Induction-generator-based system providing ... - Semantic Scholar

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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 induction-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 behavior in many aspects: voltage regulation, frequency stabilization, and reactive power compensation. The system operation strategy consists of maintaining constant synchronous frequency at the induction 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 regulation, 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 obtained results demonstrated the system is stable, robust, and an effective source of regulated three-phase voltages. Index Terms—Energy conversion, energy resources, induction generator, pulsewidth modulation inverter.

I. INTRODUCTION

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T IS FREQUENTLY stated that cage rotor induction machines (IMs) are robust, inexpensive compared with dc and wound-rotor synchronous machines, require little maintenance, and have high power-weight ratio (W/kg). Despite these favorable features, IM’s are hardly employed as generators due to their unsatisfactory voltage regulation and frequency variation, even when driven under constant speed and feeding loads which consume active power [1], [2]. Wound-rotor synchronous generators are reliable suppliers of regulated three-phase constant frequency voltage, provided the dynamic response of the speed governor is able to maintain constant rotor velocity during the occurrence of load power variations. Nevertheless, they are expensive machines due to the maintenance required by the excitation system, which contains slip rings, brushes, or rotating rectifiers, in addition to field current control circuits. Therefore, a cost-effective and technically reliable alternative to wound-rotor synchronous generators would be welcome. The aim of this investigation is to propose an induction generator (IG) application as an alternative to wound-rotor synchronous generators to be employed in low-power isolated genManuscript received February 12, 1999; revised September 20, 1999. Abstract published on the Internet April 21, 2000. This work was supported by Coordenação para o Aperfeiçoamento de Pessoal de Ensino Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP). E. Gonçalves Marra is with the School of Electrical Engineering, Federal University of Goiás, 74605-220 Goiânia, Brazil (e-mail: [email protected]). J. Antenor Pomilio is with the School of Electrical and Computer Engineering, State University of Campinas, 13081-970 Campinas, Brazil (e-mail: [email protected] ). Publisher Item Identifier S 0278-0046(00)06814-3.

Fig. 1. Capacitor-excited IG system, isolated from the utility grid.

eration systems, such as low-head microhydroelectric plants and fuel engine driven generation systems. Two distinct structures are presented. In one of these structures, the generator’s shaft speed is regulated. The other structure does not comprise speed governor, and the system acts as cogenerator, sending energy to a single-phase grid, as a strategy to control the IG terminal voltage. The cogenerator structure is appropriate to be employed in areas such as light manufacturing or agricultural areas where electric power available is only single phase. Customers in these areas may request three-phase power from the utility and find it is uneconomical for the utility to meet a relatively small threephase need [3], [4]. Both proposed systems are intended to be sources of regulated voltage with constant frequency, whose energy quality is good enough to feed sensitive loads, such as microprocessor-controlled ones. II. ISOLATED CAPACITOR-EXCITED IG SYSTEM Fig. 1 presents a system in which a capacitor-excited IG operates isolated from the utility grid. In this circumstance, the active power of the ac load affects considerably the amplitude and the frequency of the voltage at the IG terminals. In this case, the synchronous frequency is not constant, even if the rotor speed is kept constant by the action of a speed governor. Assuming that the mechanical, electrical, and magnetic losses are negligible, the electric power converted by the generator is given by the product between the rotor speed and the generator torque. Supposing the rotor speed is invariable, the increase of the active power required by the ac load yields a drop in the stator frequency, as it is the only possible way the IG can raise its rotor slip frequency and consequently elevate the torque, so that it is able to suit the load power demand. Fig. 2 illustrates qualitatively a situation in which the induction generator was feeding a unity power-factor load so that the steady-state operation point is “A.” The synchronous frequency

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MARRA AND POMILIO: IG-BASED SYSTEM

