Self-excited induction generator controlled by a VS ... - IEEE Xplore

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999

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Self-Excited Induction Generator Controlled by a VS-PWM Bidirectional Converter for Rural Applications Enes Gon¸calves Marra, Student Member, IEEE, and Jos´e Antenor Pomilio, Member, IEEE

Abstract—This paper concerns an application of a three-phase cage induction machine (IM) as a self-excited generator connected to the ac side of a voltage-source pulsewidth modulation bidirectional inverter. The generator is supposed to be driven by a lowhead unregulated shaft hydraulic turbine. The proposed system is intended to be applied in rural plants as a low-cost source of highquality ac sinusoidal regulated voltage with constant frequency. Simulation results are obtained based on the  stationary reference frame model of the IM. The experimental results demonstrated that the system presents satisfactory behavior when feeding ac loads and during the startup of induction motors. Index Terms—Induction generator, pulsewidth modulation inverter, renewable energy resources.

I. INTRODUCTION

A

N INDUCTION machine (IM) generates power when enough excitation is provided and its rotor is driven to a speed higher than that of the stator magnetic field, as a result of the mechanical prime mover action [1], [2]. The cage-rotor IM’s present a wide set of advantages, such as robustness, construction simplicity, self-protection capacity, and low short-circuit current levels. They also require little maintenance, have a high power–weight ratio (W/kg), have relatively low cost, great availability, and the ability of generating power, even when driven in variable speed. Despite all these advantages, the IM application presents important drawbacks, such as its intrinsically unsatisfactory voltage and frequency regulation characteristics and the request for an external reactive power compensation to maintain the excitation.

Paper IPCSD 99–17, presented at the 1998 IEEE Applied Power Electronics Conference and Exposition, Anaheim, CA, February 15–19, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Industrial Drives Committee of the IEEE Industry Applications Society. Manuscript released for publication February 15, 1999. This work was supported by Coordena¸ca˜ o para o Aperfei¸coamento de Pessoal de Ensino Superior, Funda¸ca˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo, and Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico. This paper was also presented at III INDUSCON, S˜ao Paulo, Brazil, September 8–10, 1998. E. G. Marra is with the School of Electrical Engineering, Federal University of Goi´as, 71605-220 Goiˆania, Brazil, and is also with the School of Electrical and Computer Engineering, State University of Campinas, 13081-970 Campinas, Brazil (e-mail: [email protected]). J. A. 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 0093-9994(99)04392-3.

Fig. 1. Induction generator system configuration using dc load control.

Electric power generation using renewable energy sources has been sought due to economical and environmental reasons, mainly since the 1970’s. In this regard, great efforts have been carried out to overcome the drawbacks of the induction generators (IG’s) by applying power electronic converters and machine control techniques [3]–[5], since the IM’s are those which best match the requirements of variable-speed generation systems. The system presented here is intended to be employed in isolated sites, where proper water resource is available, which is a typical situation in Latin America rural areas. The goal of this work is to study the cage-rotor IM as a generator, driven by a low-head unregulated shaft hydraulic turbine to provide balanced three-phase regulated voltage with constant frequency, good quality, and satisfactory reliability. The proposed system is intended to be able to feed sensitive loads, such as microprocessor-controlled ones and computers.

II. GENERATION SYSTEM The proposed system comprises an induction generator ex) and a pulsewidth cited by a three-phase capacitor bank ( modulation (PWM) bidirectional voltage-fed (VS-PWM) inverter. The IG is connected to the VS-PWM inverter ac side through filter inductances ( ), as presented in Fig. 1. ) is employed as the voltage source Moreover, a capacitor ( at the dc side of the VS-PWM inverter.

0093–9994/99$10.00  1999 IEEE

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999

Fig. 2. Torque versus rotor speed curves for an induction machine.

