908 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 4, AUGUST 2000
Induction-Generator-Based System Providing
Regulated Voltage with Constant Frequency
Enes Gonçalves Marra, Associate Member, IEEE, and José Antenor Pomilio, Member, IEEE
Abstract The electrical characteristics of an isolated induc-
tion-generator-based system are improved through the association
with a voltage-source pulsewidth modulation (PWM) inverter. The
electronic converter allows the achievement of a better system be-
havior in many aspects: voltage regulation, frequency stabilization,
and reactive power compensation. The system operation strategy
consists of maintaining constant synchronous frequency at the in-
duction generator via an association with a PWM inverter. The
system power balance and the generator voltage regulation may be
accomplished by two different means: through the rotor speed reg-
Fig. 1. Capacitor-excited IG system, isolated from the utility grid.
ulation, or by sending part of the energy stored in the inverter dc
side to the grid through a single-phase line, in case the rotor speed
is not regulated and a single-phase grid connection is available. The
eration systems, such as low-head microhydroelectric plants and
obtained results demonstrated the system is stable, robust, and an
fuel engine driven generation systems. Two distinct structures
effective source of regulated three-phase voltages.
are presented. In one of these structures, the generator s shaft
Index Terms Energy conversion, energy resources, induction
speed is regulated. The other structure does not comprise speed
generator, pulsewidth modulation inverter.
governor, and the system acts as cogenerator, sending energy
to a single-phase grid, as a strategy to control the IG terminal
voltage.
I. INTRODUCTION
The cogenerator structure is appropriate to be employed in
T IS FREQUENTLY stated that cage rotor induction ma-
areas such as light manufacturing or agricultural areas where
chines (IMs) are robust, inexpensive compared with dc and
I
electric power available is only single phase. Customers in these
wound-rotor synchronous machines, require little maintenance,
areas may request three-phase power from the utility and find it
and have high power-weight ratio (W/kg). Despite these favor-
is uneconomical for the utility to meet a relatively small three-
able features, IM s are hardly employed as generators due to
phase need [3], [4].
their unsatisfactory voltage regulation and frequency variation,
Both proposed systems are intended to be sources of regu-
even when driven under constant speed and feeding loads which
lated voltage with constant frequency, whose energy quality
consume active power [1], [2].
is good enough to feed sensitive loads, such as micropro-
Wound-rotor synchronous generators are reliable suppliers
cessor-controlled ones.
of regulated three-phase constant frequency voltage, provided
the dynamic response of the speed governor is able to main-
II. ISOLATED CAPACITOR-EXCITED IG SYSTEM
tain constant rotor velocity during the occurrence of load power
variations. Nevertheless, they are expensive machines due to
Fig. 1 presents a system in which a capacitor-excited IG op-
the maintenance required by the excitation system, which con-
erates isolated from the utility grid. In this circumstance, the ac-
tains slip rings, brushes, or rotating rectifiers, in addition to field
tive power of the ac load affects considerably the amplitude and
current control circuits. Therefore, a cost-effective and techni-
the frequency of the voltage at the IG terminals. In this case, the
cally reliable alternative to wound-rotor synchronous generators
synchronous frequency is not constant, even if the rotor speed
would be welcome.
is kept constant by the action of a speed governor.
The aim of this investigation is to propose an induction gen-
Assuming that the mechanical, electrical, and magnetic losses
erator (IG) application as an alternative to wound-rotor syn-
are negligible, the electric power converted by the generator is
chronous generators to be employed in low-power isolated gen-
given by the product between the rotor speed and the generator
torque.
Supposing the rotor speed is invariable, the increase of the
Manuscript received February 12, 1999; revised September 20, 1999. Ab-
stract published on the Internet April 21, 2000. This work was supported by active power required by the ac load yields a drop in the stator
Coordenaçćo para o Aperfeiçoamento de Pessoal de Ensino Superior (CAPES)
frequency, as it is the only possible way the IG can raise its rotor
and Fundaçćo de Amparo Ä… Pesquisa do Estado de Sćo Paulo (FAPESP).
slip frequency and consequently elevate the torque, so that it is
E. Gonçalves Marra is with the School of Electrical Engineering, Federal
University of Goiás, 74605-220 Goiânia, Brazil (e-mail: [e-mail]). able to suit the load power demand.
