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Voltage regulator for an isolated self-excited cage induction generator
Electric Power Systems Research, 24 (1992) 75-83
75
Voltage regulator for an isolated self-excited cage induction generator R. K. Mishra 400 k V Substation, UPSEB, Panki, Kanpur 208020 (India)
Bhim Singh and M. K. Vasantha Department of Electrical Engineering, University of Roorkee, Roorkee 247667 (India)
(Received November 12, 1990)
Abstract A voltage regulator for a three-phase self-excited cage induction generator (SEIG) is developed for maintaining its terminal voltage constant irrespective of the nature and amount of load. The proposed regulating scheme consists of a fixed-value capacitor bank and saturable core reactors with appropriate feedbacks. This simple static exciter at the induction machine terminals is capable of meeting its reactive power demand and regulating the terminal voltage. The developed regulator is used for a 5 hp, three-phase squirrel cage induction generator and provides inherent voltage regulation at resistive, inductive and motor loads. Extensive tests are carried out to study the steady-state and dynamic performance of the system. The relevant recorded voltage and current waveforms and other test results are given and discussed in detail. The proposed regulator is also compared with a thyristor controlled reactor type of regulator for an SEIG and is found to be superior to the latter in several aspects.
1. I n t r o d u c t i o n
Capacitor excited cage generators are becoming popular because of their numerous advantages over alternators, specially for generation of electricity in isolated places. Some of the advantages are the brushless construction of the cage rotor, small size, no separate DC source for excitation, reduced maintenance and, above all, low cost. Moreover, cage generators in this mode are capable of harnessing the electrical energy from renewable non-conventional energy sources such as wind, biogas, small hydro heads and low-grade fuels with solid-state converters [1-8]. However, the simplest way to obtain a desired voltage at its terminals for the given speed is to provide essential capacitive VAR excitation to the cage generator at different loads. A voltage regulator for these generators, therefore, must have a variable static reactive power generator, which, for this purpose, may easily be realized through a fixedvalue capacitor bank in parallel with controllable reactors. The static controlled,reactors may be achieved by using either a thyristor controlled inductor [8-10] or saturable core reactors [9-14]. 0378-7796/92/$5.00
Recently, a number of studies have been made on the suitability, analysis, control and exploitation of capacitor excited induction generators [1-8, 15-23]. The development of thyristor controlled static converters has facilitated control of the output voltage and frequency of self-excited induction generators [1-4]. A voltage regulator developed by Brennen and Abbondanti [8] uses a fixed-value capacitor in parallel with thyristor controlled inductors. All these regulators/converters, however, have harmonics and switching transients in the output voltage and generator current. Moreover, they require complicated control circuits and their short-time overload capacity is also limited due to solid-state power devices. However, reactive power compensators employing saturable core reactors are used a lot in various electrical installations [9-14]. The saturable core reactor provides robust, transientfree, controllable inductor operation at low cost with little maintenance. In this investigation, therefore, an attempt is made to develop a voltage regulator using a saturable core reactor in parallel with fixed-value capacitors for a self-excited cage induction gen© 1992 -- Elsevier Sequoia. All rights reserved
76
erator. A 5 hp, four-pole, three-phase cage generator with the developed voltage regulator is used to feed power to resistive, inductive and motor loads. Various tests are conducted to study the steadystate and dynamic performance of the system. The steady-state resu]ts with the dynamic responses for sudden application and removal of various types of load are presented and discussed in detail.
2. B a s i c principle For a capacitor self-excited induction generator, frequency and voltage are decided by the speed of the prime mover, capacitance, and load across the terminals of the generator. With a fixed value of capacitance, the terminal voltage of the generator drops with its loading. Terminal voltage can be maintained constant by using variable VAR generators as voltage regulators for the self-excited induction generators. As shown in Fig. 1, the capacitor bank, and the saturable core reactors in parallel with it, can serve the purpose of a variable VAR generator. In the present case, the saturable core reactor is to function as an auto-excited magnetic amplifier. In this scheme, two control windings are used, one for the reference setting fed from a variable DC excitation and the other as a feedback winding, excited from the derived feedback DC signal. For the purpose of feedback, the terminal voltage is stepped down to a low value and rectified by using a low rating bridge rectifier. The rectified output is connected to a feedback winding having one variable resistance in series with it.
The initial terminal voltage of the system can be adjusted to a desired value by controlling the DC signal fed to the reference winding. Loading of the generator will cause the terminal voltage to decrease, resulting in a reduced DC signal to the feedback winding which desaturates the core of the reactor, increasing its effective inductance. Hence, the current flowing to the saturable core reactor decreases, thus transferring the capacitor current to the induction generator and increasing the excitation to it. This will increase the EMF generated across the machine winding, further increasing the excitation to the feedback winding and saturation of the reactor core. Therefore, the reactor current will increase and the terminal voltage will decrease. This process of oscillation will continue for a very short period till a final steady-state condition is achieved. In this system, the effective gain is of the order of several thousands, resulting in a steady-state error of negligible value. If the load is suddenly removed, the terminal voltage of the generator will increase, causing more excitation to the feedback winding which will result in an increased saturation level of the core. Part of the capacitor current will be transferred to the saturable core reactor and the reduced excitation to the generator will result in a decrease in the induced EMF of the machine. Finally, in this case also, the terminal voltage will stabilize to the desired preset magnitude. Hence, this system will regulate the terminal voltage across the generator terminals, irrespective of load.
I m
3 ~
Jr#"
J
Three pha~'
J
balanced load
3 Prime mover
Cage induction mochinej /
3
~Opoooop, 3 phase Oelto connected satural01e core reactors
+i
II
Fig. 1. Block diagram of a three-phase self-excited induction generator with voltage regulator.
