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Practical simulation of a wind turbine driven self-excited induction generator
Energy Convers. Mgmt Vol. 34, No. 3, pp. 187-199,1993 Printed in Great Britain.All rights reserved
0196-8904/93 $6.00+ 0.00 Copyright © 1993PergamonPress Ltd
PRACTICAL SIMULATION OF A WIND TURBINE DRIVEN SELF-EXCITED INDUCTION GENERATOR S. C. TRIPATHY, M. KALANTAR and R. BALASUBRAMANIAN Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi-110 016, India (Received 18 October 1991; received for publication 20 June 1992)
Abstract--Owing to the increased emphasis on renewable resources, the development of suitable isolated power generators driven by energy sources such as wind, small hydro-electric, biogas, etc. has recently assumed greater significance. A capacitor self-excited induction generator has emerged as a suitable candidate of isolated power sources. The utilization of a three-phase squirrel cage induction motor as an autonomous self-excited induction generator is reviewed. Variations of electrical power due to changes in wind speed should be as small as possible. This is obtained by using an induction generator driven by a wind turbine. The system incorporates use of turbine blade angle pitch control to control the wind turbine speed and shaft torque. Variable pitch turbines operate efficientlyover a wider range of wind speeds. Self excitation with capacitors at the stator terminals of the induction machines is well demonstrated experimentally on a d.c. motor-induction machine set. The parameters and the excitation requirements of the induction machine, run in self-excited induction generator mode, are determined. The effects of variations in prime mover speed, terminal capacitance and the load power factor on the machine terminal voltage are studied. A problem related to the fluctuation of power and frequency is the voltage regulation, this problem is mainly associated with wind turbines coupled to induction generators. Ways of overcoming the problem of voltage and frequency control in self-excitedinduction generators have been proposed in this work. The saturable reactors in parallel or series capacitors connected with the load are suggested for voltage regulation. The system frequency deviation due to random load perturbations is then reduced, using a superconducting magnetic energy storage unit connected to the system. Wind turbine power generation
Self-excitedinduction generators
INTRODUCTION Wind turbines may be an attractive alternative in areas where fuel is usually expensive and wind regimes are particularly favorable. These c o n d i t i o n s are likely to occur in places which are remote a n d have weak a u t o n o m o u s power systems. W i n d energy systems are already p r o v e n to be a viable alternative to c o n v e n t i o n a l fuel based systems o n isolated locations, such as coastal a n d island regions in m a n y places. W i n d t u r b i n e generators are the most a d v a n c e d a m o n g the alternative energy technologies a n d are expected to find their economically viable applications in these areas where electricity costs are high. V a r i a t i o n s of electrical power due to changes in wind speed should be as small as possible. This is o b t a i n e d by using a n i n d u c t i o n generator driven by a wind turbine. F r o m the c o n t r o l p o i n t of view, unlike s y n c h r o n o u s generators, i n d u c t i o n generators are high compliance couplings between the mechanical a n d the electrical system [1]. The controlled variables are t u r b i n e speed a n d shaft torque. The c o n t r o l acts o n the t u r b i n e blade angle (pitch control). Since the t o r q u e - s p e e d characteristic o f the i n d u c t i o n generator is nearly linear in the operating regions, torque changes are reflected as speed changes. Therefore, it is possible to provide a single speed controller, acting via the blade pitch, to c o n t r o l speed as well as torque (power). The i n d u c t i o n generator has been f o u n d to be very a p p r o p r i a t e and, hence, is mostly employed, as it compares favorably with a s y n c h r o n o u s generator as follows: (i) (ii) (iii) (iv) (v)
N o s y n c h r o n i z i n g requirement, A b s e n c e of a separate d.c. source, Brushless rotor, ruggedness, Decreased u n i t cost, less m a i n t e n a n c e , It can be started as a m o t o r a n d c h a n g e d to generating m o d e b e y o n d s y n c h r o n o u s speed. 187
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A three-phase induction machine can work as a self-excited induction generator when its rotor is driven at suitable speeds by an external prime mover. Its excitation is provided by connecting a three-phase capacitor bank at the stator terminals. Over the years, several studies have been reported by numerous authors, De Mello and Hannett [2], Murthy et al. [3] and Malik and Haque [4], regarding the utilization of a squirrel cage induction motor as an autonomous self-excited induction generator. However, the self-excited induction generator has not found commercial application so far, owing to its inability to control the output voltage and frequency under varying conditions of loading. In the literature, several authors, Arrillaga and Watson [5] and Bonert and Hoops [6], have proposed use of an electronic voltage regulator to control the voltage and an a.c.--d.c.-a.c, conversion circuit to control the frequency. However, the main impediments coming in the way of using these electronic circuits are the high overall cost of the system and its required maintenance. In the present work, the use of a saturable core reactor or a series capacitor connected with the load is suggested for voltage regulation. The frequency deviation due to load perturbation is then reduced using a superconducting magnetic energy storage unit. This may be a step further towards the commercial application of these generators.
THEORY If an appropriate capacitor bank is connected across the terminals of an externally driven induction machine, an e.m.f, tends to be generated. This phenomenon is known as Capacitor Self Excitation. Induced e.m.f, and current in the windings will increase to a level governed by magnetic saturation in the machine. The capacitors provide the magnetizing VARs in the event of an external lagging power factor load and also the reactive load requirement. In order to reach a steady-state generating mode, some remnant magnetism must be present in the machine core initially. When the squirrel cage rotor of the motor is spun at constant speed by the prime mover, a small voltage is induced in the excitation winding due to the residual flux. Due to the capacitor self-excitation phenomenon, the resultant flux in the air gap is increased, which increases the voltage induced in the winding. This cumulative process continues until the voltage across the capacitor matches the induced voltage due to magnetic saturation, and a steady-state situation is reached so that no further rise in voltage or current is possible. The steady a.c. current flowing in the winding establishes an air gap flux, pulsating in time, due to which an induction of voltage takes place in the load winding via the rotating rotor. A current flows in the load winding when an external electric load is connected. As the loading is increased, the load current increases, and the terminal voltage decreases. The saturable core reactor or series capacitor connected with the load winding counters the voltage reduction so that the variation in terminal voltage from no load to full load is within acceptable limits at different operating power factors. Figure 1 shows the basic configuration of a wind turbine generator on an isolated power system. For the best performance of the system, it is required that:
"~Ae ~
Wind
power
Energy (~
sysiem
conversion
BladeI_ pitch controlFluid coupling
~+rnax ~")-" Pwtg Load
Fig. 1. Basic configuration of a wind turbine generator on an isolated power system.