J. Antenor Pomilio is with the School of Electrical and Computer Engi-
Fig. 2 illustrates qualitatively a situation in which the induc-
neering, State University of Campinas, 13081-970 Campinas, Brazil (e-mail:
tion generator was feeding a unity power-factor load so that the
[e-mail] ).
Publisher Item Identifier S 0278-0046(00)06814-3. steady-state operation point is  A. The synchronous frequency
0278 0046/00$10.00 © 2000 IEEE
MARRA AND POMILIO: IG-BASED SYSTEM 909
in the magnetization characteristic and in the excitation bank ca-
pacitive reactance.
The capacitance could be increased even more, in order to
recover the capacitive reactance . In this case, the slope of
the capacitor-bank voltage characteristic will return to its pre-
vious value, however, the steady-state operation point in the
magnetization characteristic will now be   instead of  A,
as the frequency remains . The new operation point at the
torque characteristic (Fig. 2) would depend on the behavior of
the ac load under voltage variations.
It should be highlighted that the voltage drops at the stator and
rotor resistance and leakage reactances are not the main cause
of the poor voltage and frequency regulation in the isolated IG.
The fundamental factor that affects the IG voltage regulation is
the influence of the frequency on the generator magnetization
Fig. 2. Torque-speed characteristics of the induction generator, for different
characteristic.
synchronous frequencies (f > f ).
Note that the voltage and frequency variations presented pre-
viously were caused by increments made exclusively in the ac
load active power. In case the ac load inductive reactive power
increases, the voltage reduction would be even higher, due to the
demand of capacitive reactive power from the excitation bank to
compensate for that.
Reductions at the rotor speed as a result of torque elevations,
due to a nonregulated shaft speed, would degenerate voltage and
frequency even more.
Substantial efforts have been made to overcome the poor
voltage regulation of the isolated induction generator under load
active and reactive power variations [5]. These efforts have been
concentrated on different types of voltage regulators acting
as volt ampere-reactive controllers, based on series-shunt
capacitor compounds [1], [5] [8], switched discrete capacitor
banks [9] [11], thyristor-switched inductors [12], or saturated
reactors [13], [14]. Such approaches rely on contactors, relays,
Fig. 3. Magnetization characteristics of the induction generator, for different
synchronous frequencies (f > f ). or semiconductor switches.
Although the methodologies mentioned attain valuable im-
provement in voltage regulation, they have solved the problem
( ) of the stator magnetomotive force (MMF) is equal to in
only partially, as the frequency is yet variable. Besides that, the
point  A. The point  A of the IG torque characteristic (Fig. 2)
corresponds to an equivalent steady-state point  A in the gen- generator still experiences variation in its magnetization char-
acteristic with the frequency, which leads to the requirement of
erator magnetization characteristic, as shown in Fig. 3.
a wide range of capacitance values at the excitation bank. How-
When the active power required by the ac load increases, the
ever, an excessive increase in the capacitance would deeply
synchronous frequency decreases from to , producing a
saturate the generator, leading to voltage waveform distortions.
torque increment to match the higher power demand. Thus, the
This analysis leads to the conception of a strategy which
new stable steady-state operation point is steered to point  B.
maintains constant frequency at the IG stator terminals and,
Notice that the speed governor is supposed to maintain the rotor
simultaneously, guarantees reactive power both to magnetize
speed constant.
The frequency drop to reduces the magnetization-charac- the generator and to compensate for the ac load demand.
teristic voltage ( ) in the same proportion, assuming that the The constant-frequency approach ensures that the
air-gap flux is kept constant, i.e., is constant. steady-state operation of the IG will take place following
In addition to the change in the magnetization characteristic, only one torque and magnetizing characteristic curves, both
the frequency reduction affects the capacitive reactance of the regarding the constant stator synchronous frequency.
excitation bank ( ), according to (1). and are the A generation system based on this modus operandi has to
capacitive reactance correspondent to the frequencies and comprise three indispensable parts, namely, the induction gen-
, respectively, erator itself, a voltage regulator, and a device which fixes the
frequency, magnetizes the generator, and compensates for the
(1) ac load reactive power requisites.