77
3. Development of the voltage regulator For the system shown in Fig. 1, the value of the capacitor is selected such that it must provide the excitation to the machine and feed the lagging reactive power required by the load, even under extreme cases of pure inductive load. The rating of the saturable core reactor is decided on the basis that it must absorb part of the capacitive current under no-load conditions without permitting the terminal voltage of the generator to go beyond the desired constant magnitude. The value of the capacitance required at no load by the generator can quickly be obtained from the VAR required by the machine at no load either by using design data or no-load test data of the machine. For the estimation of the rating and size of the saturable core reactor, the value of the capacitance required at full load can easily be calculated from the VAR required by the machine and the VAR required by the load. From these data, the value of the part of the capacitance to be compensated by the reactor may be obtained. Thus, the size and rating of the reactor can be estimated. It is seen experimentally that, for the 5 hp induction machine under test (details given in the Appendix), the capacitance required by the i n d u o tion generator to keep the terminal voltage constant at resistive load varies from 12.5 to 24 pF. As the self-excited induction generator is not capable of supplying reactive power to its load, the reactive power required by the load will also be supplied by the capacitor bank connected at the terminals of the induction generator. Hence, a delta-connected capacitor bank of 34 pF is selected to supply reactive power to the generator and load. To compensate the excess reactive power from the capacitor bank, saturable core reactors are connected in parallel to these capacitors. At no load, the capacitance required by the induction generator is a minimum, that is, 12.5 pF. Hence, the remaining capacitance ( 3 4 - 12.5--21.5 ~F) is to be compensated by the saturable core reactors. To compensate this capacitance, saturable core reactors of 2.7 A and 415 V will be sufficient. To construct the saturable core reactors (details given in the Appendix), an auto-excited magnetic amplifier and diodes of 10 A, 1200 V PIV rating are used with a sufficient safety factor for the load winding. In this scheme, two control windings are used, one for feedback and the other as reference (bias) winding of the saturable core reactor. The terminal voltage of the induction generator is stepped down and rectified for the feedback. To adjust the initial reference voltage, the rectified terminal voltage is
regulated by using an IC7805 regulator and is fed to the bias (reference) winding.
4. Performance characteristics The developed voltage regulator for the threephase self-excited induction generator has been tested for different types of load. The test results include the no-load characteristic and reactive power requirement of the machine for different values of load when the regulator is not used. The test results also include the steady-state and transient behaviour of the generator with the voltage regulator for different types of load. Figure 2 shows the variation of the generated voltage of the machine with capacitance at no load and rated speed. The developed induced EMF increases with increase in the value of the capacitance. The rise of the terminal voltage with capacitance is, however, limited by the saturation of the magnetic circuit of the machine. Figure 3 500
400
g o 300
5
I0
20
15 Copacilanc('(Micro F)
Fig. 2. Variation of terminal voltage with capacitance at no load and rated speed. 30--
o u ,= u
20
.g
,,,I,,J,l,,,,I 0
1000
.... 2000
3000
Ill 4000
I,I 5000
Power Output (Watts)
Fig. 3. Variation of capacitance with power output to keep Vt constant at rated speed.
78 500
500,
400'
- -
400
g
Q
2
300
30O
1
200
I
L
1ooo
I
2ooo
I
1 3000
I
,
4000
2oo
I o
5ooc
I 1ooo
I
I 2oo0
J
I 3ooo
i
I 4oo0
v,",
Power O u t . I t (Watts)
Fig. 4. Load characteristic of induction generator with voltage regulator for resistive loads.
..500 - -
Fig. 6. Load characteristic of induction generator with voltage regulator for 0.6 power factor lagging load.
500
o
400'
400
2 300
300
J
200 0
l 1000
,
I 2000
,
I 3000
,
I 4000
VA
,
200 o
I 1ooo
,
I 2000
i
I 3000
L
l 4oo0
VA
Fig. 5. Load characteristic of induction generator with voltage regulator for 0,8 power factor lagging load.
Fig. 7. Load characteristic of induction generator with voltage regulator for induction motor load.
shows the variation of reactive power in terms of capacitance with output power for resistive load at rated speed of the prime mover. It clearly shows the additional capacitive requirements at different values of load, thus representing a need for a variable reactive power source. In the present case, this requirement is met with the proposed voltage regulator. The load characteristics of the self-excited induction generator with its voltage regulator, indicating the variation of terminal voltage with output power for resistive, inductive and motor loads, are shown in Figs. 4-7. From Fig. 4, it may be seen that the developed voltage regulating system is able to maintain constant terminal voltage up to 4.05 kW at unity power factor (pf) loads, although the rated capacity of the generator is only 3.7 kW.
For inductive loads, the variation of terminal voltage with VA output of the induction generator at 0.8 and 0.6 pf lagging loads is presented in Figs. 5 and 6, respectively. From these Figures it may be observed that, for the same VA output of the induction generator, the drop in the terminal voltage for the 0.6 pf lagging load is greater than that for the same VA output for the 0.8 pf lagging load. A sudden drop in the terminal voltage above 3.4 kVA for the 0.8 pf lagging load and above 3.0 kVA for the 0.6 pf lagging load is observed. This sudden drop in voltage is due to the fixed-value capacitor bank which is required to supply reactive power to the induction generator as well as to the load along with the saturable core reactors. However, when all the capacitive power is supplied to the induction generator and the load, then the saturable core reactor current
79
Sec
(a)
Ig
20 Amperes/Div
Vt
400 V o l t s / D i v
(b)
Fig. 8. Oscillograms of the build-up of terminal voltage (upper) and generator current (lower) of the induction generator with voltage regulator at (a) no load and (b) resistive load.
?