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Fig. 2. Torque-speed characteristics of the induction generator, for different ). synchronous frequencies (

f >f

Fig. 3. Magnetization characteristics of the induction generator, for different ). synchronous frequencies (

f >f

( ) of the stator magnetomotive force (MMF) is equal to in point “A.” The point “A” of the IG torque characteristic (Fig. 2) corresponds to an equivalent steady-state point “A” in the generator magnetization characteristic, as shown in Fig. 3. When the active power required by the ac load increases, the to , producing a synchronous frequency decreases from torque increment to match the higher power demand. Thus, the new stable steady-state operation point is steered to point “B.” Notice that the speed governor is supposed to maintain the rotor speed constant. reduces the magnetization-characThe frequency drop to teristic voltage ( ) in the same proportion, assuming that the is constant. air-gap flux is kept constant, i.e., In addition to the change in the magnetization characteristic, the frequency reduction affects the capacitive reactance of the and are the excitation bank ( ), according to (1). and capacitive reactance correspondent to the frequencies , respectively, (1) Altogether, the resulting effect of increasing the ac load active power is the IG terminal-voltage reduction, due to changes

in the magnetization characteristic and in the excitation bank capacitive reactance. The capacitance could be increased even more, in order to . In this case, the slope of recover the capacitive reactance the capacitor-bank voltage characteristic will return to its previous value, however, the steady-state operation point in the magnetization characteristic will now be “ ” instead of “A,” . The new operation point at the as the frequency remains 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 characteristic. Note that the voltage and frequency variations presented previously 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, or semiconductor switches. Although the methodologies mentioned attain valuable improvement in voltage regulation, they have solved the problem only partially, as the frequency is yet variable. Besides that, the generator still experiences variation in its magnetization characteristic with the frequency, which leads to the requirement of a wide range of capacitance values at the excitation bank. Howcapacitance would deeply ever, an excessive increase in the saturate the generator, leading to voltage waveform distortions. This analysis leads to the conception of a strategy which maintains constant frequency at the IG stator terminals and, simultaneously, guarantees reactive power both to magnetize the generator and to compensate for the ac load demand. The constant-frequency approach ensures that the steady-state operation of the IG will take place following only one torque and magnetizing characteristic curves, both regarding the constant stator synchronous frequency. A generation system based on this modus operandi has to comprise three indispensable parts, namely, the induction generator itself, a voltage regulator, and a device which fixes the frequency, magnetizes the generator, and compensates for the ac load reactive power requisites. It is important to mention that a constant-frequency system like this is suitable to work driven by energy sources which cause relatively narrow ranges of speed variations, such as

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Fig. 4.


Controlled-speed-based system configuration.

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, such as wind systems. III. DESCRIPTION OF THE PROPOSED SYSTEMS Two distinct structures which are able to produce balanced three-phase regulated voltages with constant frequency are presented. Both structures employ induction generator associated with voltage-fed pulsewidth modulation (PWM) inverters, in order to establish constant frequency at the IG stator ends. One of the proposed configurations does not include a speed governor, as the elimination of the speed control yields a quite significant economy in the overall cost of the system. In this case, the IG voltage regulation is attained by consuming all exceeding power, as the speed-governor absence does not allow control of the amount of the generated power. In this case, the excess of energy, which is not consumed by the ac load, is sent to the utility grid via a single-phase line. This configuration is able to be applied in sites where there is availability of enough hydraulic energy source and a single-phase line connection to grid. The other proposed configuration employs the speed governor, in order to control the amount of the generated energy. This structure is more suitable to be applied in small fuel-engine-driven generation systems. The main goal of both proposed configurations is to feed the ac loads with satisfactory energy quality, which means providing three-phase balanced voltages, with constant frequency, sinusoidal waveform, and regulated amplitude. A. Controlled-Speed-Based System The controlled-speed-based system configuration is inherently composed of an induction generator excited by a ), connected to the ac side of three-phase capacitor bank ( a voltage-fed PWM inverter through series inductances ( ). The rotor shaft speed is controlled by a speed governor, as presented in Fig. 4. The system is isolated from grid and the starting is accomplished from the self-excitation produced by the interaction be). tween the residual flux voltage and the ac capacitive bank (