It is important to mention that a constant-frequency system
Altogether, the resulting effect of increasing the ac load ac- like this is suitable to work driven by energy sources which
tive power is the IG terminal-voltage reduction, due to changes cause relatively narrow ranges of speed variations, such as
910 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 4, AUGUST 2000
After startup, the IG provides the energy required to charge
and to supply the losses. The PWM inverter control circuit
is also fed by , by means of a forward dc dc converter.
The fundamental frequency of the PWM inverter output
voltage is maintained constant at 60 Hz, yielding a constant-fre-
quency busbar at the IG leads.
The IG terminal voltage waveform is sinusoidal due to the ac-
tion of the filter, which attenuates the high-frequency
voltage components.
The capacitance is rated to match the IG self-excitation
requisites during the startup. After the definition of , is
rated to set the filter cutoff frequency ( ) (for example, one
decade below the switching frequency of the PWM inverter) as
Fig. 4. Controlled-speed-based system configuration.
in
(2)
microhydroelectric plants and fuel engine plants. Therefore,
this approach is not adequate for systems where the speed
variation is the basis to achieve profitable energy conversion,
The speed governor role is to set rotor speed so that the IG
such as wind systems.
produces enough power to supply the ac loads, the system
losses, and the PWM inverter control circuits, as well as to
keep properly charged.
III. DESCRIPTION OF THE PROPOSED SYSTEMS
In this system, the rotor speed is variable and has to be set
to suit the IG power requirements, conversely to synchronous
Two distinct structures which are able to produce balanced
generator systems where the rotor speed is kept constant. Conse-
three-phase regulated voltages with constant frequency are pre-
quently, the governor speed reference value ( ) is made vari-
sented. Both structures employ induction generator associated
able in the present system.
with voltage-fed pulsewidth modulation (PWM) inverters, in
In case the electric power produced by the IG is not enough
order to establish constant frequency at the IG stator ends.
to match the consumed power, the PWM inverter dc capacitor
One of the proposed configurations does not include a speed
( ) is the only source from where the ac loads can take power.
governor, as the elimination of the speed control yields a quite
Thus, the consumption of part of the energy stored in would
significant economy in the overall cost of the system. In this
produce a decrease in the dc-link voltage ( ) up to the system
case, the IG voltage regulation is attained by consuming all ex-
collapse. Similarly, an excess of generated power with relation
ceeding power, as the speed-governor absence does not allow
to the ac load power would be stored in , causing the unlim-
control of the amount of the generated power. In this case, the
ited increase of . Therefore, is a suitable parameter to
excess of energy, which is not consumed by the ac load, is sent
indicate the system power balance and it can be employed as the
to the utility grid via a single-phase line. This configuration is
control variable of the speed governor. Thus, the speed-governor
able to be applied in sites where there is availability of enough
control operates to maintain tracking a reference value, in
hydraulic energy source and a single-phase line connection to
order to attain the system s power balance.
grid.
Assuming the synchronous frequency at the induction gen-
The other proposed configuration employs the speed gov-
erator stator is kept constant by the PWM inverter action, the
ernor, in order to control the amount of the generated energy.
speed governor affects the voltage amplitude as well as the
This structure is more suitable to be applied in small fuel-en-
generator terminal voltage at the proposed system (Fig. 4).
gine-driven generation systems.
As acts as a voltage source to the PWM inverter, a good
The main goal of both proposed configurations is to feed
voltage regulation is obtained at the IG leads by keeping in-
the ac loads with satisfactory energy quality, which means pro-
variable, since the only difference between the voltages at the IG
viding three-phase balanced voltages, with constant frequency,
and at the PWM inverter ac terminals is the voltage drop at the
sinusoidal waveform, and regulated amplitude.
series inductance ( ). Provided is assessed to filter voltage
components at the switching frequency and higher frequencies,
A. Controlled-Speed-Based System
the voltage drop at 60 Hz is quite small. Hence, a good voltage
The controlled-speed-based system configuration is in- regulation and the system power balance are both achieved when
herently composed of an induction generator excited by a the speed governor maintains constant .
three-phase capacitor bank ( ), connected to the ac side of Considering the PWM inverter allows bidirectional power
a voltage-fed PWM inverter through series inductances ( ). flow, the capability to compensate for reactive power is a natural
The rotor shaft speed is controlled by a speed governor, as consequence of the system configuration and operation mode.
presented in Fig. 4. Therefore, when is kept constant, the generator voltage is
The system is isolated from grid and the starting is accom- regulated, even when feeding dominantly reactive loads. Nev-
plished from the self-excitation produced by the interaction be- ertheless, the PWM inverter should be properly rated to support
tween the residual flux voltage and the ac capacitive bank ( ). the reactive power load flow.