Fig. 9. Waveforms of the induction generator with voltage regulator for sudden application and removal of resistive load: Vt = terminal voltage, Ig = generator current, Im = saturable core reactor current and I I = load current.
is almost zero. Then, the voltage regulator loses its control and the system voltage starts to drop. The variation of terminal voltage with VA output of the induction generator for a dynamic load (induction motor load) is presented in Fig. 7. The induction generator with the voltage regulating system is able to start and to r u n a 3 hp cage induction motor. The p h e n o m e n o n of voltage build-up of the induction generator with its voltage regulating system at no load and under loaded conditions has been observed and the corresponding recorded oscillograms are shown in Figs. 8(a) and (b). It is seen that, during the process of
build-up of the voltage, the transient rise in the terminal voltage above the preset value of the regulator is greater for the no-load condition t h a n for the loaded condition of the generator. The transient performance of the cage generator with its regulating system, in terms of the variation of the terminal voltage along with the generator current, saturable core reactor current and load current has also been obtained and is shown in Figs. 9-11. At the sudden application of load, a small dip, and at the sudden removal of load, a small rise in the terminal voltage are observed, but it stabilizes after a few cycles of the generator o u t p u t supply. From Fig. 10, a
80
~
Im
!~
:~)ii;
30. O 0 " A ~ e r • s
18.00 Amperes
;:::!~:i
6 . 3 0 Amperes
Fig. 10. Waveforms of the induction g e n e r a t o r with voltage r e g u l a t o r for s u d d e n application and removal of induction load: Vt = t e r m i n a l voltage, Ig = g e n e r a t o r c u r r e n t , 1,, = s a t u r a b l e core r e a c t o r c u r r e n t and I l = load current.
:'~
!
....
:i
.........
........
Fig. 11. Waveforms of the i n d u c t i o n g e n e r a t o r with voltage r e g u l a t o r for sudden application and removal of dynamic load (induction m o t o r load): Vt = t e r m i n a l voltage, I~ = g e n e r a t o r c u r r e n t , I m = s a t u r a b l e core r e a c t o r c u r r e n t and I] = load current.
sudden c h a n g e in s a t u r a b l e core r e a c t o r c u r r e n t with the sudden a p p l i c a t i o n and removal of inductive load indicates the shifting of reactive p o w e r from the s a t u r a b l e core r e a c t o r to the i n d u c t i v e load and vice versa. A 3 hp i n d u c t i o n m o t o r has also been started as the dynamic load on the induction g e n e r a t o r with its voltage r e g u l a t o r and the r e c o r d e d waveforms of the voltage and c u r r e n t are s h o w n in Fig. 11. At the time of starting of the i n d u c t i o n
motor, the required reactive p o w e r shifts from the s a t u r a b l e core r e a c t o r to the i n d u c t i o n motor, which, after starting the i n d u c t i o n motor, again shifts gradually to the s a t u r a b l e core reactor. Figure 12 shows the same p h e n o m e n o n w h e n the i n d u c t i o n motor is s t a r t e d by the i n d u c t i o n generator w h e n it is a l r e a d y feeding p o w e r to the inductive load c o n n e c t e d at the g e n e r a t o r terminals. During these processes, the terminal voltage of the g e n e r a t o r remains almost constant.
81
)eres
~eres
Fig. 12~ W a v e f o r m s of t h e i n d u c t i o n g e n e r a t o r w i t h v o l t a g e r e g u l a t o r for s u d d e n a p p l i c a t i o n a n d r e m o v a l of d y n a m i c load ( i n d u c t i o n m o t o r load) w i t h i n d u c t i v e load: Vt = t e r m i n a l voltage, Ig = g e n e r a t o r c u r r e n t , I m = s a t u r a b l e core r e a c t o r c u r r e n t a n d Il = load current.
5. Comparison of the proposed voltage regulator with the thyristorized type of voltage regulator In the thyristorized type of voltage regulator of self-excited cage generators, a unit of thyristor controlled inductors with feedback controllers and trigger circuits is used as the controllable inductor in parallel with fixed-value capacitor banks across the generator terminals, whereas in the proposed voltage regulator of the cage generator, a unit of saturable core reactors with a small feedback circuit is used, instead of the controllable inductors, in parallel with fixedvalue capacitor banks. The size, ~veight and cost of the saturable core reactors are the same as for the inductors used in thyristorized regulators. However, the cost of the thyristors, their trigger circuits and controllers is greater for the thyristorized regulator t h a n for the proposed regulator. Owing to the complicated control circuit and trigger unit of thyristors, the reliability and robustness of thyristorized regulators are poor compared with those of saturable core reactor regulators. A unique feature is the existence of self-excitation of the generator observed for the proposed regulator; even under loaded conditions, the voltage builds up when the generator starts and is sustained even after a short-circuit at the generator terminals. This is not normally
observed with thyristorized voltage regulators because of the poor self-excitation of capacitor excited induction generators, so these generators were not in use for long. With the proposed regulator of the self-excited cage generators, it is hoped t h a t these will find application as a source of power in emergencies and in isolated places because of the certain existence of self-excitation of the machine, even under extreme conditions. Moreover, the quality of the o u t p u t voltage is better with the proposed regulator due to the absence of switching transients and harmonics which are caused in the voltage with the thyristorized type of voltage regulator of cage generators. The dynamic performance of the system with the developed voltage regulator is better t h a n that of the thyristorized regulator because there are fewer oscillations in the o u t p u t voltage waveform on sudden application and removal of loads. A summary of the comparative features of the proposed regulator and the thyristorized regulator is given in Table 1.