After startup, the IG provides the energy required to charge and to supply the losses. The PWM inverter control circuit , by means of a forward dc–dc converter. is also fed by The fundamental frequency of the PWM inverter output voltage is maintained constant at 60 Hz, yielding a constant-frequency busbar at the IG leads. The IG terminal voltage waveform is sinusoidal due to the acfilter, which attenuates the high-frequency tion of the voltage components. capacitance is rated to match the IG self-excitation The , is requisites during the startup. After the definition of rated to set the filter cutoff frequency ( ) (for example, one decade below the switching frequency of the PWM inverter) as in (2) The speed governor role is to set rotor speed so that the IG produces enough power to supply the ac loads, the system losses, and the PWM inverter control circuits, as well as to properly charged. keep In this system, the rotor speed is variable and has to be set to suit the IG power requirements, conversely to synchronous generator systems where the rotor speed is kept constant. Conse) is made variquently, the governor speed reference value ( able in the present system. In case the electric power produced by the IG is not enough to match the consumed power, the PWM inverter dc capacitor ) is the only source from where the ac loads can take power. ( would Thus, the consumption of part of the energy stored in produce a decrease in the dc-link voltage ( ) up to the system collapse. Similarly, an excess of generated power with relation , causing the unlimto the ac load power would be stored in . Therefore, is a suitable parameter to ited increase of indicate the system power balance and it can be employed as the control variable of the speed governor. Thus, the speed-governor tracking a reference value, in control operates to maintain order to attain the system’s power balance. Assuming the synchronous frequency at the induction generator stator is kept constant by the PWM inverter action, the voltage amplitude as well as the speed governor affects the generator terminal voltage at the proposed system (Fig. 4). acts as a voltage source to the PWM inverter, a good As involtage regulation is obtained at the IG leads by keeping variable, since the only difference between the voltages at the IG and at the PWM inverter ac terminals is the voltage drop at the is assessed to filter voltage series inductance ( ). Provided components at the switching frequency and higher frequencies, the voltage drop at 60 Hz is quite small. Hence, a good voltage regulation and the system power balance are both achieved when . the speed governor maintains constant Considering the PWM inverter allows bidirectional power flow, the capability to compensate for reactive power is a natural consequence of the system configuration and operation mode. is kept constant, the generator voltage is Therefore, when regulated, even when feeding dominantly reactive loads. Nevertheless, the PWM inverter should be properly rated to support the reactive power load flow.


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This kind of system is suitable to be employed mainly in microhydroelectric 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 attributes 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 Fig. 5. Variable-speed-based system configuration.

The energy stored in is vital to improve the system’s capability to support extreme transient conditions, such as induction motor startups and high-power load steps. As a result, the is system’s transient behavior becomes more robust when suitably rated. Although Fig. 4 presents a proportional–integral (PI) gain for the dc-voltage-loop error amplifier, other compensators can be employed as an alternative to improve the system’s phase and gain margins. B. Ungoverned-Speed-Based System Similarly to the controlled-speed-based system, the ungoverned-speed-based system also relies on a voltage-fed PWM inverter to improve the induction generator electrical characteristics, as presented in Fig. 5. This system does not include a speed governor, hence, the generated power is fully determined by the prime mover and the energy source availability. In this case, the system startup can be accomplished either from the self-excitation produced by the rotor residual flux or , with energy obtained from the utility grid via a charging ), consingle-phase diode rectifier in series with a resistor ( nected in parallel with the current inverter, as shown in Fig. 5. Since there is no direct control upon the amount of the genercontrol is accomplished by means of sending ated power, the the excess of energy, which is not consumed by the ac load, to the utility grid through the current inverter and a single-phase line. This sort of system is intended to be driven by nonregulated-shaft-speed hydraulic turbines. Therefore, it is necessary to guarantee the existence of a coordination between the IG and the turbine torque characteristics, so that the shaft speed does not cause a rotor slip frequency higher than the rated value, at the point relative to the maximum generated power. The PWM inverter dc side is asynchronously connected to the single-phase utility grid through a current inverter (CI) (Fig. 5). Thus, the system works as a cogenerator for the utility. A buck dc–dc converter operates as a high-power-factor regulator, ensuring that the current sent to grid is properly phased with the utility terminal voltage, and attains practically sinusoidal waveform.

