IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 4, OCTOBER 2008 2207

STATCOM Controls for a Self-Excited InductionGenerator Feeding Random Loads

Woei-Luen Chen, Member, IEEE, Yung-Hsiang Lin, Hrong-Sheng Gau, and Chia-Hung Yu

Abstract—Voltage and frequency fluctuations due to randomload variation are the most important power-quality problem ina self-excited induction generator (SEIG) system. Traditionally,instantaneous voltage regulation can be achieved using staticsynchronous compensators (STATCOMs) to condition the reac-tive power flow in a power distribution system. However, theirwidespread use might be limited by temporary loss of synchro-nization owing to system frequency fluctuations. In this paper, anadvanced, laboratory STATCOM was constructed based on therelative rotation speed theory, which is essential in estimating thefrequency deviation and assisting the STATCOM in synchronismwith system frequency. As a result, the stator voltage fluctuationdue to random load variations can be eliminated and effectiveregulations in generator speed and mechanical power are thusable to be guaranteed. Both simulation and experimental resultsare in agreement with the proposed controller, leading to theconclusion that simultaneous voltage and frequency control forSEIG feeding the balanced three-phase loads is feasible.

Index Terms—Frequency control, self-excited induction genera-tors (SEIG), static synchronous compensator (STATCOM), voltagecontrol, voltage-sourced inverter (VSI).

NOMENCLATURE

A. General

Voltage in instantaneous, magnitude, and vectornotation.Inverter output voltage in instantaneous notation.

Current in instantaneous notation.

Relative angular displacement for a specific vector,where .Absolute angular displacement for a specific vector.

Angular frequency (angular speed).

Flux linkage.

Reactance and resistance.

Input mechanical power and output active powerfor the IG.Electrical and mechanical torques.

State vector.

Manuscript received April 30, 2007; revised October 22, 2007. First pub-lished May 7, 2008; current version published September 24, 2008. This workwas supported in part by the National Science Council of the R.O.C. under Con-tract NSC-96-2622-E-182-014-CC3 and in part by the Green Technology Re-search Center of Chang Gung University. Paper no. TPWRD-00234-2007.

The authors are with Chang Gung University, Tao-Yuan 333, Taiwan, R.O.C.Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPWRD.2008.923160

Output vector.

Input vector.

State and output feedback gain matrices.

B. Subscripts

Quantities in -axis and -axis.

Stator and rotor.

DC link.

Load.

Base quantity.

Fixed shunt capacitor.

Coupling transformer and filter for the STATCOM.

Vector or matrix for the augmented system.

0 Initial operating point.

Reference value.

C. Superscripts

Matrix transpose operator.

I. INTRODUCTION

T HE cage-rotor induction machine can be employed as ei-ther a motor or a generator in industry applications be-

cause of its simplicity, ruggedness, reliability, efficiency and lowcost advantages. As a grid-connected generator, the lagging vardemand of the IG is supplied by the grid utility and hence, thestator frequency is dominated by the grid frequency, irrespectiveof the rotor speed. This makes it suitable for variable speed ap-plications such as low-head hydro and wind energy conversionsystems.

The rotating IG may remain excited even after the line voltageis disconnected. The self-excitation process is initiated while theelectromotive force (emf) in the stator coil, related to the air-gapflux and rotor speed, is induced by the residual magnetism of thespinning rotor. By acting on the induced emf across the externalcapacitors, the magnetizing current is created and thus enhancesthe air-gap flux. To maintain the excitation at specific statorvoltage, a moderate amount of capacitance must be controlledto accommodate the various load conditions. Since the statoremf is induced by the relative motion of the air-gap flux waveand the stator coil, the stator circuit frequency is proportional tothe rotor speed. Therefore, unlike the grid-connected IG whosestator frequency is synchronized with that of the utility grid, the

0885-8977/$25.00 © 2008 IEEE

2208 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 4, OCTOBER 2008

rotor speed needs to be controlled to maintain the excitation ata specific stator frequency for a self-excited IG (SEIG). Withvoltage-frequency control feasibility, IGs are being consideredas an alternative choice to the well-developed synchronous gen-erator. The efforts toward simultaneous SEIG voltage and fre-quency control were presented in [1]–[6].

An electric load governor (ELG) was proposed in [1]–[4],using an impedance controller at the generator terminals to con-sume the surplus power in the generation system so that theload impedance seen by the IG at its terminals is always thesame. The voltage and frequency at the generator terminal willthen be constant as well. In another type of ELG approach,based on the steady-state behavior of the slip-ring IG [5], con-stant stator voltage and frequency over a wide range of speedswas achieved using rotor resistance control. Only a small tol-erable error in frequency and voltage was observed following aload change. Although the ELG can provide good stator voltageand frequency regulation, a mechanical speed governor is stillnecessary in practical applications in increasing load situations.Moreover, to attain constant stator voltage and frequency overa wide range of speeds and loads, the installed ELG capacity iscomparable to that of the IG, thus confining the generator ca-pacity to a relatively low rating. In addition, the ELG is not ap-plicable to fuel-engine-driven IG system from the economicalviewpoint.

An alternative way of attaining frequency control is using aspeed governor to control the amount of power generation [6].A voltage-sourced inverter has been employed to regulate thestator voltage and implement the frequency control strategy. DCcapacitor voltage at the inverter dc side, which is an indicator ofsystem power balance, was employed as the control variable forthe speed governor under stand-alone operation. This structureis more suitable for application in fuel-engine-driven IG sys-tems.