The fundamental frequency of the VS-PWM inverter output voltage is kept constant at 60 Hz and, as a result, a constant frequency busbar is created at the IG leads. Therefore, the IG operates according to the 60-Hz torque speed characteristic, as shown in Fig. 2. It is assumed the IG is driven by a hydraulic turbine without any mechanical speed governor, leading to variable rotor speed. The electric power generated is determined by the hydraulic turbine mechanical power, as well as by the association of the speed characteristics for the IG and the hydraulic torque turbine, provided the fundamental frequency is kept constant at the VS-PWM output. Since the stator electric frequency is maintained constant at 60 Hz, the IG torque characteristic is governed exclusively by the 60-Hz torque curve. Thus, the generated power is determined by rotor-shaft speed, which is defined by the prime mover torque characteristic curve. Considering that the use of speed governors increases significantly the cost of the system and that low cost is highly desirable, the shaft speed is not regulated. Therefore, the generated power is settled by the common point belonging to both IG and turbine torque characteristics. The existence of a coordination between the torque speed characteristics for the IG and the turbine is an obligatory condition for the system. This condition avoids the condition that the maximum power produced by the turbine occurs on a speed corresponding to a slip greater than the IM rated slip. For instance, point “A” in Fig. 2 is a possible operation point for the system with a nonregulated speed hydraulic turbine. The amount of electric power produced by the IG is, therefore, completely independent on the ac load power. Thus, the electric power produced has to be completely consumed to maintain the balance between generated and consumed power. In case the ac load is not enough to match the generated power, tank the so-called exceeding power will be stored in the voltage ( ). capacitor, yielding an increase in acts as voltage source for the VS-PWM inverter, As the value of the inverter ac voltage will be constant whether and the modulation index do not vary. As a result, both

a good voltage regulation is obtained at the generator leads by maintaining invariable, since the only difference between the voltages at the IG and at the VS-PWM inverter terminals is the small voltage drop at the series inductance ( ). Hence, the exceeding power has to be consumed to achieve a good voltage regulation for the generator. The exceeding power can be consumed by a controllable load situated either in the dc side of the VS-PWM inverter or in the ac side, connected to the IG terminals. These alternatives are respectively named dc-load control and ac-load control of voltage. In both cases, is the feedback signal the which describes the balance between the generated and the indicates that the electric consumed power. An increase in power generated is higher than the power of the ac load indicates connected to the IG. Conversely, a decrease in a deficit in the generated power. sample is compared to a reference, producing an The error signal which is used to define the amount of power to be consumed by either the ac or the dc controllable load. The system startup is carried out through the induction generator self-excitation process. The self-excitation is the result of the interaction between the voltage provided by IM residual magnetic flux and the three-phase capacitor excitation ). bank ( Only in the case of loss of the IM residual flux, a simple 6-V alkaline-battery set, switched by a push button and connected to the IG leads through a 6–220-V 15–W transformer would be able to provide voltage pulses required to start the excitation process. After the startup, the IG provides the energy required to and to supply the active losses during the steadycharge also supplies the energy required by state operation. the VS-PWM control circuit, by means of a forward dc–dc converter. and The second-order low-pass filter composed of guarantees sinusoidal waveform at the IG leads. The is based exclusively on the IG selfassessment of excitation requirements. The cutoff frequency ( ) should be chosen one decade below the VS-PWM inverter switching frequency, to obtain a 40-dB attenuation for the voltage components at the switching frequency. is then obtained by substituting and in

(1)

In the system herein proposed, the induction generator is the device responsible for converting the turbine mechanical power into electrical power. Moreover, the VS-PWM inverter is responsible for another three important roles, namely, fixing a constant frequency for the IG, providing a mechanism of voltage regulation to the system, and compensating for the reactive power required by both the IG and the ac load. The PWM-converter action as var compensator is a natural consequence of the converter operation with constant fundamental frequency and regulated voltage.

MARRA AND POMILIO: SELF-EXCITED IG CONTROLLED BY A VS-PWM BIDIRECTIONAL CONVERTER

Considering that capacitor is the converter voltage source, the voltage of which is maintained constant, and that the converter allows the bidirectional power flux, the generator terminal voltage is regulated, even when feeding highly inductive loads, such as induction motors. capacitor bank could be withdrawn after Although the the system startup without affecting the generator magnetization, it is not done, taking into account the necessity of filter. Moreover, the excitation bank decreases the the demand for reactive power from the VS-PWM converter. The assessment of the VS-PWM inverter rated power has to take into account the maximum active power that will pass through the inverter, as well as the amount of reactive power to be compensated by the converter. The absence of mechanical speed governors together with the control simplicity are fundamental characteristics which makes the system outstandingly cost attractive. The low cost, the robustness, and the satisfactory energy quality are the main advantages of this induction generator system. Hence, even the employment of dissipating loads as control is admitted, with the aim of maintaining part of the system’s advantageous characteristics. A. DC-Load Voltage Control The dc-load voltage control consists of utilizing dc controllable loads connected at the VS-PWM converter dc side to consume the exceeding power. The dc loads may be employed in applications such as cogeneration [6], battery charging, heating, or an association of these options. The system shown in Fig. 1 employs a dc load for heating purposes. In that case, the dc controllable load comprises a ) switched by . resistor ( control is attained by comparing the sample The with a reference using a hysteresis comparator, the output switching (control block, Fig. 1). The of which controls in an allowable hysteresis limits should be set to maintain variation range, in order to avoid voltage variation (flicker effect) in the ac side. B. AC-Load Voltage Control The ac-load voltage control is an alternative which makes use of an ac controllable load, connected to the IG leads. Thus, the ac load is suitably controlled to consume the exceeding power. A system which employs ac-load voltage control is presented in Fig. 3. In that case, the ac controllable load consists of a three-phase delta-connected resistive load, where each sample is applied as the branch is SCR controlled. The