6. Conclusions A n e w type of voltage regulator for a self-excited cage generator has been developed using a saturable core reactor to act as a controllable static VAR generator. Based on experimental in-
82 TABLE 1. Comparison of the thyristorized type of voltage regulator with the saturable core reactor type of voltage regulator for a self-excited cage generator Item
Voltage regulator Thyristorized
Saturable core reactor
(SCR) Weight Cost
Reliability Technology Possibility of self-excitation Voltage waveform Robustness
Short-time overload capacity Complexity
Voltage regulation
Dynamic performance
Size
Same (inductors + thyristor unit) High (inductor + thyristors + control circuits) Lower owing to complicated circuits Sophisticated Low; little under loaded conditions Contains switching transients and harmonics Less robust owing to complex control circuits and thyristors Low owing to thyristors having low overload capability Complex owing to complex control circuits and thyristors Same with suitable design
More oscillations in voltage at sudden application and removal of load Larger
vestigations, it is concluded that the proposed voltage regulator is capable of regulating the terminal voltage with resistive, inductive and motor loads. The voltage build-up phenomenon has been found to be reliable even under loaded conditions at start-up. The dynamic response is also observed to be satisfactory and there is no possibility of failure of self-excitation, even at the start-up of a motor load. The proposed voltage regulator for the cage generator has been found superior to the thyristor controlled regulator in terms of cost, robustness, reliability and dynamic performance. It is hoped that the self-excited cage generator with the developed voltage regulator will find good applications as a source of electric power in isolated places, as a portable generator at construction sites in emergency, and to harness energy from non-conventional energy sources.
Same (SCRs + diodes) Low (SCRs + diodes + small feedback circuits) Greater owing to robust circuit Simple Certain, even at load Smooth and sinusoidal Robust as the cage generator Very high, as for the generator Simple, like inductors
Same owing to very high gain and almost zero steady-state error Fewer oscillations in voltage with load disturbance Smaller
Appendix Details of generator 3.7kW, 415V, 7.1A, four-pole, 50Hz, threephase delta-connected squirrel cage induction machine.
Details of voltage regulator (a) Saturable core reactor. (i) Output winding: 415 V, AC, SWG-19 wire, 900 turns. (ii) Reference (bias) winding: DC, SWG-29 wire, 1000 turns. (iii) Feedback winding: DC, SWG-29 wire, 500 turns. (iv) Core: toroidal (CRGO), outer diameter 150 mm, inner diameter 39 mm, thickness 39. (b) Diodes: PIV-1200 V, 10 A. (c) Terminal capacitor: 34~F/phase, 415V, 50 Hz, oil-filled AC power capacitor.
83
References 1 D. W. Novotny, D. G. Gritter and G. H. Studtmann, Self excitation in inverter driven induction machines, IEEE Trans., PAS-96 (1977) 1117-1125. 2 J. Arrillaga and D. B. Watson, Static power conversion from self excited induction generators, Proc. Inst. Electr. Eng., 125 (1978) 743-746. 3 D. B. Watson, J. Arrillaga and T. Densem, Controllable d.c. power supply from wind driven self excited induction machines, Proc. Inst. Electr. Eng., 126 (1979) 12451248. 4 O. P. Malik, D. Divan, S. S. Murthy, T. Grant and P. Walsh, A solid state voltage regulator for self excited induction generators, Proc. IEEE Industrial and Commercial Power Systems Conf., Milwaukee, OR, USA, 1983. 5 D. B. Watson and R. M. Watson, Microprocessor control of a self excited induction generator, Int. J. Electr. Eng. Educ., 22 (1985) 69-82. 6 C. H. Watanabe and A. N. Barreto, Self excited induction generator/forced-commutated rectifier system operating as a d.c, power supply, Proc. Inst. Electr. Eng., Part B, 134 (1987) 255 260. 7 G. Raina and O. P. Malik, Wind energy copversion using a self excited induction generator, IEEE Trans., PAS.102 (1983) 3933-3936. 8 M. B. Brennen and A. Abbondanti, Static exciters for induction generators, IEEE Trans., IA-13 (1977)422-428. 9 R. M. Mathur, Static Compensators for Reactive Power Control, Context, 1984. 10 T. J. E. Miller, Reactive Power Control in Electric Systems, Wiley, New York, 1982. 11 M. G. Say, Magnetic Amplifiers and Saturable Reactors, Newnes, London, 1954.
12 W. A. Geyger, Magnetic Amplifier Circuits, Basic Principles, Characteristics and Applications, McGraw-Hill, New York, 1957. 13 C. S. Siskind, Electrical Control Systems in Industry, McGraw-Hill, New York, 1963. 14 E. H. Frost-Smith, The Theory and Design of Magnetic Amplifiers, Chapman and Hall, London, 1966. 15 S. S. Murthy, O. P. Malik and A. K. Tandon, Analysis of self excited induction generators, Proc. Inst. Electr. Eng., Part C, 129 (1982) 260-265. 16 A. K. Tandon, S. S. Murthy and G. J. Berg, Steady state analysis of capacitor self excited induction generator, IEEE Trans., PAS-103 (1984) 612-618. 17 J. M. Elder, J. T. Boys and J. L. Woodward, Self excited induction machine as a small low cost generator, Proc. Inst. Electr. Eng., Part C, 131 (1984) 33-40. 18 S. S. Y. Narayanan and V. J. Johny, Contributions to the steady state analysis of wind-turbine driven self excited induction generators, IEEE Trans., EC-1 (1986) 169-176. 19 N. H. Malik and S. E. Haque, Steady state analysis and performance of an isolated self excited induction generator, IEEE Trans., EC-1 (1986) 134 139. 20 N. Ammasaigounden, M. Subbiah and M. R. Krishnamurthy, Wind driven self excited pole changing induction generators, Proc. Inst. Electr. Eng., Part B, 133 (1986) 315--321. 21 L. Quazene and G. McPherson, Jr., Analysis of the isolated induction generator, IEEE Trans., PAS-102(1983) 2793-2798. 22 S. S. Murthy, B. P. Singh, C. Nagamani and K. V. V. Satya Narayana, Studies on the use of conventional induction motor as self excited induction generator, IEEE Trans., EC-3 (1988) 842 848. 23 E. Bim, J. Szajner and ¥. Burian, Voltage compensation of an induction generator with long shunt connection, IEEE Trans., EC-4 (1989) 526-530.
