, S.S. (1983) Slip-power recovery scheme employing a fully controlled converter with half controlled characteristics. Proceedings of the IEE, pt. B, 130, 1 (Jan. 1983), 33-38. Shepherd, W., and Stanway, J. (1969) Slip power recovery in an induction motor by the use of a thyristor inverter. IEEE Transactions on Industry Applications, IA-5, 1 (Jan./ Feb. 1969), 74-82. Murphy, J.M.D. (1973) Thyristor Control of AC Motors. New York: Pergamon Press, 1973. Fitzgerald, A.E., Kingsley, C., Jr., and Kusko, A. (1971) Electric Machinery, 3rd ed. New York: McGraw-Hill, 1971.

[8]

[9] [10]

[11] [12] [13]

Velayudhan, C., Bundell, J.H., and Leary, B.G. (1983) An adaptive rotor resistance control for wind driven slip-ring induction generator. Presented at the International Electrical, Electronics Conference and Exposition, Toronto, Ont., Canada, Sept. 1983. Lavi, A., and Polge, R.J. (1966) Induction motor speed control with static inverter in the rotor. IEEE Transactions on Power Apparatus and Systems, PAS85, 1 (Jan. 1966), 76-84. Schaefer, J. (1965) Rectifier Circuits-Theory and Design. New York: Wiley, 1965. Zissos, D. (1979) Problems and Solutions in Logic Design. New York: Oxford, 1979. MCS-85 User's Manual, Intel Corporation, Santa Clara, Calif., Sept. 1978. MCS-48 User's Manual, Intel Corporation, Santa Clara, Calif., July 1978. Givens, K., Dehler, G., and Hancock, G. (1983) Romanaid. Manual for Romanaid available on Vax 11/780. Department of Electrical Engineering, University of Calgary, Calgary, Alta., Canada, 1983.

G. Raina received the B.E.E. degree from the University of Kashmir in 1978, the M.Sc. degree in engineering from Panjabi University in 1980, and the Ph.D. degree in electrical engineering from the University of Calgary, Calgary, Alta., Canada, in 1985. During 1980-1981 he was a lecturer at Panjabi University and worked with a consulting organization. He is currently a senior engineer in the Power System Analysis Section of Harris Corporation, Controls and Composition Division, Melbourne, Fla. He is presently involved in the development of energy management systems and advanced power application programs.

O.P. Malik (M'66-SM'69) graduated in electrical engineering from Delhi Polytechnic, India, in 1952. He received the M.E. degree in electrical machine design from the University of Roorkee, India, in 1962 and the Ph.D. degree from the University of London, London, England, and D.I.C. from the Imperial College of Science and Technology, London, both in 1965. From 1952 to 1961 he worked with electric utilities in India on various aspects of design, construction, and operation of power systems. For one year he was a Confederation of British Industries Scholar in the United Kingdom. In 1965 he worked with the English Electric Company in England. He is now in Canada, where he taught for two years at the University of Windsor and is at present at the University of Calgary. Dr. Malik is a Fellow of the Institution of Electrical Engineers (London), a member of the Canadian Electrical Association, and the American Society for Engineering Education. He is a registered Professional Engineer in the Provinces of Alberta and Ontario, Canada. 210

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-22, NO. 3 MARCH 1986