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Fig. 3. Induction generator system configuration using ac load control.

feedback signal for a proportional–integral (PI) control that produces the SCR drive reference. The ac controllable load can be further used in other applications, like pumping water to reservoirs or in irrigation systems. The main advantage of the ac-load control over the dc one is that the former avoids the flow of active power current through the VS-PWM converter. Consequently, the VS-PWM inverter rated power can be decreased. III. INDUCTION MACHINE MODEL AND PARAMETERS In the analysis carried out in this paper, the machine is represented by a three-phase model which expresses the rotor quantities referred to a reference frame along the magnetic stationary axes of the stator windings, henceforth called frame. The complete fundamentals and equations of this model are presented in [7] and [8]. The main advantages of this representation are the following: 1) equivalent circuit with the same number of terminals as the machine; 2) simulation using programs which employ circuit representation; and 3) stator transformation. variables are not affected by the The following assumptions are made for the determination stationary frame machine model: 1) rotor winding of the variables are referred to the stator; 2) machine is not saturated and MMF is harmonic free; and 3) iron losses are neglected. stationary reference axes are, respectively, coinThe cident with the magnetic axes of the abc phase windings of the stator. The reference frame transformation is obtained from (2)

(3)

(4)

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999

TABLE I PARAMETERS OF THE INDUCTION MACHINE AT 60 Hz

where and

and are variables in the stationary reference frames, respectively. is the to reference frames transformation matrix which may be divided into a matrix for stator transand another one for the rotor transformation formation , as presented by (3) and (4), shown at the bottom of the is the time-dependent rotor angular previous page, where position in electrical radians. The stator quantities are not affected by the frame transformation. As a result, the notation for stator variables is kept as reference frame. originally used for the The application of (2)–(4) to write the machine mathematical model yields

Fig. 4. (a) voltage.

VDC

voltage, (b) inverter line voltage, and (c) the

SDC

drive

(5) (6) (7) (8) (9) (10) subscript indicates stator variables and The subscript indicates rotor variables, both referred to the stationary frame. and are, respectively, the stator is the linkage and rotor windings leakage inductances. is the air-gap torque. inductance between stator and rotor. and are stator voltage and current vectors, is the rotor current vector. and respectively. are the stator and rotor windings magnetic flux vectors, respectively. and are the respective stator and rotor windvector appears as a consequence ings resistance. The to frame transformation. is the electrical of the angular speed of the rotor. The mechanical system equation is

(11)

where is the IM number of poles, is the viscous friction constant, and

is the rotor inertia, is the load torque.

Fig. 5. (a) VDC voltage, (b) load line current, and (c) the SDC drive voltage.

The stationary frame induction machine model is governed by (5)–(11). IV. SIMULATION RESULTS The IG-based system employing dc-load voltage control, stationary presented in Fig. 1, was simulated based on the frame model for the IM representation. The PSpice program was the software tool utilized for the simulations. The IM parameters are presented in Table I, and the further F, values of the other components are F, mH, and . . Fig. 4(a) shows the dc-load voltage control effect on sample with the dc The hysteretical comparison of the drive voltage [Fig. 4(c)]. voltage reference yields the Under simulation circumstances, the hysteresis control setup to vary from 325 to 330 V. limits allowed Fig. 5 presents the simulation of a transient load connection, the power of which corresponds to 90% of the IG rated power. The system startup took place from 0 to 50 ms. Thereupon, a three-phase resistive bank is connected at 80 ms and held on until 150 ms. During the interval, the load is kept connected, rate of increase [Fig. 5(a)] is low and did not the need to be closed [Fig. 5(c)]. After the load disconnection, the load line current extinguishes [Fig. 5(b)], resulting in storage , hence increasing . Therefore, of energy in switching frequency [Fig. 5(c)] increases, aiming to maintain in the allowed variation range.