MARRA AND POMILIO: IG-BASED SYSTEM 911
This kind of system is suitable to be employed mainly in mi-
crohydroelectric plants whose rated power is lower than 50 kW,
such as rural sites where there are both enough hydraulic energy
source and a single-phase grid connection available. This at-
tributes are normally found in the north-central region of Brazil
and other Latin America rural areas.
The system rated power is limited by the availability of
low-cost turbines suitable to operate with nonregulated shaft.
Furthermore, the single-phase line has to be rated to receive all
the generated power if necessary.
IV. SIMULATION RESULTS
The controlled-speed-based system simulation was carried
Fig. 5. Variable-speed-based system configuration.
out for a 50-hp induction generator, assisted by the PSpice pro-
gram, using a three-stationary-axes model ( model) to rep-
resent the induction machine. This system experiences a more
The energy stored in is vital to improve the system s ca-
critical dynamic response than the ungoverned-speed-based
pability to support extreme transient conditions, such as induc-
system, due to the closed-loop speed-control dynamics in-
tion motor startups and high-power load steps. As a result, the
volved. Thus, the controlled-speed-based system simulation is
system s transient behavior becomes more robust when is
a more suitable method to probe the system feasibility.
suitably rated.
The 50-hp cage-rotor induction machine parameters referred
Although Fig. 4 presents a proportional integral (PI) gain for
to the stator are presented in Table I [15], where , , ,
the dc-voltage-loop error amplifier, other compensators can be
and are the stator and rotor windings respective resistances
employed as an alternative to improve the system s phase and
and leakage inductances, is the air-gap magnetization induc-
gain margins.
tance, and is the rotor inertia.
The system was simulated using proportional constant equal
B. Ungoverned-Speed-Based System
to 0.5 and integral constant equal to 5 ms ( k ,
Similarly to the controlled-speed-based system, the un-
k , and nF, in Fig. 4), the inverter switching
governed-speed-based system also relies on a voltage-fed
frequency was 5 kHz, mH, F, and
PWM inverter to improve the induction generator electrical
mF. The dc-link reference voltage ( ) was set to 650 V.
characteristics, as presented in Fig. 5. This system does not
Fig. 6 presents the ac voltage at the IG terminals, the rotor
include a speed governor, hence, the generated power is
speed in radians per second, and the ac-load line current ob-
fully determined by the prime mover and the energy source
tained from simulation of an ac-load step transient connection.
availability.
After the startup process and an interval running under no
In this case, the system startup can be accomplished either
load, the system was submitted to an ac-load step at 800 ms.
from the self-excitation produced by the rotor residual flux or
The ac load was composed of a -connected resistance bank,
charging , with energy obtained from the utility grid via a
rated at about 40% of the generator rated power. The ac load
single-phase diode rectifier in series with a resistor ( ), con-
was kept connected up to 1.5 s, when the system returned to the
nected in parallel with the current inverter, as shown in Fig. 5.
previous no-load condition. It was verified that the system was
Since there is no direct control upon the amount of the gener- able to maintain the generator terminal voltage during a severe
ated power, the control is accomplished by means of sending
load transient. The closed-loop speed control acted in order to
the excess of energy, which is not consumed by the ac load, to
adjust the rotor speed so that the generator could suit the ac-load
the utility grid through the current inverter and a single-phase
power requirements.
line.