TRIPATHYet
al.:
SELF-EXCITEDINDUCTIONGENERATOR ls
189
VL,F
I1
Induction generator
Prime mover
|Ct Capacitor
Load
Fig. 2. Self-excitedinductiongeneratorsystem. (i) Variations of electrical power due to changes in wind speed should be as small as possible. (ii) The wind turbine generator should always operate within its design limits. (iii) The wind turbine generator should participate in the short term, as well as the long term, adjustment of generated power that occurs when there is a change in load. (iv) A wind turbine generator with a given set of controls should be able to operate in a small power system where it represents the only, or the major, source of power as well as in a large power system where it represents an insignificant part of the total power. The generating system, Fig. 2, consists of an induction machine, driven by a prime mover having a three-phase terminal capacitor to provide self excitation and the load. As the load varies randomly, the capacitor has to be varied to obtain the desired voltage regulation. Measurement of equivalent circuit parameters The equivalent circuit parameters of a three-phase induction generator can be measured from the following tests performed on a 1.75 kW, delta connected, 415 V, 3 A, 2 pole, 50 Hz, squirrel cage induction motor. d.c. resistance measurement. The d.c. resistance of the stator winding per phase can be obtained by applying a low value of d.c. voltage across its terminals and measuring the current, and the ratio between them gives the resistance. The value of stator resistance obtained is multiplied by a correction factor of 1.1-1.3 to account for temperature and skin effects under normal a.c. operation. Blocked rotor test. A three-phase supply is connected to the stator terminals. The rotor is blocked, and the voltage is increased gradually from zero so that rated current flows in the winding. V~, I~ and Wsc are measured. At slip S = - 1 , the voltage required to circulate full load current is very low, therefore, core loss and magnetizing current are neglected. We will have the following relations Z~
=
V~/I~:,
p~=
X~ = X,~ + X,r, x~ = ( z ~
-
W ~ I I, ~2
X,~=X,r=X=I2
(1)
R~) '/~
Rr = R~ - Rs The blocked rotor test equivalent circuit is shown in Fig. 3(a). Synchronous speed test. Here, the rotor is not blocked and is allowed to move freely without external load, so that the torque delivered is zero. To ensure zero slip, the rotor is driven at the (a)
(b) Rs
JXls
JXlr
Rs
Is c
l
JXls Is
Rr
Fig. 3. (a) Blocked rotor test equivalent circuit; (b) synchronous speed test equivalent circuit.
lm IXm
190
TRIPATHY et al.:
SELF-EXCITED INDUCTION GENERATOR
synchronous speed corresponding to line frequency (i.e. F = 1) using an external prime mover. The phasor equation is, Vg = Es - (Rs + jX,~)I~.
(2)
The stator impedance drop is neglected due to low stator current on no load. Therefore
V,-- Es,
P~=P~-IL{2R~
P s = V~Iscos tp,
R~=lVgl2/P~
II=l=lV, I/e~,
(3)
1I,,,l=(I2s-I~) '/2
Xm= l V, I/lIml (a)
1.2
1.0
l
0.8
-4 ~
0.6
0.4
0.2
I
0
1.2-
2
I
3
I
I
I
I
I
4 5 6 X m (p.u. ohm)
I
7
8
9
10
(b)
1.o -
0.8
>. =. 0.6 LL 0.4 0.2
0
I
{
I
I
I
I
0.2
0.4
0.6
0.8
1.0
1.2
Im
(p.u. A)
Fig. 4. (a) Vadation of air gap voltage with magnetizing reactance; (b) variation of air gap voltage with magnetizing current.
TRIPATHY
et al.: SELF-EXCITED INDUCTION GENERATOR
191
The synchronous speed equivalent circuit is shown in Fig. 3(b). F r o m these tests, one can determine the magnetization characteristics. Calculations are done, using computer simulation, and the following graphs are drawn, as shown in Fig. 4. (a)
Vg/F vs "~m (b) Vg/Fv s I m.
The saturated portion of this characteristic can be linearized and expressed in the form:
Vg/F = K 1 - K2Xm
(4)
where K~ a n d / ( 2 depend upon the design of the machine.
Experimental set-up and machine parameters The induction machine of the rating given in the previous section was coupled to a separately excited d.c. drive m o t o r to provide different constant speeds. A three-phase capacitor bank was connected to the machine terminals to obtain self-excited generator action. A variable three-phase RL load was used to load the induction generator. The parameters and constants for this machine, as measured in the laboratory, are given below Base Base Base Base Base Base
voltage = rated phase voltage = 415 V current = rated phase current = 1.732 A impedance = base voltage/base current = 240 f~ power = base voltage • base current = 720 W frequency = rated frequency = 50 Hz speed = 3000 rpm.
The measured machine parameters are Stator Rotor Stator Rotor
resistance per phase = Rs = 0.0825 p.u. f~ resistance per phase = Rr = 0.0562 p.u. f~ leakage reactance per phase = X~s = 0.198 p.u. f~ leakage reactance per phase = X~r = 0.198 p.u. ~.
To determine the magnetizing reactance at different air gap voltages Vg/F, the machine was driven at synchronous speed by the d.c. motor, and the input impedance per phase was measured at different input voltages. As we need the variation of Xm with the air gap flux, proportional to Vg/F, it is necessary to calculate the air gap voltage by subtracting the voltage drop in the stator leakage impedance from the input voltage. Xm at each voltage is obtained by subtracting the stator leakage impedance from the measured input impedance. Figure 4(a) shows the experimental results relating Vg/Fwith Xm. The variation of Vg/Fwith Xm will be non-linear due to magnetic saturation. To simplify the analysis, the variation under the saturated region can be linearized using the appropriate curve drawn in Fig. 4(a). Referring to Fig. 4(a), we have
V~/F = a x 415
-
[-'~m
-
c x415 (b x 240)] d x 240
(5)
where base voltage = 415 V and base impedance = 240 f~. After substituting values for a, b, c and d in the above relation, we get
Vs/F= 1.1 x 4 1 5 - [Xm - (1.75 x 240)] = 518.7 - 0.148Xm.
0.6 × 415 7.0 x 240 (6)
For this particular machine, the values of K~ and/£2 are found to be 518.7 and 0.148, respectively. CONTROLLER
CONCEPT
It is desired that the induction generator provide a constant terminal voltage and frequency under varying loads. In practice, a drop in both the terminal voltage and frequency with increasing load is an observed feature. In the present work, an attempt has been made to improve the voltage and frequency regulation of a self-excited induction generator.
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TRIPATHY et al.:
SELF-EXCITED INDUCTION GENERATOR
+ O
T
-
O
d.c.
220V
i )
!
[
i Shunt I~k/KI capacitor ~ _ _ . ~
Saturablecore reactor
Load
Fig, 5. Schematic diagram of self-excited induction generator with saturable core reactor.