75
Voltage regulator for an isolated self-excited cage induction generator R. K. Mishra 400 k V Substation, UPSEB, Panki, Kanpur 208020 (India)
Bhim Singh and M. K. Vasantha Department of Electrical Engineering, University of Roorkee, Roorkee 247667 (India)
(Received November 12, 1990)
Abstract A voltage regulator for a three-phase self-excited cage induction generator (SEIG) is developed for maintaining its terminal voltage constant irrespective of the nature and amount of load. The proposed regulating scheme consists of a fixed-value capacitor bank and saturable core reactors with appropriate feedbacks. This simple static exciter at the induction machine terminals is capable of meeting its reactive power demand and regulating the terminal voltage. The developed regulator is used for a 5 hp, three-phase squirrel cage induction generator and provides inherent voltage regulation at resistive, inductive and motor loads. Extensive tests are carried out to study the steady-state and dynamic performance of the system. The relevant recorded voltage and current waveforms and other test results are given and discussed in detail. The proposed regulator is also compared with a thyristor controlled reactor type of regulator for an SEIG and is found to be superior to the latter in several aspects.
1. I n t r o d u c t i o n
Capacitor excited cage generators are becoming popular because of their numerous advantages over alternators, specially for generation of electricity in isolated places. Some of the advantages are the brushless construction of the cage rotor, small size, no separate DC source for excitation, reduced maintenance and, above all, low cost. Moreover, cage generators in this mode are capable of harnessing the electrical energy from renewable non-conventional energy sources such as wind, biogas, small hydro heads and low-grade fuels with solid-state converters [1-8]. However, the simplest way to obtain a desired voltage at its terminals for the given speed is to provide essential capacitive VAR excitation to the cage generator at different loads. A voltage regulator for these generators, therefore, must have a variable static reactive power generator, which, for this purpose, may easily be realized through a fixedvalue capacitor bank in parallel with controllable reactors. The static controlled,reactors may be achieved by using either a thyristor controlled inductor [8-10] or saturable core reactors [9-14]. 0378-7796/92/$5.00
Recently, a number of studies have been made on the suitability, analysis, control and exploitation of capacitor excited induction generators [1-8, 15-23]. The development of thyristor controlled static converters has facilitated control of the output voltage and frequency of self-excited induction generators [1-4]. A voltage regulator developed by Brennen and Abbondanti [8] uses a fixed-value capacitor in parallel with thyristor controlled inductors. All these regulators/converters, however, have harmonics and switching transients in the output voltage and generator current. Moreover, they require complicated control circuits and their short-time overload capacity is also limited due to solid-state power devices. However, reactive power compensators employing saturable core reactors are used a lot in various electrical installations [9-14]. The saturable core reactor provides robust, transientfree, controllable inductor operation at low cost with little maintenance. In this investigation, therefore, an attempt is made to develop a voltage regulator using a saturable core reactor in parallel with fixed-value capacitors for a self-excited cage induction gen© 1992 -- Elsevier Sequoia. All rights reserved
76
erator. A 5 hp, four-pole, three-phase cage generator with the developed voltage regulator is used to feed power to resistive, inductive and motor loads. Various tests are conducted to study the steadystate and dynamic performance of the system. The steady-state resu]ts with the dynamic responses for sudden application and removal of various types of load are presented and discussed in detail.
2. B a s i c principle For a capacitor self-excited induction generator, frequency and voltage are decided by the speed of the prime mover, capacitance, and load across the terminals of the generator. With a fixed value of capacitance, the terminal voltage of the generator drops with its loading. Terminal voltage can be maintained constant by using variable VAR generators as voltage regulators for the self-excited induction generators. As shown in Fig. 1, the capacitor bank, and the saturable core reactors in parallel with it, can serve the purpose of a variable VAR generator. In the present case, the saturable core reactor is to function as an auto-excited magnetic amplifier. In this scheme, two control windings are used, one for the reference setting fed from a variable DC excitation and the other as a feedback winding, excited from the derived feedback DC signal. For the purpose of feedback, the terminal voltage is stepped down to a low value and rectified by using a low rating bridge rectifier. The rectified output is connected to a feedback winding having one variable resistance in series with it.
The initial terminal voltage of the system can be adjusted to a desired value by controlling the DC signal fed to the reference winding. Loading of the generator will cause the terminal voltage to decrease, resulting in a reduced DC signal to the feedback winding which desaturates the core of the reactor, increasing its effective inductance. Hence, the current flowing to the saturable core reactor decreases, thus transferring the capacitor current to the induction generator and increasing the excitation to it. This will increase the EMF generated across the machine winding, further increasing the excitation to the feedback winding and saturation of the reactor core. Therefore, the reactor current will increase and the terminal voltage will decrease. This process of oscillation will continue for a very short period till a final steady-state condition is achieved. In this system, the effective gain is of the order of several thousands, resulting in a steady-state error of negligible value. If the load is suddenly removed, the terminal voltage of the generator will increase, causing more excitation to the feedback winding which will result in an increased saturation level of the core. Part of the capacitor current will be transferred to the saturable core reactor and the reduced excitation to the generator will result in a decrease in the induced EMF of the machine. Finally, in this case also, the terminal voltage will stabilize to the desired preset magnitude. Hence, this system will regulate the terminal voltage across the generator terminals, irrespective of load.
I m
3 ~
Jr#"
J
Three pha~'
J
balanced load
3 Prime mover
Cage induction mochinej /
3
~Opoooop, 3 phase Oelto connected satural01e core reactors
+i
II
Fig. 1. Block diagram of a three-phase self-excited induction generator with voltage regulator.