As the prime mover is not able to produce negative torque to
This sort of system is intended to be driven by nonregu- brake the rotor, a dc-link resistance was employed to avoid over-
lated-shaft-speed hydraulic turbines. Therefore, it is necessary
voltages during the occurrence of disconnections of ac loads
to guarantee the existence of a coordination between the IG and
rated at significant power values, similarly to what is done in
the turbine torque characteristics, so that the shaft speed does
motor drives. A 5- resistance was then set to be switched on
not cause a rotor slip frequency higher than the rated value, at
when the DC voltage exceeds 670 V and, once connected, to
the point relative to the maximum generated power.
be switched off when the dc voltage returns to 650 V. Since the
The PWM inverter dc side is asynchronously connected to the purpose of the dc-link resistance is to avoid overvoltages under
single-phase utility grid through a current inverter (CI) (Fig. 5). transient episodes, this does not operate under normal circum-
Thus, the system works as a cogenerator for the utility. stances, when the nondissipative speed control is intended to
A buck dc dc converter operates as a high-power-factor reg- maintain constant dc voltage.
ulator, ensuring that the current sent to grid is properly phased It was observed that the system simulation demanded a
with the utility terminal voltage, and attains practically sinu- long computation time due to the concurrent high switching
soidal waveform. frequency (5 kHz) and mechanical time constants involved.
912 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 4, AUGUST 2000
TABLE I
INDUCTION MACHINE PARAMETERS
Fig. 7. (a) PWM-inverter line voltage. (b) IG terminal line voltage.
Fig. 6. (a) IG terminal line voltage. (b) Rotor speed. (c) AC-load line curren.
V. EXPERIMENTAL RESULTS
Both controlled-speed and ungoverned-speed IG-based
systems previously described were implemented, employing
a three-phase 1/2-hp induction machine with four poles, and
rated voltage of 220 V in delta connection. Moreover, the PWM
inverter switching frequency was 5 kHz, uF , and
mH, while is rated to produce satisfactory dynamic
behavior during both steady-state and transient conditions.
A. Controlled-Speed-Based System Results
The controlled-speed-based system was set up experimen-
tally, employing a dc motor as the system prime mover. The
dc motor was independently excited and driven by a controlled
Fig. 8. Experimental relation between the IG rotor speed (r/min) and the
ac-load power {pu).
rectifier. The system also attained F at the PWM
inverter dc side.
Fig. 7 shows the IG and the PWM inverter terminal line volt-
ages in steady state. Observe that the filter was ef-
fective in preventing the IG line voltage from the presence of
high-frequency components.
The variation of the IG rotor speed with the ac-load active
power is indicated in Fig. 8. Notice that the speed governor
raises the rotor speed, as the ac load power increases, causing
an augment in the rotor slip frequency, so that more power is
produced by the IG to suit the ac load demand.
Fig. 9 indicates the startup of an eight-pole induction machine
whose rated values are 220 V (delta connection) and 70% of the
IG rated power. The induction motor was directly connected to
the IG leads at the startup. During the motor starting, part of
the energy stored in is employed in the motor acceleration.
This causes a voltage sag which is subsequently eliminated due
to the speed controller action ( control).
The maximum voltage sag allowed at the ac loads should be
Fig. 9. (a) IG terminal line voltage and (b) induction motor line current, during
a decisive guideline to assess the rated value of . the motor startup.
MARRA AND POMILIO: IG-BASED SYSTEM 913
Fig. 10. Line current from the IG including (a) C (I ), (b) PWM-inverter
line current (I ), and (c) induction motor line current (I ), during the motor
Fig. 11. (a) Single-phase line terminal voltage. (b) Current sent to the utility.
disconnection.
(c) Frequency spectrum of the current sent to utility.
The PWM inverter capability to compensate for the ac-load
reactive power requisites is evidenced by the record of the line
currents at the PWM inverter and at the induction motor, pre-
sented in Fig. 10. These currents were registered during the in-
duction motor disconnection. The motor was operating without
mechanical load at its shaft, leading to a strongly inductive line
current.
Fig. 10 presents the line currents at the PWM inverter output
( ), at the induction motor ( ) , and the resulting current from
the association of the IG with the excitation capacitor bank ( ),
whose adopted positive directions are those presented in Fig. 4.
The current comprises the switching-frequency compo-
nent as well as the component relative to the active power con-
sumed by the load and the PWM converter (losses). After the
IM disconnection, the current component relative to the motor
losses is extinguished.
The IM magnetization current is provided by the PWM in-
verter. As a result, the converter line current ( ) decays with
Fig. 12. (a) IG terminal voltage. (b) Current sent to the utility. (c) Line current
the disconnection of the motor (Fig. 10), which was the only ac at the ac load.
load fed by the system.