Voltage control using saturable core reactor Voltage regulation is the most severe problem faced by a self-excited induction generator. To overcome this problem, the use of a saturable core reactor, connected in parallel with the load, is investigated in this work. The current taken by the saturable core reactor is very small at rated voltage, but as the voltage across the reactor increases, the current drawn by the reactor increases. If the voltage reaches the value on which it saturates, then the saturable reactor draws a large value of current and prevents further voltage rise. So, if this saturable core reactor is connected in parallel with the load, it will stabilize voltage at the saturation voltage. At rated output and rated terminal voltage, the saturable core reactor draws zero current, but if the load is reduced, the voltage rises and the reactor starts drawing current through its inductance, hence, reduces the voltage. As the load reduces to no load, this saturable core reactor saturates and prevents the voltage to rise. Figure 5 shows the schematic diagram of the self-excited induction generator with the saturable core reactor connected in parallel with the load. Voltage control using series capacitor In order to maintain the terminal voltage constant, the capacitive VARs have to be adjusted as the load on the self-excited induction generator varies. The series capacitor, in addition to its ability to correct steady-state voltage conditions, has the advantage of being instantaneous and correcting voltage fluctuations quite fast. In order to investigate the effect of series capacitors on the performance of the self-excited induction generator, a standard 1.75 kW induction motor was considered. For a given machine, the proper value of the series capacitor will depend upon the power factor of the load, speed of the machine and the excitation capacitance. A number of suitable capacitors can be used to provide discrete steps of varying reactive power to meet the varying load demand of reactive power by the self-excited induction generator. Figure 6 shows the schematic diagram of the self-excited induction generator with voltage control using the series capacitor. The values of the series and parallel capacitors, theoretically, can be obtained from the voltage saturation curve of the machine. The slope of the tangent to the knee of the curve gives the value of capacitive reactance (Xp) of the parallel capacitor. Also, the slope of the tangent to the upper part of the saturation curve, as shown in Fig. 4(b), gives the capacitive reactance (Xs) value of the series capacitor (Bim et al. [7]). Frequency control using SMES unit The frequency deviation due to load perturbation is reduced by using a superconducting magnetic energy storage unit. During high wind speed and less load demand, the excess energy will be stored in the magnets with superconductive windings of the SMES unit. Such superconducting magnetic energy storage systems would consist of a superconducting inductor, a helium refrigerator, dewar system to keep the temperature well below the critical temperature for the superconductor, and
TRIPATHY et al.:
SELF-EXCITED INDUCTION G E N E R A T O R
193
o
o
T
,
d.c.
220V
T
!
o
Tt _
i
VA.,AC
Shunt ~____~~ capacitor
o I
--
"/ Load
Fig. 6. Schematic diagram of self-excited induction generator with series capacitor.
an a.c./d.c, converter. Charging and discharging is achieved by varying the d.c. voltage, applied to the inductor, through a wide range of positive and negative values. This can be achieved by controlling the delay angle of commutation. The prime mover speed is kept constant using wind turbine blade angle pitch control. A total power set point is selected which can be adjusted t¥om zero to its maximum value. This power set point will also take into account the capacity of the SMES unit. When wind power rises above the power set point and the SMES unit is fully charged, the pitch control system begins operating to maintain an average power equal to the set point. The pitch control system consists of a power measurement transducer, a manual power set point control, a proportional plus integral feedback function, and a hydraulic actuator which varies the pitch of the blades. Turbine blade pitch control has a significant impact on the dynamic behaviour of the system. This type of control only exists in horizontal axis machines. Variable pitch turbines operate efficiently over a wider range of wind speeds than fixed pitch machines. However, cost and complexity are higher. The system model with SMES connected into it is fully discussed by Tripathy et al. [8]. PERFORMANCE OF THE SEIG ON LOAD WITH tl.c. MOTOR From the tests performed on 1.75 kW, three-phase induction machine, the following significant observations can be made. Variation of shunt capacitance with load Figure 7 shows the variation of shunt capacitance from no load to full load, keeping the terminal voltage constant, for different operating load power factors. The shunt capacitor is connected in parallel with the stator main winding to ensure self excitation. Therefore, keeping the terminal voltage constant, the minimum value of capacitor (Cmi,) required to ensure self excitation at 1.0, 0.8 and 0.6 power factor loads are 12, 15 and 18 #F/phase respectively. However, to improve the system performance, the value of capacitor should be chosen slightly above (Cmi,). AS it is seen from Fig. 7, the value of shunt capacitor required to obtain the self excitation in the machine is increased as the power factor changes from unity to a lower value. Similarly, for the same power factor load, the value of the shunt capacitor required for self excitation is increased from no load to full load. Variation of voltage with load Figure 8 shows the variation of voltage from no load to full load for different operating load power factors. It is seen that the value of the shunt capacitor (slightly more than Cram) required to cause self excitation in the machine at unity power factor load is 16 #F/phase. For this value of capacitor, the machine is self-excited at almost rated speed (i.e. 3000 rpm). This observation
194
TRIPATHY et al.:
SELF-EXCITED INDUCTION GENERATOR
50
I 40 -
p.f. = 0.6 "N~
p.f. = 0.8 • .=
~
,
a0
v
II) 0 c-
O
o. 0
20
10
I 0
0.2
0.4
0.6
0.8
1.0
Output power (p.u. W) Fig. 7. Shunt capacitance variation with load for different power factors.
indicates that the minimum value of capacitor required for self excitation to take place changes to a higher value as the voltage increases. For the load power factor of 0.8 and 0.6, the values of the shunt capacitor obtained are 20 and 24 #F/phase, respectively. It is found that the terminal voltage gets reduced from 460 to 360 in the case of the unity power factor load. Therefore, the voltage regulation in this case is 27.7%. The voltage regulation thus obtained for 0.8 and 0.6 power factor loads are 31.4 and 35.4%, respectively. Therefore, it is noticed that the voltage regulation is better for the higher power factor loads. Figure 9 shows the variation of the terminal voltage with load by increasing the shunt capacitor. It has been observed that the machine terminal voltage rises as the value of the shunt capacitor is increased for the same power factor load. 1.2
1.1
~, 1.o
~
0.9
0
> 0.sF-
/
l O.7
P'f" = 0.6
/
0.6 I
0
I
I
I
I
1
0,2
0.4
0.6
0.8
1,0
Output power (p.u. W) Fig. 8. Voltage variation with load for different power factors.
TRIPATHY
et al.: SELF-EXCITED INDUCTION GENERATOR
195
1.2 -
1.1
> ~.
1.0 20 ~F
~ o
0.9
16~F
~
>
14p_F
0.8 --
0.7 0
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
Output power (p.u. W)
Fig. 9. Voltage variation with load for different shunt capacitor values.
Effect of speed variation To study the effect of speed variation, the curves of Fig. 10 are compared• It is seen that the voltage regulation obtained by keeping the prime mover speed constant from no load to full load is 27.7%. When the prime mover speed is allowed to change with the load, the voltage regulation falls to 31.4%, hence showing that the better regulation can be achieved if the speed of the prime mover is kept constant. Effect of saturable core reactor Figure 11 shows the effect of the saturable core reactor on voltage regulation• It is seen that, at constant prime mover speed, the voltage regulation from no load to full load (at different power factors) obtained without the saturable core reactor is 33%. By adding the saturable core reactor in parallel with the load, the voltage regulation is improved to 16.8%, as shown in Fig. l l(a). 1.2 -
1.1 A >
-
~
n
t
speed
1.0
g ~
>o
0.9
Variablespeed
0.8 --
0.7 0.6 0
I 0.2
I 0.4
I 0.6
I
I
0.8
1.0
Output power (p.u. W)
Fig. I0. Effect of prime-mover speed on voltage regulation.