77
3. Development of the voltage regulator For the system shown in Fig. 1, the value of the capacitor is selected such that it must provide the excitation to the machine and feed the lagging reactive power required by the load, even under extreme cases of pure inductive load. The rating of the saturable core reactor is decided on the basis that it must absorb part of the capacitive current under no-load conditions without permitting the terminal voltage of the generator to go beyond the desired constant magnitude. The value of the capacitance required at no load by the generator can quickly be obtained from the VAR required by the machine at no load either by using design data or no-load test data of the machine. For the estimation of the rating and size of the saturable core reactor, the value of the capacitance required at full load can easily be calculated from the VAR required by the machine and the VAR required by the load. From these data, the value of the part of the capacitance to be compensated by the reactor may be obtained. Thus, the size and rating of the reactor can be estimated. It is seen experimentally that, for the 5 hp induction machine under test (details given in the Appendix), the capacitance required by the i n d u o tion generator to keep the terminal voltage constant at resistive load varies from 12.5 to 24 pF. As the self-excited induction generator is not capable of supplying reactive power to its load, the reactive power required by the load will also be supplied by the capacitor bank connected at the terminals of the induction generator. Hence, a delta-connected capacitor bank of 34 pF is selected to supply reactive power to the generator and load. To compensate the excess reactive power from the capacitor bank, saturable core reactors are connected in parallel to these capacitors. At no load, the capacitance required by the induction generator is a minimum, that is, 12.5 pF. Hence, the remaining capacitance ( 3 4 - 12.5--21.5 ~F) is to be compensated by the saturable core reactors. To compensate this capacitance, saturable core reactors of 2.7 A and 415 V will be sufficient. To construct the saturable core reactors (details given in the Appendix), an auto-excited magnetic amplifier and diodes of 10 A, 1200 V PIV rating are used with a sufficient safety factor for the load winding. In this scheme, two control windings are used, one for feedback and the other as reference (bias) winding of the saturable core reactor. The terminal voltage of the induction generator is stepped down and rectified for the feedback. To adjust the initial reference voltage, the rectified terminal voltage is
regulated by using an IC7805 regulator and is fed to the bias (reference) winding.
4. Performance characteristics The developed voltage regulator for the threephase self-excited induction generator has been tested for different types of load. The test results include the no-load characteristic and reactive power requirement of the machine for different values of load when the regulator is not used. The test results also include the steady-state and transient behaviour of the generator with the voltage regulator for different types of load. Figure 2 shows the variation of the generated voltage of the machine with capacitance at no load and rated speed. The developed induced EMF increases with increase in the value of the capacitance. The rise of the terminal voltage with capacitance is, however, limited by the saturation of the magnetic circuit of the machine. Figure 3 500
400
g o 300
5
I0
20
15 Copacilanc('(Micro F)
Fig. 2. Variation of terminal voltage with capacitance at no load and rated speed. 30--
o u ,= u
20
.g
,,,I,,J,l,,,,I 0
1000
.... 2000
3000
Ill 4000
I,I 5000
Power Output (Watts)
Fig. 3. Variation of capacitance with power output to keep Vt constant at rated speed.
78 500
500,
400'
- -
400
g
Q
2
300
30O
1
200
I
L
1ooo
I
2ooo
I
1 3000
I
,
4000
2oo
I o
5ooc
I 1ooo
I
I 2oo0
J
I 3ooo
i
I 4oo0
v,",
Power O u t . I t (Watts)
Fig. 4. Load characteristic of induction generator with voltage regulator for resistive loads.
..500 - -
Fig. 6. Load characteristic of induction generator with voltage regulator for 0.6 power factor lagging load.
500
o
400'
400
2 300
300
J
200 0
l 1000
,
I 2000
,
I 3000
,
I 4000
VA
,
200 o
I 1ooo
,
I 2000
i
I 3000
L
l 4oo0
VA
Fig. 5. Load characteristic of induction generator with voltage regulator for 0,8 power factor lagging load.
Fig. 7. Load characteristic of induction generator with voltage regulator for induction motor load.
shows the variation of reactive power in terms of capacitance with output power for resistive load at rated speed of the prime mover. It clearly shows the additional capacitive requirements at different values of load, thus representing a need for a variable reactive power source. In the present case, this requirement is met with the proposed voltage regulator. The load characteristics of the self-excited induction generator with its voltage regulator, indicating the variation of terminal voltage with output power for resistive, inductive and motor loads, are shown in Figs. 4-7. From Fig. 4, it may be seen that the developed voltage regulating system is able to maintain constant terminal voltage up to 4.05 kW at unity power factor (pf) loads, although the rated capacity of the generator is only 3.7 kW.
For inductive loads, the variation of terminal voltage with VA output of the induction generator at 0.8 and 0.6 pf lagging loads is presented in Figs. 5 and 6, respectively. From these Figures it may be observed that, for the same VA output of the induction generator, the drop in the terminal voltage for the 0.6 pf lagging load is greater than that for the same VA output for the 0.8 pf lagging load. A sudden drop in the terminal voltage above 3.4 kVA for the 0.8 pf lagging load and above 3.0 kVA for the 0.6 pf lagging load is observed. This sudden drop in voltage is due to the fixed-value capacitor bank which is required to supply reactive power to the induction generator as well as to the load along with the saturable core reactors. However, when all the capacitive power is supplied to the induction generator and the load, then the saturable core reactor current
79
Sec
(a)
Ig
20 Amperes/Div
Vt
400 V o l t s / D i v
(b)
Fig. 8. Oscillograms of the build-up of terminal voltage (upper) and generator current (lower) of the induction generator with voltage regulator at (a) no load and (b) resistive load.
?