The system s voltage regulation demonstrated to be satisfac-
The experimental results obtained from the variable-speed
tory, as the steady-state values of the IG terminal voltage varied
system implementation are similar to those obtained for the con-
from 226 to 224 V, when the ac resistive load power varied from
trolled-speed system, regarding the voltage waveform, the capa-
no load to the rated power.
bility to compensate for the ac load reactive power, and to sup-
The IG voltage could be even made constant if an ac voltage
port induction-motor startups, directly connected to the gener-
feedback was used to compensated the voltage drop in , by
ator leads.
changing the modulation index of the PWM signal.
The single-phase line terminal voltage and the current sent
to the utility grid are presented in Fig. 11. Notice the current is
B. Ungoverned-Speed-Based System Results
in phase with voltage and its waveform is approximately sinu-
The experimental setup of the variable-speed system was car- soidal, as pointed out by the low harmonic content in the cur-
ried out based on the system configuration presented in Fig. 5, rent-frequency spectrum.
which was set up using F, mH, and The record of a sequence of ac-load transients is presented
switching frequency of 25 kHz at the buck dc dc converter. in Fig. 12. The ac load is composed of balanced three-phase
The L4981 integrated circuit, normally employed to drive a light-bulb sets, which are disconnected at three different in-
power-factor preregulator in ac dc converter applications [16], stants. As a result, the power provided by the IG varies from
was used to control the buck converter. Hence, the current sent 120% of the generator rated power to no load.
to the utility grid through the current inverter is a sinusoidal Observe that each reduction at the ac-load power causes an
waveform and phased with the line voltage. increase at the current sent to the utility through the current
914 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 4, AUGUST 2000
inverter (Fig. 12). No relevant influence was detected at the IG REFERENCES
terminal voltage during the transient occurrences.
[1] D. E. Bassett and M. F. Potter,  Capacitive excitation for induction gen-
erators, AIEE Trans., vol. 54, pp. 540 543, May 1935.
The system presented satisfactory voltage regulation results
[2] C. F. Wagner,  Self-excitation of induction motors, AIEE Trans., vol.
in steady-state operation, namely, 1.8% when feeding purely
58, pp. 47 51, Feb. 1939.
resistive loads and 2.2% when feeding three-phase inductive
[3] W. A. Moncrief,  Practical application and selection of single-phase to
three-phase converters, in Proc. IEEE-IAS Rural Electric Power Conf.,
loads. In addition, no effect was detected at the terminal voltage
1996, pp. D3/1 D3/9.
when both sorts of loads were made unbalanced.
[4] M. O. Durham and R. Ramakumar,  Power system balancers for an in-
duction generator, IEEE Trans. Ind. Applicat., vol. 23, pp. 1067 1072,
Nov./Dec. 1987.
VI. CONCLUSION
[5] L. Wang and S. Jina-Yi,  Effects of long-shunt and short-shunt connec-
tions on voltage variations of a self-excited induction generator, IEEE
This paper has contributed toward proposing a constant-fre-
Trans. Energy Conversion, vol. 12, pp. 368 374, Dec. 1997.
quency operation mode to isolated induction generator systems,
[6] T. F. Chan,  Analysis of self-excited induction generators using an iter-
as an approach to improve their voltage regulation and main- ative method, IEEE Trans. Energy Conversion, vol. 10, pp. 502 507,
Sept. 1995.
tain constant synchronous frequency. Moreover, the systems are
[7] ,  Steady-state analysis of self-excited induction generators, IEEE
able to feed inductive ac loads, as well as to support direct-con-
Trans. Energy Conversion, vol. 9, pp. 288 296, June 1994.
nection startups of induction motors. The proposed strategy is
[8] E. Bim, J. Szajner, and Y. Burian,  Voltage compensation of an induction
generator with long-shunt connection, IEEE Trans. Energy Conversion,
compatible with both controlled-speed or variable-speed opera-
vol. 4, pp. 526 530, Sept. 1989.
tion modes.
[9] T. F. Chan,  Capacitance requirements of self-excited induction genera-
The adopted methodology is nowadays feasible and cost ef- tors, IEEE Trans. Energy Conversion, vol. 8, pp. 304 311, June 1993.
[10] N. H. Malik and A. A. Mazi,  Capacitive requirements for isolated
fective at power levels mentioned before, due to the outstanding
self-excited induction generators, IEEE Trans. Energy Conversion,
evolution experienced by power semiconductor devices and
vol. EC-2, pp. 62 69, Mar. 1987.
power converter technology.