196
TRIPATHYet al.: SELF-EXCITEDINDUCTIONGENERATOR 1.2 I(a) 1.1
>
~ With saturable
1.0
ID
~
O.9-
Withoutsaturable ,/ core reactor 0.8 --
0.7
I
L
I
I
0.2
0.4
0.6
0.8
~.o I (b) 0.8 --
.o.o ; o~ n 0.4
fj // /
Withsaturable core reactor
0.2
0
L
I
I
I
0.2
0.4
0.6
0.8
Output power (p.u. W) Fig. 11. (a) Voltage regulation with and without saturable core reactor; (b) power factor variation with load with and without saturable core reactor.
Figure 11 (b) shows the variation of power factor with load for the case with and without the saturable core reactor connected to the SEIG. It is shown that, for the same value of excitation capacitor, the machine loading can be increased with the saturable core reactor.
Effect of series capacitor By obtaining the load characteristics of the self-excited induction generator without the series capacitor in the circuit, the voltage regulation at unity power factor load was found to be 27.7% as shown in Fig. 8. When a capacitor of 80 # F/phase was connected in series with the unity power factor load, the voltage regulation improved to 10% as shown in Fig. 12(a). The test was carried out by replacing the 80#F/phase series capacitor with capacitors of 100 and 60 /z F/phase. However, regulation in the case of 100#F/phase was found to be 12.6%, and in the case of 60/tF/phase, it was found to be 15.3% as shown in Fig. 12(b). The test was repeated by replacing the unity power factor load with a 0.8 power factor load. Figure 13 shows the load characteristics
TRIPATHY et
1.2 t
SELF-EXCITED INDUCTION GENERATOR
al.:
197
(a)
1.1
>
1.o
•'-' >0
0.9
i capacitor
0.8 -
0.7
I 0.2
o
1.1o-
1.05 -
I 0.4
I 0.6
I 0.8
I 1.o
(b)
1oo ~tF
>
d. 1 .oo t~
>0 0.95
0.90
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
Output power (p.u. W) Fig. 12. (a) Voltage regulation with and without seriescapacitor for u.p.f, load; (b) voltage variation with u.p.f, load for different series capacitor values•
of the self-excited induction generator at 0.8 power factor load. It is seen that the voltage regulation with series capacitors of 120, 100 and 80 t~F/phase are 13.9, 11.4 and 16.8%, respectively. Figure 14 shows the variation in terminal voltage from no load to full load with speed variation at 0.8 power factor load. When the speed is allowed to vary, the voltage regulation obtained is 12.8%. This indicates that the voltage regulation will be better with constant speed of the prime mover and is about 10% for this particular machine. CONCLUSIONS The main objective of this investigation was to examine the possibility of using a self-excited induction generator for wind power generation. The results of the investigation reported confirmed the fact that a normally designed three-phase squirrel cage induction motor can be used as a wind turbine driven self-excited induction generator for supplying the load demand.
198
TRIPATHY et al.: 1.2
-
1.1
SELF-EXCITED INDUCTION GENERATOR
(a)
m
With
>
series
1.0
r
~D O) tl:l
"-' 0 >
0.9
--
.I" Without series capacitor 0.8
--
0.7
I 0.2
0
~.10-
1.05
I 0.4
I 0.6
I 0.8
(b)
--
t.00 1 O0 N,F 0
>
0.95
~
--
120 gF
80 gF
0.90
0
0.2
0.4
0.6
0.8
Output power (p.u. W) Fig. 13. (a) Voltage regulation with and without series capacitor for 0,8 p,f. load; (b) voltage regulation with 0.8 p,f. load for different series capacitor values.
The load characteristics of the three-phase self-excited induction generator for wind power isolated stand-alone application based on the capacitor self-excitation phenomenon are presented here. Experiments were conducted on a d.c. motor-induction machine set to determine the parameters and the excitation requirements of the induction machine in the self-excited generator mode. It is shown that the values of the shunt capacitor required to cause self excitation in the machine at 1.0, 0.8 and 0.6 power factor load are 16, 20 and 24 #F/phase, respectively. The effect of varying the prime mover speed on the performance of the induction generator has also been examined, and it has been found that a speed variation of 6.5% changes the voltage regulation from 27.7 to 31.4%. The study has also demonstrated the effectiveness of using a saturable core reactor for improvement of voltage regulation. It has been found that the use of a saturable core reactor in parallel with the load improves voltage regulation from 33 to 16.8%. The study has further shown that there is a tremendous improvement in voltage regulation, to 10%, by connecting a
TRIPATHY et al.: SELF-EXCITED INDUCTION GENERATOR
199
1.2 -
1.1
--
> 1.0 5 m o~ O
Variable speed
0.9
>
0.8
0.7 0
I
I
t
I
0.2
0.4
0.6
0.8
O u t p u t p o w e r (p.u. W)
Fig. 14. Effect of speed on voltage regulation for 0.8 p.f. load with series capacitor of 100 #F. p r o p e r t h r e e - p h a s e c a p a c i t o r b a n k in series with the load. T h e frequency d e v i a t i o n due to l o a d p e r t u r b a t i o n is then reduced, using a s u p e r c o n d u c t i n g m a g n e t i c energy storage unit. D u e to their simplicity, ruggedness a n d low cost o f c o n s t r u c t i o n , squirrel cage i n d u c t i o n m a c h i n e s offer a relatively inexpensive alternative to a.c. g e n e r a t i o n using wind p o w e r in the s t a n d - a l o n e m o d e o r in parallel with o t h e r c o n v e n t i o n a l sources, like diesel in a h y b r i d w i n d - d i e s e l isolated p o w e r system. REFERENCES
I. 2. 3. 4. 5. 6. 7. 8.
E. N. Hinrichsen, IEEE Trans. Power Apparatus Syst. PAS-103, 886 (April 1984). F. P. De Mello and L. N. Hannett, IEEE Trans. Power Apparatus Syst. PAS-100, 2610 (May 1981). S. S. Murthy, O. P. Malik and A. K. Tandon, IEE Proe. 129, Pt C, No. 6 (November 1982). N. H. Malik and S. E. Haque, IEEE Trans. Energy Convers. EC-I, 133 (September 1986). J. Arrillaga and D. B. Watson, IEE Proe. 125, 743 (August 1978). R. Bonert and G. Hoops, IEEE Trans. Energy Convers. 5, 28 (March 1990). E. Bim, J. Szajner and Y. Burian, IEEE Trans. Energy Convers. 4, 526 (September 1989). S. C. Tripathy, M. Kalantar and R. Balasubramanian, Dynamics and stability of wind and diesel turbine generators with superconducting magnetic energy storage unit on an isolated power system. IEEE Winter Power Meeting, Paper No. 136-2-EC (February 1991).