MARRA AND POMILIO: SELF-EXCITED IG CONTROLLED BY A VS-PWM BIDIRECTIONAL CONVERTER

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Fig. 8. (a) Line currents at the IG terminals, (b) at the VS-PWM inverter terminals, and (c) at the ac load. Vert.: 2 A/div. Horiz.: 10 ms/div. Fig. 6. (a) Line voltages at the VS-PWM inverter terminals and (b) at the IG leads. Vert.: 250 V/div. Horiz.: 2 ms/div.

Fig. 7. (a) Line currents at the IG terminals, (b) at the VS-PWM inverter terminals, and (c) at the ac load. Vert.: 2 A/div. Horiz.: 10 ms/div.

V. EXPERIMENTAL RESULTS Experimental results were obtained from the system presented in Fig. 1 using an IM, the parameters of which are shown in Table I. A commercially available insulated-gatebipolar-transistor (IGBT)-based inverter, switched at 5 kHz using the vector modulation technique, was employed as the bidirectional VS-PWM inverter. The other additional components have the same values as those used for simulation. The IG startup was obtained through the self-excitation promoted by the interaction between the residual rotor flux and capacitor excitation bank. During the IG excitation, the capacitor is charged with energy provided by the IG. the After the self-excitation process, both IG terminal voltage and were suitably established. Fig. 6(a) presents the line voltages at the VS-PWM output terminals, and Fig. 6(b) presents the line voltages at the IG. It is noticeable that the generator voltage is sinusoidal due to the filter action. Thus, the inverter switching-frequency voltage components are not present at the IG leads. The line currents at the IG terminals [Figs. 7(a) and 8(a)], at the VS-PWM inverter output [Figs. 7(b) and 8(b)], and at the

ac load [Figs. 7(c) and 8(c)] are presented in cases where the ac load power is 90% of the IG rated power (Fig. 7) and when no ac load is connected (Fig. 8). In both mentioned cases, the IG line current is not affected by the ac load power during the steady-state operation. A comparison between load and no-load circumstances (Figs. 7 and 8) shows that the ac-load disconnection causes a significant increase at the VS-PWM inverter line current, as . a result of the increase in the exceeding power sent to The IG terminal line current [Figs. 7(a) and 8(a)] consists of the VS-PWM inverter line current [Figs. 7(b) and 8(b)], the ac-load line current [Figs. 7(c) and 8(c)], and the IG line excitation current. In the present case, the IG excitation current is as significant as the active load line current, due to the low IM rated power. The IG excitation current is expected to be less significant as the system rated power increases. The high-frequency oscillations observed in the inverter line current are caused by the PWM switching. These highand , so that they are frequency currents flow through not present in the IG stator windings. The system configuration based on the ac-load voltage control was also realized, employing a delta-connected SCRcontrolled resistive load, as indicated in Fig. 3. A sequence of connections of three stages of deltaconnected bulbs, which makes the ac load vary from no load to IG rated power, is exhibited in Fig. 9. It is observable that, as the ac-load line current increases [Fig. 9(c)], the SCR firing angle reference also increases [Fig. 9(a)], as a result of the PI controller action; hence, the ac controllable load phase current [Fig. 9(b)] decreases. Therefore, the SCR-controlled load consumes the exceeding power derived from the difference between the IG power and the ac load power. A startup of an induction motor rated at 67% of the IG rated power was executed by directly connecting the motor to the IG leads. Both ac- and dc-load control presented the same performance in the accomplishment of the motor starting. voltage, Fig. 10(b) the IG terminal Fig. 10(a) presents line voltage, and Fig. 10(c) the induction motor line current during the startup.

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999

Fig. 9. (a) SCR firing angle reference voltage (5 V/div), (b) phase current through a branch of the delta-connected ac SCR controlled load (1 A/div), and (c) ac load line current (1 A/div). Horiz.: 100 ms/div.

Fig. 10. (a) VDC voltage (100 V/div), (b) IG terminal line voltage (250 V/div), and (c) induction motor line current (5 A/div). Horiz.: 50 ms/div.

Note that the IG line voltage follows the same profile as voltage. Part of the energy stored in is employed in the induction motor acceleration, causing a voltage sag in and, consequently, at the IG terminals. Thus, the power of the induction motors to be started by the system, as well as the maximum allowed voltage sag, are essential guidelines capacitance. to assess also Besides supplying energy to the motor inertia, compensates for the motor magnetizing current and the ac-load is rated to reactive power requirements, considering that match only the IG excitation requirements. The voltage regulation steady-state profiles for resistive and inductive loads are shown in Fig. 11. The system was analyzed while it was feeding both resistive and inductive ac loads, and satisfactory voltage regulations of 1.8% for resistive, and 2.2% for reactive loads, were observed. An even better voltage regulation is expected for the system which employs ac-load voltage control, as there is no active power flow to the converter dc side, leading to lower voltage drop at the filter inductance.