The controlled-speed-based system simulation was carried out for a 50-hp induction generator, assisted by the PSpice promodel) to repgram, using a three-stationary-axes model ( resent the induction machine. This system experiences a more critical dynamic response than the ungoverned-speed-based system, due to the closed-loop speed-control dynamics involved. Thus, the controlled-speed-based system simulation is a more suitable method to probe the system feasibility. The 50-hp cage-rotor induction machine parameters referred , to the stator are presented in Table I [15], where , , are the stator and rotor windings respective resistances and is the air-gap magnetization inducand leakage inductances, tance, and is the rotor inertia. The system was simulated using proportional constant equal k , to 0.5 and integral constant equal to 5 ms ( k , and nF, in Fig. 4), the inverter switching mH, F, and frequency was 5 kHz, mF. The dc-link reference voltage ( ) was set to 650 V. Fig. 6 presents the ac voltage at the IG terminals, the rotor speed in radians per second, and the ac-load line current obtained from simulation of an ac-load step transient connection. After the startup process and an interval running under no load, the system was submitted to an ac-load step at 800 ms. The ac load was composed of a -connected resistance bank, rated at about 40% of the generator rated power. The ac load was kept connected up to 1.5 s, when the system returned to the previous no-load condition. It was verified that the system was able to maintain the generator terminal voltage during a severe load transient. The closed-loop speed control acted in order to adjust the rotor speed so that the generator could suit the ac-load power requirements. As the prime mover is not able to produce negative torque to brake the rotor, a dc-link resistance was employed to avoid overvoltages during the occurrence of disconnections of ac loads rated at significant power values, similarly to what is done in motor drives. A 5- resistance was then set to be switched on when the DC voltage exceeds 670 V and, once connected, to be switched off when the dc voltage returns to 650 V. Since the purpose of the dc-link resistance is to avoid overvoltages under transient episodes, this does not operate under normal circumstances, when the nondissipative speed control is intended to maintain constant dc voltage. It was observed that the system simulation demanded a long computation time due to the concurrent high switching frequency (5 kHz) and mechanical time constants involved.

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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 uF , and inverter switching frequency was 5 kHz, 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 experimentally, employing a dc motor as the system prime mover. The dc motor was independently excited and driven by a controlled F at the PWM rectifier. The system also attained inverter dc side. Fig. 7 shows the IG and the PWM inverter terminal line voltfilter was efages in steady state. Observe that the 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 is employed in the motor acceleration. the energy stored in This causes a voltage sag which is subsequently eliminated due control). to the speed controller action ( The maximum voltage sag allowed at the ac loads should be . a decisive guideline to assess the rated value of

Fig. 8. Experimental relation between the IG rotor speed (r/min) and the ac-load power {pu).

Fig. 9. (a) IG terminal line voltage and (b) induction motor line current, during the motor startup.


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 disconnection.

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, presented in Fig. 10. These currents were registered during the induction 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. current comprises the switching-frequency compoThe nent as well as the component relative to the active power consumed 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 inverter. As a result, the converter line current ( ) decays with the disconnection of the motor (Fig. 10), which was the only ac load fed by the system. The system’s voltage regulation demonstrated to be satisfactory, as the steady-state values of the IG terminal voltage varied from 226 to 224 V, when the ac resistive load power varied from no load to the rated power. The IG voltage could be even made constant if an ac voltage , by feedback was used to compensated the voltage drop in changing the modulation index of the PWM signal. B. Ungoverned-Speed-Based System Results The experimental setup of the variable-speed system was carried out based on the system configuration presented in Fig. 5, F, mH, and which was set up using switching frequency of 25 kHz at the buck dc–dc converter. The L4981 integrated circuit, normally employed to drive a power-factor preregulator in ac–dc converter applications [16], was used to control the buck converter. Hence, the current sent to the utility grid through the current inverter is a sinusoidal waveform and phased with the line voltage.