E C M 34/3---D
0196-8904/93 $6.00+ 0.00 Copyright © 1993PergamonPress Ltd
PRACTICAL SIMULATION OF A WIND TURBINE DRIVEN SELF-EXCITED INDUCTION GENERATOR S. C. TRIPATHY, M. KALANTAR and R. BALASUBRAMANIAN Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi-110 016, India (Received 18 October 1991; received for publication 20 June 1992)
Abstract--Owing to the increased emphasis on renewable resources, the development of suitable isolated power generators driven by energy sources such as wind, small hydro-electric, biogas, etc. has recently assumed greater significance. A capacitor self-excited induction generator has emerged as a suitable candidate of isolated power sources. The utilization of a three-phase squirrel cage induction motor as an autonomous self-excited induction generator is reviewed. Variations of electrical power due to changes in wind speed should be as small as possible. This is obtained by using an induction generator driven by a wind turbine. The system incorporates use of turbine blade angle pitch control to control the wind turbine speed and shaft torque. Variable pitch turbines operate efficientlyover a wider range of wind speeds. Self excitation with capacitors at the stator terminals of the induction machines is well demonstrated experimentally on a d.c. motor-induction machine set. The parameters and the excitation requirements of the induction machine, run in self-excited induction generator mode, are determined. The effects of variations in prime mover speed, terminal capacitance and the load power factor on the machine terminal voltage are studied. A problem related to the fluctuation of power and frequency is the voltage regulation, this problem is mainly associated with wind turbines coupled to induction generators. Ways of overcoming the problem of voltage and frequency control in self-excitedinduction generators have been proposed in this work. The saturable reactors in parallel or series capacitors connected with the load are suggested for voltage regulation. The system frequency deviation due to random load perturbations is then reduced, using a superconducting magnetic energy storage unit connected to the system. Wind turbine power generation
Self-excitedinduction generators
INTRODUCTION Wind turbines may be an attractive alternative in areas where fuel is usually expensive and wind regimes are particularly favorable. These c o n d i t i o n s are likely to occur in places which are remote a n d have weak a u t o n o m o u s power systems. W i n d energy systems are already p r o v e n to be a viable alternative to c o n v e n t i o n a l fuel based systems o n isolated locations, such as coastal a n d island regions in m a n y places. W i n d t u r b i n e generators are the most a d v a n c e d a m o n g the alternative energy technologies a n d are expected to find their economically viable applications in these areas where electricity costs are high. V a r i a t i o n s of electrical power due to changes in wind speed should be as small as possible. This is o b t a i n e d by using a n i n d u c t i o n generator driven by a wind turbine. F r o m the c o n t r o l p o i n t of view, unlike s y n c h r o n o u s generators, i n d u c t i o n generators are high compliance couplings between the mechanical a n d the electrical system [1]. The controlled variables are t u r b i n e speed a n d shaft torque. The c o n t r o l acts o n the t u r b i n e blade angle (pitch control). Since the t o r q u e - s p e e d characteristic o f the i n d u c t i o n generator is nearly linear in the operating regions, torque changes are reflected as speed changes. Therefore, it is possible to provide a single speed controller, acting via the blade pitch, to c o n t r o l speed as well as torque (power). The i n d u c t i o n generator has been f o u n d to be very a p p r o p r i a t e and, hence, is mostly employed, as it compares favorably with a s y n c h r o n o u s generator as follows: (i) (ii) (iii) (iv) (v)
N o s y n c h r o n i z i n g requirement, A b s e n c e of a separate d.c. source, Brushless rotor, ruggedness, Decreased u n i t cost, less m a i n t e n a n c e , It can be started as a m o t o r a n d c h a n g e d to generating m o d e b e y o n d s y n c h r o n o u s speed. 187
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A three-phase induction machine can work as a self-excited induction generator when its rotor is driven at suitable speeds by an external prime mover. Its excitation is provided by connecting a three-phase capacitor bank at the stator terminals. Over the years, several studies have been reported by numerous authors, De Mello and Hannett [2], Murthy et al. [3] and Malik and Haque [4], regarding the utilization of a squirrel cage induction motor as an autonomous self-excited induction generator. However, the self-excited induction generator has not found commercial application so far, owing to its inability to control the output voltage and frequency under varying conditions of loading. In the literature, several authors, Arrillaga and Watson [5] and Bonert and Hoops [6], have proposed use of an electronic voltage regulator to control the voltage and an a.c.--d.c.-a.c, conversion circuit to control the frequency. However, the main impediments coming in the way of using these electronic circuits are the high overall cost of the system and its required maintenance. In the present work, the use of a saturable core reactor or a series capacitor connected with the load is suggested for voltage regulation. The frequency deviation due to load perturbation is then reduced using a superconducting magnetic energy storage unit. This may be a step further towards the commercial application of these generators.
THEORY If an appropriate capacitor bank is connected across the terminals of an externally driven induction machine, an e.m.f, tends to be generated. This phenomenon is known as Capacitor Self Excitation. Induced e.m.f, and current in the windings will increase to a level governed by magnetic saturation in the machine. The capacitors provide the magnetizing VARs in the event of an external lagging power factor load and also the reactive load requirement. In order to reach a steady-state generating mode, some remnant magnetism must be present in the machine core initially. When the squirrel cage rotor of the motor is spun at constant speed by the prime mover, a small voltage is induced in the excitation winding due to the residual flux. Due to the capacitor self-excitation phenomenon, the resultant flux in the air gap is increased, which increases the voltage induced in the winding. This cumulative process continues until the voltage across the capacitor matches the induced voltage due to magnetic saturation, and a steady-state situation is reached so that no further rise in voltage or current is possible. The steady a.c. current flowing in the winding establishes an air gap flux, pulsating in time, due to which an induction of voltage takes place in the load winding via the rotating rotor. A current flows in the load winding when an external electric load is connected. As the loading is increased, the load current increases, and the terminal voltage decreases. The saturable core reactor or series capacitor connected with the load winding counters the voltage reduction so that the variation in terminal voltage from no load to full load is within acceptable limits at different operating power factors. Figure 1 shows the basic configuration of a wind turbine generator on an isolated power system. For the best performance of the system, it is required that:
"~Ae ~
Wind
power
Energy (~
sysiem
conversion
BladeI_ pitch controlFluid coupling
~+rnax ~")-" Pwtg Load
Fig. 1. Basic configuration of a wind turbine generator on an isolated power system.