[11] N. H. Malik and S. E. Haque,  Steady state analysis and performance of
an isolated self-excited induction generator, IEEE Trans. Energy Con-
The proposed strategy is intended to be applied in generation
version, vol. EC-1, pp. 134 140, Sept. 1986.
systems whose primary energy source produces a narrow range
[12] M. B. Brennen and A. Abbondanti,  Static exciters for induction genera-
of variation at the generator shaft speed, such as low-head micro- tors, IEEE Trans. Ind. Applicat., vol. 13, pp. 422 428, Sept./Oct. 1977.
[13] S. M. Alghuwainem,  Steady-state analysis of a self-excited induction
hydroelectric plants and fuel-engine-driven generation systems.
generator self-regulated by a shunt saturable reactor, in Conf. Rec. IEEE
Two distinct system configurations were presented based
IEMDC 97, Milwaukee, WI, May 1997, pp. MB1-10.1 MB1-10.3.
on the constant-synchronous-frequency operation mode. One
[14] B. C. Doxey,  Theory and applications of capacitors excited induction
generators, Engineer, vol. 216, pp. 893 897, Nov. 1963.
of the proposed configurations employs the speed governor,
[15] S. B. Dewan, G. R. Slemon, and A. Straughen, Power Semiconductor
in order to suit the IG power to the ac-load power requisites,
Drives. New York: Wiley, 1984.
while the other proposed configuration does not use any speed
[16] C. A. Ayres and I. Barbi,  A family of converters for UPS produc-
tion burn-in energy recovery, IEEE Trans Power Electron., vol. 12, pp.
governor. The latter structure makes use of a single-phase
615 622, July 1997.
grid-connection to match the system s generated power to the
ac-load power demand. In this case, the system operates as
cogenerator, sending the exceeding generated power to the
Enes Gonçalves Marra (S 95 A 99) was born
utility grid.
in Brazil in 1966. He received the B.S. and M.S.
In both systems, the PWM inverter guarantees constant fre- degrees in electrical engineering from the Federal
University of Uberlândia, Uberlândia, Brazil, and
quency at the IG leads and provide the means to the proposed
the Doctoral degree in electrical engineering from
systems compensate for the ac-load reactive power requisites.
the State University of Campinas, Campinas, Brazil,
Furthermore, the capacitor at the PWM inverter is a fast-re- in 1989, 1993, and 1999, respectively.
Since 1993, he has been a Lecturer in the School of
covery energy storage device, which improves the robustness
Electrical Engineering, Federal University of Goiás,
of the IG system to support severe transients, such as induction
Goiânia, Brazil. His research interests include elec-
motor startups.
trical drives, power electronics applications, and re-
newable energy sources.
Both systems were also confirmed to be robust and stable
when submitted to sequential ac-load steps as well as during
the induction motor starting.
José Antenor Pomilio (M 93) received the Bach-
The PWM inverter dc voltage control ( control), exerted
elor s, Master s, and Doctoral degrees in electrical
by the speed governor, indicated that it was an effective, fast, and
engineering from the State University of Campinas,
reliable technique to obtain power balance, and to regulate the Campinas, Brazil, in 1983, 1986, and 1991, respec-
tively.
amplitude of the IG terminal voltage, in controlled-speed-based
From 1988 to 1991, he was Head of the Power
systems.
Electronics Group at the Brazilian Synchrotron
The variable-speed-based system operation as a cogenerator, Laboratory. Since 1991, he has been an Assistant
Professor in the School of Electrical and Computer
sending unity power factor sinusoidal current to the utility grid
Engineering, State University of Campinas. In
through a single-phase line, demonstrated that it was an effective
1993 1994, he held a postdoctoral position in the
control strategy for . This approach allows the elimination Electrical Engineering Department, University of Padova, Padova, Italy. His
main interests are switching-mode power supplies and electrical drives. He is
of the speed governor, leading to significant cost saving when
Vice-President of the Brazilian Power Electronics Society.
applied to microhydroelectric plants whose rated power is lower
Dr. Pomilio is currently the IEEE Power Electronics Society Liaison to Re-
than 50 kW. gion 9.