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Fig. 11. (a) Single-phase line terminal voltage. (b) Current sent to the utility. (c) Frequency spectrum of the current sent to utility.

Fig. 12. (a) IG terminal voltage. (b) Current sent to the utility. (c) Line current at the ac load.

The experimental results obtained from the variable-speed system implementation are similar to those obtained for the controlled-speed system, regarding the voltage waveform, the capability to compensate for the ac load reactive power, and to support induction-motor startups, directly connected to the generator leads. The single-phase line terminal voltage and the current sent to the utility grid are presented in Fig. 11. Notice the current is in phase with voltage and its waveform is approximately sinusoidal, as pointed out by the low harmonic content in the current-frequency spectrum. The record of a sequence of ac-load transients is presented in Fig. 12. The ac load is composed of balanced three-phase light-bulb sets, which are disconnected at three different instants. As a result, the power provided by the IG varies from 120% of the generator rated power to no load. Observe that each reduction at the ac-load power causes an increase at the current sent to the utility through the current

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inverter (Fig. 12). No relevant influence was detected at the IG terminal voltage during the transient occurrences. The system presented satisfactory voltage regulation results in steady-state operation, namely, 1.8% when feeding purely resistive loads and 2.2% when feeding three-phase inductive loads. In addition, no effect was detected at the terminal voltage when both sorts of loads were made unbalanced. VI. CONCLUSION This paper has contributed toward proposing a constant-frequency operation mode to isolated induction generator systems, as an approach to improve their voltage regulation and maintain constant synchronous frequency. Moreover, the systems are able to feed inductive ac loads, as well as to support direct-connection startups of induction motors. The proposed strategy is compatible with both controlled-speed or variable-speed operation modes. The adopted methodology is nowadays feasible and cost effective at power levels mentioned before, due to the outstanding evolution experienced by power semiconductor devices and power converter technology. The proposed strategy is intended to be applied in generation systems whose primary energy source produces a narrow range of variation at the generator shaft speed, such as low-head microhydroelectric plants and fuel-engine-driven generation systems. Two distinct system configurations were presented based on the constant-synchronous-frequency operation mode. One of the proposed configurations employs the speed governor, in order to suit the IG power to the ac-load power requisites, while the other proposed configuration does not use any speed governor. The latter structure makes use of a single-phase 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 utility grid. In both systems, the PWM inverter guarantees constant frequency at the IG leads and provide the means to the proposed systems compensate for the ac-load reactive power requisites. capacitor at the PWM inverter is a fast-reFurthermore, the covery energy storage device, which improves the robustness of the IG system to support severe transients, such as induction motor startups. 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. control), exerted The PWM inverter dc voltage control ( by the speed governor, indicated that it was an effective, fast, and reliable technique to obtain power balance, and to regulate the amplitude of the IG terminal voltage, in controlled-speed-based systems. The variable-speed-based system operation as a cogenerator, sending unity power factor sinusoidal current to the utility grid through a single-phase line, demonstrated that it was an effective . This approach allows the elimination control strategy for of the speed governor, leading to significant cost saving when applied to microhydroelectric plants whose rated power is lower than 50 kW.