TRIPATHYet
al.:
SELF-EXCITEDINDUCTIONGENERATOR ls
189
VL,F
I1
Induction generator
Prime mover
|Ct Capacitor
Load
Fig. 2. Self-excitedinductiongeneratorsystem. (i) Variations of electrical power due to changes in wind speed should be as small as possible. (ii) The wind turbine generator should always operate within its design limits. (iii) The wind turbine generator should participate in the short term, as well as the long term, adjustment of generated power that occurs when there is a change in load. (iv) A wind turbine generator with a given set of controls should be able to operate in a small power system where it represents the only, or the major, source of power as well as in a large power system where it represents an insignificant part of the total power. The generating system, Fig. 2, consists of an induction machine, driven by a prime mover having a three-phase terminal capacitor to provide self excitation and the load. As the load varies randomly, the capacitor has to be varied to obtain the desired voltage regulation. Measurement of equivalent circuit parameters The equivalent circuit parameters of a three-phase induction generator can be measured from the following tests performed on a 1.75 kW, delta connected, 415 V, 3 A, 2 pole, 50 Hz, squirrel cage induction motor. d.c. resistance measurement. The d.c. resistance of the stator winding per phase can be obtained by applying a low value of d.c. voltage across its terminals and measuring the current, and the ratio between them gives the resistance. The value of stator resistance obtained is multiplied by a correction factor of 1.1-1.3 to account for temperature and skin effects under normal a.c. operation. Blocked rotor test. A three-phase supply is connected to the stator terminals. The rotor is blocked, and the voltage is increased gradually from zero so that rated current flows in the winding. V~, I~ and Wsc are measured. At slip S = - 1 , the voltage required to circulate full load current is very low, therefore, core loss and magnetizing current are neglected. We will have the following relations Z~
=
V~/I~:,
p~=
X~ = X,~ + X,r, x~ = ( z ~
-
W ~ I I, ~2
X,~=X,r=X=I2
(1)
R~) '/~
Rr = R~ - Rs The blocked rotor test equivalent circuit is shown in Fig. 3(a). Synchronous speed test. Here, the rotor is not blocked and is allowed to move freely without external load, so that the torque delivered is zero. To ensure zero slip, the rotor is driven at the (a)
(b) Rs
JXls
JXlr
Rs
Is c
l
JXls Is
Rr
Fig. 3. (a) Blocked rotor test equivalent circuit; (b) synchronous speed test equivalent circuit.
lm IXm
190
TRIPATHY et al.:
SELF-EXCITED INDUCTION GENERATOR
synchronous speed corresponding to line frequency (i.e. F = 1) using an external prime mover. The phasor equation is, Vg = Es - (Rs + jX,~)I~.
(2)
The stator impedance drop is neglected due to low stator current on no load. Therefore
V,-- Es,
P~=P~-IL{2R~
P s = V~Iscos tp,
R~=lVgl2/P~
II=l=lV, I/e~,
(3)
1I,,,l=(I2s-I~) '/2
Xm= l V, I/lIml (a)
1.2
1.0
l
0.8
-4 ~
0.6
0.4
0.2
I
0
1.2-
2
I
3
I
I
I
I
I
4 5 6 X m (p.u. ohm)
I
7
8
9
10
(b)
1.o -
0.8
>. =. 0.6 LL 0.4 0.2
0
I
{
I
I
I
I
0.2
0.4
0.6
0.8
1.0
1.2
Im
(p.u. A)
Fig. 4. (a) Vadation of air gap voltage with magnetizing reactance; (b) variation of air gap voltage with magnetizing current.
TRIPATHY
et al.: SELF-EXCITED INDUCTION GENERATOR
191
The synchronous speed equivalent circuit is shown in Fig. 3(b). F r o m these tests, one can determine the magnetization characteristics. Calculations are done, using computer simulation, and the following graphs are drawn, as shown in Fig. 4. (a)
Vg/F vs "~m (b) Vg/Fv s I m.
The saturated portion of this characteristic can be linearized and expressed in the form:
Vg/F = K 1 - K2Xm
(4)
where K~ a n d / ( 2 depend upon the design of the machine.
Experimental set-up and machine parameters The induction machine of the rating given in the previous section was coupled to a separately excited d.c. drive m o t o r to provide different constant speeds. A three-phase capacitor bank was connected to the machine terminals to obtain self-excited generator action. A variable three-phase RL load was used to load the induction generator. The parameters and constants for this machine, as measured in the laboratory, are given below Base Base Base Base Base Base
voltage = rated phase voltage = 415 V current = rated phase current = 1.732 A impedance = base voltage/base current = 240 f~ power = base voltage • base current = 720 W frequency = rated frequency = 50 Hz speed = 3000 rpm.
The measured machine parameters are Stator Rotor Stator Rotor
resistance per phase = Rs = 0.0825 p.u. f~ resistance per phase = Rr = 0.0562 p.u. f~ leakage reactance per phase = X~s = 0.198 p.u. f~ leakage reactance per phase = X~r = 0.198 p.u. ~.
To determine the magnetizing reactance at different air gap voltages Vg/F, the machine was driven at synchronous speed by the d.c. motor, and the input impedance per phase was measured at different input voltages. As we need the variation of Xm with the air gap flux, proportional to Vg/F, it is necessary to calculate the air gap voltage by subtracting the voltage drop in the stator leakage impedance from the input voltage. Xm at each voltage is obtained by subtracting the stator leakage impedance from the measured input impedance. Figure 4(a) shows the experimental results relating Vg/Fwith Xm. The variation of Vg/Fwith Xm will be non-linear due to magnetic saturation. To simplify the analysis, the variation under the saturated region can be linearized using the appropriate curve drawn in Fig. 4(a). Referring to Fig. 4(a), we have
V~/F = a x 415
-
[-'~m
-
c x415 (b x 240)] d x 240
(5)
where base voltage = 415 V and base impedance = 240 f~. After substituting values for a, b, c and d in the above relation, we get
Vs/F= 1.1 x 4 1 5 - [Xm - (1.75 x 240)] = 518.7 - 0.148Xm.
0.6 × 415 7.0 x 240 (6)
For this particular machine, the values of K~ and/£2 are found to be 518.7 and 0.148, respectively. CONTROLLER
CONCEPT
It is desired that the induction generator provide a constant terminal voltage and frequency under varying loads. In practice, a drop in both the terminal voltage and frequency with increasing load is an observed feature. In the present work, an attempt has been made to improve the voltage and frequency regulation of a self-excited induction generator.
192
TRIPATHY et al.:
SELF-EXCITED INDUCTION GENERATOR
+ O
T
-
O
d.c.
220V
i )
!
[
i Shunt I~k/KI capacitor ~ _ _ . ~
Saturablecore reactor
Load
Fig, 5. Schematic diagram of self-excited induction generator with saturable core reactor.