Fig. 9. Waveforms of the induction generator with voltage regulator for sudden application and removal of resistive load: Vt = terminal voltage, Ig = generator current, Im = saturable core reactor current and I I = load current.
is almost zero. Then, the voltage regulator loses its control and the system voltage starts to drop. The variation of terminal voltage with VA output of the induction generator for a dynamic load (induction motor load) is presented in Fig. 7. The induction generator with the voltage regulating system is able to start and to r u n a 3 hp cage induction motor. The p h e n o m e n o n of voltage build-up of the induction generator with its voltage regulating system at no load and under loaded conditions has been observed and the corresponding recorded oscillograms are shown in Figs. 8(a) and (b). It is seen that, during the process of
build-up of the voltage, the transient rise in the terminal voltage above the preset value of the regulator is greater for the no-load condition t h a n for the loaded condition of the generator. The transient performance of the cage generator with its regulating system, in terms of the variation of the terminal voltage along with the generator current, saturable core reactor current and load current has also been obtained and is shown in Figs. 9-11. At the sudden application of load, a small dip, and at the sudden removal of load, a small rise in the terminal voltage are observed, but it stabilizes after a few cycles of the generator o u t p u t supply. From Fig. 10, a
80
~
Im
!~
:~)ii;
30. O 0 " A ~ e r • s
18.00 Amperes
;:::!~:i
6 . 3 0 Amperes
Fig. 10. Waveforms of the induction g e n e r a t o r with voltage r e g u l a t o r for s u d d e n application and removal of induction load: Vt = t e r m i n a l voltage, Ig = g e n e r a t o r c u r r e n t , 1,, = s a t u r a b l e core r e a c t o r c u r r e n t and I l = load current.
:'~
!
....
:i
.........
........
Fig. 11. Waveforms of the i n d u c t i o n g e n e r a t o r with voltage r e g u l a t o r for sudden application and removal of dynamic load (induction m o t o r load): Vt = t e r m i n a l voltage, I~ = g e n e r a t o r c u r r e n t , I m = s a t u r a b l e core r e a c t o r c u r r e n t and I] = load current.
sudden c h a n g e in s a t u r a b l e core r e a c t o r c u r r e n t with the sudden a p p l i c a t i o n and removal of inductive load indicates the shifting of reactive p o w e r from the s a t u r a b l e core r e a c t o r to the i n d u c t i v e load and vice versa. A 3 hp i n d u c t i o n m o t o r has also been started as the dynamic load on the induction g e n e r a t o r with its voltage r e g u l a t o r and the r e c o r d e d waveforms of the voltage and c u r r e n t are s h o w n in Fig. 11. At the time of starting of the i n d u c t i o n
motor, the required reactive p o w e r shifts from the s a t u r a b l e core r e a c t o r to the i n d u c t i o n motor, which, after starting the i n d u c t i o n motor, again shifts gradually to the s a t u r a b l e core reactor. Figure 12 shows the same p h e n o m e n o n w h e n the i n d u c t i o n motor is s t a r t e d by the i n d u c t i o n generator w h e n it is a l r e a d y feeding p o w e r to the inductive load c o n n e c t e d at the g e n e r a t o r terminals. During these processes, the terminal voltage of the g e n e r a t o r remains almost constant.
81
)eres
~eres
Fig. 12~ W a v e f o r m s of t h e i n d u c t i o n g e n e r a t o r w i t h v o l t a g e r e g u l a t o r for s u d d e n a p p l i c a t i o n a n d r e m o v a l of d y n a m i c load ( i n d u c t i o n m o t o r load) w i t h i n d u c t i v e load: Vt = t e r m i n a l voltage, Ig = g e n e r a t o r c u r r e n t , I m = s a t u r a b l e core r e a c t o r c u r r e n t a n d Il = load current.
5. Comparison of the proposed voltage regulator with the thyristorized type of voltage regulator In the thyristorized type of voltage regulator of self-excited cage generators, a unit of thyristor controlled inductors with feedback controllers and trigger circuits is used as the controllable inductor in parallel with fixed-value capacitor banks across the generator terminals, whereas in the proposed voltage regulator of the cage generator, a unit of saturable core reactors with a small feedback circuit is used, instead of the controllable inductors, in parallel with fixedvalue capacitor banks. The size, ~veight and cost of the saturable core reactors are the same as for the inductors used in thyristorized regulators. However, the cost of the thyristors, their trigger circuits and controllers is greater for the thyristorized regulator t h a n for the proposed regulator. Owing to the complicated control circuit and trigger unit of thyristors, the reliability and robustness of thyristorized regulators are poor compared with those of saturable core reactor regulators. A unique feature is the existence of self-excitation of the generator observed for the proposed regulator; even under loaded conditions, the voltage builds up when the generator starts and is sustained even after a short-circuit at the generator terminals. This is not normally
observed with thyristorized voltage regulators because of the poor self-excitation of capacitor excited induction generators, so these generators were not in use for long. With the proposed regulator of the self-excited cage generators, it is hoped t h a t these will find application as a source of power in emergencies and in isolated places because of the certain existence of self-excitation of the machine, even under extreme conditions. Moreover, the quality of the o u t p u t voltage is better with the proposed regulator due to the absence of switching transients and harmonics which are caused in the voltage with the thyristorized type of voltage regulator of cage generators. The dynamic performance of the system with the developed voltage regulator is better t h a n that of the thyristorized regulator because there are fewer oscillations in the o u t p u t voltage waveform on sudden application and removal of loads. A summary of the comparative features of the proposed regulator and the thyristorized regulator is given in Table 1.