REFERENCES [1] D. E. Bassett and M. F. Potter, “Capacitive excitation for induction generators,” AIEE Trans., vol. 54, pp. 540–543, May 1935. [2] C. F. Wagner, “Self-excitation of induction motors,” AIEE Trans., vol. 58, pp. 47–51, Feb. 1939. [3] W. A. Moncrief, “Practical application and selection of single-phase to three-phase converters,” in Proc. IEEE-IAS Rural Electric Power Conf., 1996, pp. D3/1–D3/9. [4] M. O. Durham and R. Ramakumar, “Power system balancers for an induction generator,” IEEE Trans. Ind. Applicat., vol. 23, pp. 1067–1072, Nov./Dec. 1987. [5] L. Wang and S. Jina-Yi, “Effects of long-shunt and short-shunt connections on voltage variations of a self-excited induction generator,” IEEE Trans. Energy Conversion, vol. 12, pp. 368–374, Dec. 1997. [6] T. F. Chan, “Analysis of self-excited induction generators using an iterative method,” IEEE Trans. Energy Conversion, vol. 10, pp. 502–507, Sept. 1995. , “Steady-state analysis of self-excited induction generators,” IEEE [7] Trans. Energy Conversion, vol. 9, pp. 288–296, June 1994. [8] E. Bim, J. Szajner, and Y. Burian, “Voltage compensation of an induction generator with long-shunt connection,” IEEE Trans. Energy Conversion, vol. 4, pp. 526–530, Sept. 1989. [9] T. F. Chan, “Capacitance requirements of self-excited induction generators,” IEEE Trans. Energy Conversion, vol. 8, pp. 304–311, June 1993. [10] N. H. Malik and A. A. Mazi, “Capacitive requirements for isolated self-excited induction generators,” IEEE Trans. Energy Conversion, vol. EC-2, pp. 62–69, Mar. 1987. [11] N. H. Malik and S. E. Haque, “Steady state analysis and performance of an isolated self-excited induction generator,” IEEE Trans. Energy Conversion, vol. EC-1, pp. 134–140, Sept. 1986. [12] M. B. Brennen and A. Abbondanti, “Static exciters for induction generators,” IEEE Trans. Ind. Applicat., vol. 13, pp. 422–428, Sept./Oct. 1977. [13] S. M. Alghuwainem, “Steady-state analysis of a self-excited induction generator self-regulated by a shunt saturable reactor,” in Conf. Rec. IEEE IEMDC’97, Milwaukee, WI, May 1997, pp. MB1-10.1–MB1-10.3. [14] B. C. Doxey, “Theory and applications of capacitors excited induction generators,” Engineer, vol. 216, pp. 893–897, Nov. 1963. [15] S. B. Dewan, G. R. Slemon, and A. Straughen, Power Semiconductor Drives. New York: Wiley, 1984. [16] C. A. Ayres and I. Barbi, “A family of converters for UPS production burn-in energy recovery,” IEEE Trans Power Electron., vol. 12, pp. 615–622, July 1997.

Enes Gonçalves Marra (S’95–A’99) was born in Brazil in 1966. He received the B.S. and M.S. degrees in electrical engineering from the Federal University of Uberlândia, Uberlândia, Brazil, and the Doctoral degree in electrical engineering from the State University of Campinas, Campinas, Brazil, in 1989, 1993, and 1999, respectively. Since 1993, he has been a Lecturer in the School of Electrical Engineering, Federal University of Goiás, Goiânia, Brazil. His research interests include electrical drives, power electronics applications, and renewable energy sources.

José Antenor Pomilio (M’93) received the Bachelor‘s, Master‘s, and Doctoral degrees in electrical engineering from the State University of Campinas, Campinas, Brazil, in 1983, 1986, and 1991, respectively. From 1988 to 1991, he was Head of the Power Electronics Group at the Brazilian Synchrotron Laboratory. Since 1991, he has been an Assistant Professor in the School of Electrical and Computer Engineering, State University of Campinas. In 1993–1994, he held a postdoctoral position in the Electrical Engineering Department, University of Padova, Padova, Italy. His main interests are switching-mode power supplies and electrical drives. He is Vice-President of the Brazilian Power Electronics Society. Dr. Pomilio is currently the IEEE Power Electronics Society Liaison to Region 9.