Voltage control using saturable core reactor Voltage regulation is the most severe problem faced by a self-excited induction generator. To overcome this problem, the use of a saturable core reactor, connected in parallel with the load, is investigated in this work. The current taken by the saturable core reactor is very small at rated voltage, but as the voltage across the reactor increases, the current drawn by the reactor increases. If the voltage reaches the value on which it saturates, then the saturable reactor draws a large value of current and prevents further voltage rise. So, if this saturable core reactor is connected in parallel with the load, it will stabilize voltage at the saturation voltage. At rated output and rated terminal voltage, the saturable core reactor draws zero current, but if the load is reduced, the voltage rises and the reactor starts drawing current through its inductance, hence, reduces the voltage. As the load reduces to no load, this saturable core reactor saturates and prevents the voltage to rise. Figure 5 shows the schematic diagram of the self-excited induction generator with the saturable core reactor connected in parallel with the load. Voltage control using series capacitor In order to maintain the terminal voltage constant, the capacitive VARs have to be adjusted as the load on the self-excited induction generator varies. The series capacitor, in addition to its ability to correct steady-state voltage conditions, has the advantage of being instantaneous and correcting voltage fluctuations quite fast. In order to investigate the effect of series capacitors on the performance of the self-excited induction generator, a standard 1.75 kW induction motor was considered. For a given machine, the proper value of the series capacitor will depend upon the power factor of the load, speed of the machine and the excitation capacitance. A number of suitable capacitors can be used to provide discrete steps of varying reactive power to meet the varying load demand of reactive power by the self-excited induction generator. Figure 6 shows the schematic diagram of the self-excited induction generator with voltage control using the series capacitor. The values of the series and parallel capacitors, theoretically, can be obtained from the voltage saturation curve of the machine. The slope of the tangent to the knee of the curve gives the value of capacitive reactance (Xp) of the parallel capacitor. Also, the slope of the tangent to the upper part of the saturation curve, as shown in Fig. 4(b), gives the capacitive reactance (Xs) value of the series capacitor (Bim et al. [7]). Frequency control using SMES unit The frequency deviation due to load perturbation is reduced by using a superconducting magnetic energy storage unit. During high wind speed and less load demand, the excess energy will be stored in the magnets with superconductive windings of the SMES unit. Such superconducting magnetic energy storage systems would consist of a superconducting inductor, a helium refrigerator, dewar system to keep the temperature well below the critical temperature for the superconductor, and
TRIPATHY et al.:
SELF-EXCITED INDUCTION G E N E R A T O R
193
o
o
T
,
d.c.
220V
T
!
o
Tt _
i
VA.,AC
Shunt ~____~~ capacitor
o I
--
"/ Load
Fig. 6. Schematic diagram of self-excited induction generator with series capacitor.
an a.c./d.c, converter. Charging and discharging is achieved by varying the d.c. voltage, applied to the inductor, through a wide range of positive and negative values. This can be achieved by controlling the delay angle of commutation. The prime mover speed is kept constant using wind turbine blade angle pitch control. A total power set point is selected which can be adjusted t¥om zero to its maximum value. This power set point will also take into account the capacity of the SMES unit. When wind power rises above the power set point and the SMES unit is fully charged, the pitch control system begins operating to maintain an average power equal to the set point. The pitch control system consists of a power measurement transducer, a manual power set point control, a proportional plus integral feedback function, and a hydraulic actuator which varies the pitch of the blades. Turbine blade pitch control has a significant impact on the dynamic behaviour of the system. This type of control only exists in horizontal axis machines. Variable pitch turbines operate efficiently over a wider range of wind speeds than fixed pitch machines. However, cost and complexity are higher. The system model with SMES connected into it is fully discussed by Tripathy et al. [8]. PERFORMANCE OF THE SEIG ON LOAD WITH tl.c. MOTOR From the tests performed on 1.75 kW, three-phase induction machine, the following significant observations can be made. Variation of shunt capacitance with load Figure 7 shows the variation of shunt capacitance from no load to full load, keeping the terminal voltage constant, for different operating load power factors. The shunt capacitor is connected in parallel with the stator main winding to ensure self excitation. Therefore, keeping the terminal voltage constant, the minimum value of capacitor (Cmi,) required to ensure self excitation at 1.0, 0.8 and 0.6 power factor loads are 12, 15 and 18 #F/phase respectively. However, to improve the system performance, the value of capacitor should be chosen slightly above (Cmi,). AS it is seen from Fig. 7, the value of shunt capacitor required to obtain the self excitation in the machine is increased as the power factor changes from unity to a lower value. Similarly, for the same power factor load, the value of the shunt capacitor required for self excitation is increased from no load to full load. Variation of voltage with load Figure 8 shows the variation of voltage from no load to full load for different operating load power factors. It is seen that the value of the shunt capacitor (slightly more than Cram) required to cause self excitation in the machine at unity power factor load is 16 #F/phase. For this value of capacitor, the machine is self-excited at almost rated speed (i.e. 3000 rpm). This observation
194
TRIPATHY et al.:
SELF-EXCITED INDUCTION GENERATOR
50
I 40 -
p.f. = 0.6 "N~
p.f. = 0.8 • .=
~
,
a0
v
II) 0 c-
O
o. 0
20
10
I 0
0.2
0.4
0.6
0.8
1.0
Output power (p.u. W) Fig. 7. Shunt capacitance variation with load for different power factors.
indicates that the minimum value of capacitor required for self excitation to take place changes to a higher value as the voltage increases. For the load power factor of 0.8 and 0.6, the values of the shunt capacitor obtained are 20 and 24 #F/phase, respectively. It is found that the terminal voltage gets reduced from 460 to 360 in the case of the unity power factor load. Therefore, the voltage regulation in this case is 27.7%. The voltage regulation thus obtained for 0.8 and 0.6 power factor loads are 31.4 and 35.4%, respectively. Therefore, it is noticed that the voltage regulation is better for the higher power factor loads. Figure 9 shows the variation of the terminal voltage with load by increasing the shunt capacitor. It has been observed that the machine terminal voltage rises as the value of the shunt capacitor is increased for the same power factor load. 1.2
1.1
~, 1.o
~
0.9
0
> 0.sF-
/
l O.7
P'f" = 0.6
/
0.6 I
0
I
I
I
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1
0,2
0.4
0.6
0.8
1,0
Output power (p.u. W) Fig. 8. Voltage variation with load for different power factors.
TRIPATHY
et al.: SELF-EXCITED INDUCTION GENERATOR
195
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1.1
> ~.
1.0 20 ~F
~ o
0.9
16~F
~
>
14p_F
0.8 --
0.7 0
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
Output power (p.u. W)
Fig. 9. Voltage variation with load for different shunt capacitor values.
Effect of speed variation To study the effect of speed variation, the curves of Fig. 10 are compared• It is seen that the voltage regulation obtained by keeping the prime mover speed constant from no load to full load is 27.7%. When the prime mover speed is allowed to change with the load, the voltage regulation falls to 31.4%, hence showing that the better regulation can be achieved if the speed of the prime mover is kept constant. Effect of saturable core reactor Figure 11 shows the effect of the saturable core reactor on voltage regulation• It is seen that, at constant prime mover speed, the voltage regulation from no load to full load (at different power factors) obtained without the saturable core reactor is 33%. By adding the saturable core reactor in parallel with the load, the voltage regulation is improved to 16.8%, as shown in Fig. l l(a). 1.2 -
1.1 A >
-
~
n
t
speed
1.0
g ~
>o
0.9
Variablespeed
0.8 --
0.7 0.6 0
I 0.2
I 0.4
I 0.6
I
I
0.8
1.0
Output power (p.u. W)
Fig. I0. Effect of prime-mover speed on voltage regulation.