6. Conclusions A n e w type of voltage regulator for a self-excited cage generator has been developed using a saturable core reactor to act as a controllable static VAR generator. Based on experimental in-
82 TABLE 1. Comparison of the thyristorized type of voltage regulator with the saturable core reactor type of voltage regulator for a self-excited cage generator Item
Voltage regulator Thyristorized
Saturable core reactor
(SCR) Weight Cost
Reliability Technology Possibility of self-excitation Voltage waveform Robustness
Short-time overload capacity Complexity
Voltage regulation
Dynamic performance
Size
Same (inductors + thyristor unit) High (inductor + thyristors + control circuits) Lower owing to complicated circuits Sophisticated Low; little under loaded conditions Contains switching transients and harmonics Less robust owing to complex control circuits and thyristors Low owing to thyristors having low overload capability Complex owing to complex control circuits and thyristors Same with suitable design
More oscillations in voltage at sudden application and removal of load Larger
vestigations, it is concluded that the proposed voltage regulator is capable of regulating the terminal voltage with resistive, inductive and motor loads. The voltage build-up phenomenon has been found to be reliable even under loaded conditions at start-up. The dynamic response is also observed to be satisfactory and there is no possibility of failure of self-excitation, even at the start-up of a motor load. The proposed voltage regulator for the cage generator has been found superior to the thyristor controlled regulator in terms of cost, robustness, reliability and dynamic performance. It is hoped that the self-excited cage generator with the developed voltage regulator will find good applications as a source of electric power in isolated places, as a portable generator at construction sites in emergency, and to harness energy from non-conventional energy sources.
Same (SCRs + diodes) Low (SCRs + diodes + small feedback circuits) Greater owing to robust circuit Simple Certain, even at load Smooth and sinusoidal Robust as the cage generator Very high, as for the generator Simple, like inductors
Same owing to very high gain and almost zero steady-state error Fewer oscillations in voltage with load disturbance Smaller
Appendix Details of generator 3.7kW, 415V, 7.1A, four-pole, 50Hz, threephase delta-connected squirrel cage induction machine.
Details of voltage regulator (a) Saturable core reactor. (i) Output winding: 415 V, AC, SWG-19 wire, 900 turns. (ii) Reference (bias) winding: DC, SWG-29 wire, 1000 turns. (iii) Feedback winding: DC, SWG-29 wire, 500 turns. (iv) Core: toroidal (CRGO), outer diameter 150 mm, inner diameter 39 mm, thickness 39. (b) Diodes: PIV-1200 V, 10 A. (c) Terminal capacitor: 34~F/phase, 415V, 50 Hz, oil-filled AC power capacitor.
83
References 1 D. W. Novotny, D. G. Gritter and G. H. Studtmann, Self excitation in inverter driven induction machines, IEEE Trans., PAS-96 (1977) 1117-1125. 2 J. Arrillaga and D. B. Watson, Static power conversion from self excited induction generators, Proc. Inst. Electr. Eng., 125 (1978) 743-746. 3 D. B. Watson, J. Arrillaga and T. Densem, Controllable d.c. power supply from wind driven self excited induction machines, Proc. Inst. Electr. Eng., 126 (1979) 12451248. 4 O. P. Malik, D. Divan, S. S. Murthy, T. Grant and P. Walsh, A solid state voltage regulator for self excited induction generators, Proc. IEEE Industrial and Commercial Power Systems Conf., Milwaukee, OR, USA, 1983. 5 D. B. Watson and R. M. Watson, Microprocessor control of a self excited induction generator, Int. J. Electr. Eng. Educ., 22 (1985) 69-82. 6 C. H. Watanabe and A. N. Barreto, Self excited induction generator/forced-commutated rectifier system operating as a d.c, power supply, Proc. Inst. Electr. Eng., Part B, 134 (1987) 255 260. 7 G. Raina and O. P. Malik, Wind energy copversion using a self excited induction generator, IEEE Trans., PAS.102 (1983) 3933-3936. 8 M. B. Brennen and A. Abbondanti, Static exciters for induction generators, IEEE Trans., IA-13 (1977)422-428. 9 R. M. Mathur, Static Compensators for Reactive Power Control, Context, 1984. 10 T. J. E. Miller, Reactive Power Control in Electric Systems, Wiley, New York, 1982. 11 M. G. Say, Magnetic Amplifiers and Saturable Reactors, Newnes, London, 1954.
12 W. A. Geyger, Magnetic Amplifier Circuits, Basic Principles, Characteristics and Applications, McGraw-Hill, New York, 1957. 13 C. S. Siskind, Electrical Control Systems in Industry, McGraw-Hill, New York, 1963. 14 E. H. Frost-Smith, The Theory and Design of Magnetic Amplifiers, Chapman and Hall, London, 1966. 15 S. S. Murthy, O. P. Malik and A. K. Tandon, Analysis of self excited induction generators, Proc. Inst. Electr. Eng., Part C, 129 (1982) 260-265. 16 A. K. Tandon, S. S. Murthy and G. J. Berg, Steady state analysis of capacitor self excited induction generator, IEEE Trans., PAS-103 (1984) 612-618. 17 J. M. Elder, J. T. Boys and J. L. Woodward, Self excited induction machine as a small low cost generator, Proc. Inst. Electr. Eng., Part C, 131 (1984) 33-40. 18 S. S. Y. Narayanan and V. J. Johny, Contributions to the steady state analysis of wind-turbine driven self excited induction generators, IEEE Trans., EC-1 (1986) 169-176. 19 N. H. Malik and S. E. Haque, Steady state analysis and performance of an isolated self excited induction generator, IEEE Trans., EC-1 (1986) 134 139. 20 N. Ammasaigounden, M. Subbiah and M. R. Krishnamurthy, Wind driven self excited pole changing induction generators, Proc. Inst. Electr. Eng., Part B, 133 (1986) 315--321. 21 L. Quazene and G. McPherson, Jr., Analysis of the isolated induction generator, IEEE Trans., PAS-102(1983) 2793-2798. 22 S. S. Murthy, B. P. Singh, C. Nagamani and K. V. V. Satya Narayana, Studies on the use of conventional induction motor as self excited induction generator, IEEE Trans., EC-3 (1988) 842 848. 23 E. Bim, J. Szajner and ¥. Burian, Voltage compensation of an induction generator with long shunt connection, IEEE Trans., EC-4 (1989) 526-530.