Fig. 11. Voltage regulation for inductive and resistive loads.

In case the voltage at the generator leads was sensed, the PWM modulation index could be adjusted by closed-loop control to guarantee constant voltage at the load. Meanwhile, the resistive and inductive loads were fed; they were first balanced and subsequently unbalanced, maintaining the same power. In fact, no influence was noticed in the IG terminal voltage and frequency when feeding unbalanced loads with relation to balanced ones. In addition, the system proved to be able to feed three-phase diode rectifiers. The harmonic current components produced by the rectifier flow through three possible paths, namely, the bank, the VS-PWM inverter passing through , and the IG stator windings. The way the harmonic currents are distributed among these paths depends on the impedance they represent at each harmonic frequency. While stable when feeding the diode rectifier, the IG terminal voltage waveform was slightly distorted due to the circulation of part of the harmonic currents present in the diode line current, through the IG stator windings.

VI. CONCLUSIONS This paper has presented a cage-rotor IG system implementation, completely isolated from the utility grid, intended to supply rural sites or isolated areas where there are proper water resources available. The system was demonstrated to be able to feed resistive and inductive, balanced and unbalanced loads with regulated voltage, constant frequency, and satisfactory energy quality. The absence of a mechanical speed governor together with the control simplicity, which needs to sample only the voltage, and the self-excited system startup are remarkable features that makes the system cost effective, robust, and reliable. The IG is the system’s conversion device, while the VSPWM inverter is responsible for keeping constant frequency at IG leads, compensating for the ac-load reactive power requisites, and guaranteeing the IG voltage regulation. The system robustness was certified by the direct startup of a three-phase induction motor rated at 67% of the IG power.

MARRA AND POMILIO: SELF-EXCITED IG CONTROLLED BY A VS-PWM BIDIRECTIONAL CONVERTER

Both ac- and dc-load control of were demonstrated to be effective means of maintaining the IG voltage regulation. proved to be a faithful parameter to indicate Moreover, the system power balance state. as a feedback Alternatively, it is possible to employ signal to control the prime mover power, such as in a dieselmotor-driven system. ACKNOWLEDGMENT The authors wish to thank R. M. M. Martinez for his help with the simulations. REFERENCES [1] D. E. Basset 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] J. Arrilaga and D. B. Watson, “Static power conversion from selfexcited induction generators,” Proc. Inst. Elect. Eng., vol. 125, no. 8, pp. 743–746, 1978. [4] M. G. Simoes, B. K. Bose, and R. J. Spiegel, “Fuzzy logic based intelligent control of a variable speed cage machine wind generation system,” in Proc. PESC’95, 1995, pp. 389–395. [5] C. B. Jacobina, E. R. C. Silva, A. M. N. Lima, and R. L. A. Ribeiro, “Induction generator static systems with a reduced number of components,” in Proc. 31st IEEE-IAS Annu. Meeting, San Diego, CA, 1996, pp. 432–439. [6] E. G. Marra and J. A. Pomilio, “Self-excited induction generator controlled by a VS-PWM converter providing high power-factor current to a single-phase grid,” in Proc. IECON’98, Aachen, Germany, 1998, pp. 703–708. [7] R. Szcesny and M. A. Ronkowski, “New equivalent circuit approach to simulation of converter–induction machine associations,” in Proc. EPE’91, Florence, Italy, 1991, pp. 4/356–4/361.

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[8] R. M. M. Martinez, “A contribution to the study and design of a threelevel voltage source three-phase inverter,” M.Sc. thesis, School Elect. Comput. Eng., State Univ. Campinas, Campinas, Brazil, Aug. 1997.

Enes Gon¸calves Marra (S’96) was born in Brazil in 1966. He received the B.S. and M.S. degrees from the Federal University of Uberlˆandia, Uberlˆandia, Brazil, in 1989 and 1993, respectively. He is currently working towards the Ph.D. degree at the State University of Campinas, Campinas, Brazil. Since 1993, he has been a Lecturer in the School of Electrical Engineering, Federal University of Goi´as, Goiˆania, Brazil. His research interests include electrical drives, power electronics applications, and renewable energy sources.

Jos´e 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.