196
TRIPATHYet al.: SELF-EXCITEDINDUCTIONGENERATOR 1.2 I(a) 1.1
>
~ With saturable
1.0
ID
~
O.9-
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0.7
I
L
I
I
0.2
0.4
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~.o I (b) 0.8 --
.o.o ; o~ n 0.4
fj // /
Withsaturable core reactor
0.2
0
L
I
I
I
0.2
0.4
0.6
0.8
Output power (p.u. W) Fig. 11. (a) Voltage regulation with and without saturable core reactor; (b) power factor variation with load with and without saturable core reactor.
Figure 11 (b) shows the variation of power factor with load for the case with and without the saturable core reactor connected to the SEIG. It is shown that, for the same value of excitation capacitor, the machine loading can be increased with the saturable core reactor.
Effect of series capacitor By obtaining the load characteristics of the self-excited induction generator without the series capacitor in the circuit, the voltage regulation at unity power factor load was found to be 27.7% as shown in Fig. 8. When a capacitor of 80 # F/phase was connected in series with the unity power factor load, the voltage regulation improved to 10% as shown in Fig. 12(a). The test was carried out by replacing the 80#F/phase series capacitor with capacitors of 100 and 60 /z F/phase. However, regulation in the case of 100#F/phase was found to be 12.6%, and in the case of 60/tF/phase, it was found to be 15.3% as shown in Fig. 12(b). The test was repeated by replacing the unity power factor load with a 0.8 power factor load. Figure 13 shows the load characteristics
TRIPATHY et
1.2 t
SELF-EXCITED INDUCTION GENERATOR
al.:
197
(a)
1.1
>
1.o
•'-' >0
0.9
i capacitor
0.8 -
0.7
I 0.2
o
1.1o-
1.05 -
I 0.4
I 0.6
I 0.8
I 1.o
(b)
1oo ~tF
>
d. 1 .oo t~
>0 0.95
0.90
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
Output power (p.u. W) Fig. 12. (a) Voltage regulation with and without seriescapacitor for u.p.f, load; (b) voltage variation with u.p.f, load for different series capacitor values•
of the self-excited induction generator at 0.8 power factor load. It is seen that the voltage regulation with series capacitors of 120, 100 and 80 t~F/phase are 13.9, 11.4 and 16.8%, respectively. Figure 14 shows the variation in terminal voltage from no load to full load with speed variation at 0.8 power factor load. When the speed is allowed to vary, the voltage regulation obtained is 12.8%. This indicates that the voltage regulation will be better with constant speed of the prime mover and is about 10% for this particular machine. CONCLUSIONS The main objective of this investigation was to examine the possibility of using a self-excited induction generator for wind power generation. The results of the investigation reported confirmed the fact that a normally designed three-phase squirrel cage induction motor can be used as a wind turbine driven self-excited induction generator for supplying the load demand.
198
TRIPATHY et al.: 1.2
-
1.1
SELF-EXCITED INDUCTION GENERATOR
(a)
m
With
>
series
1.0
r
~D O) tl:l
"-' 0 >
0.9
--
.I" Without series capacitor 0.8
--
0.7
I 0.2
0
~.10-
1.05
I 0.4
I 0.6
I 0.8
(b)
--
t.00 1 O0 N,F 0
>
0.95
~
--
120 gF
80 gF
0.90
0
0.2
0.4
0.6
0.8
Output power (p.u. W) Fig. 13. (a) Voltage regulation with and without series capacitor for 0,8 p,f. load; (b) voltage regulation with 0.8 p,f. load for different series capacitor values.
The load characteristics of the three-phase self-excited induction generator for wind power isolated stand-alone application based on the capacitor self-excitation phenomenon are presented here. Experiments were conducted on a d.c. motor-induction machine set to determine the parameters and the excitation requirements of the induction machine in the self-excited generator mode. It is shown that the values of the shunt capacitor required to cause self excitation in the machine at 1.0, 0.8 and 0.6 power factor load are 16, 20 and 24 #F/phase, respectively. The effect of varying the prime mover speed on the performance of the induction generator has also been examined, and it has been found that a speed variation of 6.5% changes the voltage regulation from 27.7 to 31.4%. The study has also demonstrated the effectiveness of using a saturable core reactor for improvement of voltage regulation. It has been found that the use of a saturable core reactor in parallel with the load improves voltage regulation from 33 to 16.8%. The study has further shown that there is a tremendous improvement in voltage regulation, to 10%, by connecting a
TRIPATHY et al.: SELF-EXCITED INDUCTION GENERATOR
199
1.2 -
1.1
--
> 1.0 5 m o~ O
Variable speed
0.9
>
0.8
0.7 0
I
I
t
I
0.2
0.4
0.6
0.8
O u t p u t p o w e r (p.u. W)
Fig. 14. Effect of speed on voltage regulation for 0.8 p.f. load with series capacitor of 100 #F. p r o p e r t h r e e - p h a s e c a p a c i t o r b a n k in series with the load. T h e frequency d e v i a t i o n due to l o a d p e r t u r b a t i o n is then reduced, using a s u p e r c o n d u c t i n g m a g n e t i c energy storage unit. D u e to their simplicity, ruggedness a n d low cost o f c o n s t r u c t i o n , squirrel cage i n d u c t i o n m a c h i n e s offer a relatively inexpensive alternative to a.c. g e n e r a t i o n using wind p o w e r in the s t a n d - a l o n e m o d e o r in parallel with o t h e r c o n v e n t i o n a l sources, like diesel in a h y b r i d w i n d - d i e s e l isolated p o w e r system. REFERENCES
I. 2. 3. 4. 5. 6. 7. 8.
E. N. Hinrichsen, IEEE Trans. Power Apparatus Syst. PAS-103, 886 (April 1984). F. P. De Mello and L. N. Hannett, IEEE Trans. Power Apparatus Syst. PAS-100, 2610 (May 1981). S. S. Murthy, O. P. Malik and A. K. Tandon, IEE Proe. 129, Pt C, No. 6 (November 1982). N. H. Malik and S. E. Haque, IEEE Trans. Energy Convers. EC-I, 133 (September 1986). J. Arrillaga and D. B. Watson, IEE Proe. 125, 743 (August 1978). R. Bonert and G. Hoops, IEEE Trans. Energy Convers. 5, 28 (March 1990). E. Bim, J. Szajner and Y. Burian, IEEE Trans. Energy Convers. 4, 526 (September 1989). S. C. Tripathy, M. Kalantar and R. Balasubramanian, Dynamics and stability of wind and diesel turbine generators with superconducting magnetic energy storage unit on an isolated power system. IEEE Winter Power Meeting, Paper No. 136-2-EC (February 1991).
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