Another important issue for the SEIG is that undesiredself-excitation may cause severe over-voltage and therebystress the machine insulation [7]. To avoid undesired self-ex-citation and achieve continuous voltage regulation, staticsynchronous compensators (STATCOMs) have been used,given in literatures [8]–[12]. This also seems to be a goodchoice for the SEIG. However, the STATCOM may suffer fromtemporary loss of synchronization because the severe frequencyfluctuation caused by the random load variation in the SEIGsystem would perturb the phase-locked loop (PLL) circuit usedto provide the synchronizing signal in [8]–[12]. In this work,the advanced STATCOM has one major advantage over thetraditional STATCOM in that it is almost totally immune tofrequency fluctuations due to real-time frequency deviationmeasurement using the proposed relative rotation speed theory.

This investigation enables STATCOM operation in a fre-quency-perturbed system and proposes a simultaneous voltageand frequency controller for the SEIG system in standby powersource applications such as diesel-engine-driven generatorsystems. The presented controller is validated by experimentalresults.

II. SYSTEM MODELS

Fig. 1 depicts a one-line diagram of an SEIG system in whicha voltage-frequency controller is utilized to regulate the me-

chanical power and the reactive power through a speed governorand an advanced STATCOM. The steady-state reactive powerrequirement for the isolated IG is supplied mainly by a fixedshunt capacitor bank.

Since the system frequency and the amount of reactive powerrequired for excitation varies with the generator speed, the ad-vanced STATCOM, which allows a wide range of system fre-quencies is proposed to achieve continuous voltage regulationunder various load conditions for an SEIG system.

A. Induction Generator Model

The per unit flux-linkages for the stator and rotor circuits ofthe induction generator described in - and -axes are as follows[13]–[15]:

(1)

(2)

(3)

(4)

where a synchronous reference frame rotating at an electricalangular speed corresponding to the predefined reference frame,herein denoted as , is adopted.

The electromechanical torque in per unit can be written interms of stator flux linkages and currents as

(5)

The corresponding torque balance equation is given by

(6)

where is the per unit mechanical torque, and and arethe equivalent inertia constant and the equivalent damping con-stant of the isolated induction generator system, respectively.

B. STATCOM Model

For a balanced three-phase system, the STATCOM model canbe described in per unit using the variables in - and -axessynchronous reference frame as [10], [11]

(7)

(8)

In Fig. 1, the instantaneous powers at the ac- and dc-sides of thevoltage-sourced inverter are equal, giving the following powerbalance equation:

(9)

The per unit dc-side circuit equation is

(10)

where is used to represent the inverter switching loss.

CHEN et al.: STATCOM CONTROLS FOR A SELF-EXCITED INDUCTION GENERATOR FEEDING RANDOM LOADS 2209

Fig. 1. System configuration.

The per unit equations of the fixed shunt capacitor and localload are given in the Appendix.

III. STEADY STATE ANALYSIS OF AN SEIG WITH CONSTANT

STATOR VOLTAGE

For a rotating machine, in order that an electric current be pro-duced in the stator conductors, an electric field must be built init. The mechanical power can be therefore converted into elec-trical power while the electric field is elevated using machine ex-citation. The isolated IG differs greatly from the grid-connectedIG in steady-state characteristic due to the diverse machine ex-citation processes. For a grid-connected IG, a synchronous ro-tating field will be produced by the stator winding excited fromthe grid utility, thus rotor current is then induced at the slip fre-quency due to the relative motion of the stator flux and the rotorconductor. When the rotor is driven to a speed higher than that ofthe stator rotating field, the mechanical power can be transferredacross the air gap to the stator in electrical form. In contrast tothe grid-connected IG, the isolated IG excited from the residualmagnetism of the iron core will induce a rotating field in thestator windings at a frequency that depends on the rotor speedand stator voltage. This has led to an interaction problem of greatcomplexity between the active power and reactive power whilestudying the steady-state performance of the isolated IG. Oneeffective and reasonable way to simplify the steady-state anal-ysis is to decouple the active power and reactive power throughthe constant stator voltage assumption the STATCOM employedin this system is capable of dynamic voltage regulation.

Figs. 2 and 3 present the steady-state analysis results for theconstant-voltage IG. As shown in Fig. 2, the generator speed

is proportional to the stator frequency under a specific load.Thus, stator frequency can be regulated through IG speed con-trol when the stator voltage is kept constant by the STATCOMunder varying load conditions. Since the stator voltage is fixedthe active power generation is almost irrespective of the statorfrequency except for a small increment providing the losses forthe STATCOM and the IG, as shown in Fig. 3(a). It is also ob-served from Fig. 3(b) that the greater the isolated IG load, themore reactive power consumption is necessary for the isolatedIG. In addition, for a specific load condition ( p.u.),the reactive power consumption of the isolated IG depends onthe stator frequency, as illustrated in Fig. 3(b). In this work, it isevident that for the SEIG with constant stator voltage,

Fig. 2. Dependence of generator speed of an SEIG on active output power withstator frequency as a parameter.

Fig. 3. Dependence of (a) active power and (b) reactive power of an SEIG onstator frequency with terminal load, r , as a parameter.

the shunt magnetizing reactance would consume more reac-tive power than the series leakage reactance over the lower fre-quency, whereas the series leakage reactance would dominate itwhen the stator frequency is greater than 62 Hz.

IV. CONTROL STRATEGIES BASED ON RELATIVE ROTATION

SPEED THEORY

Apart from the reactive power compensation, the advancedSTATCOM, employing the relative rotation speed theory, may

2210 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 4, OCTOBER 2008

Fig. 4. Geometric representation of the relative rotation speed theory.

be used for the purpose of providing the voltage-frequency de-coupled control to the isolated IG system. Before explaining therelative rotation speed theory at the beginning of this section, aclear distinction is made between the “relative angular displace-ment” and “absolute angular displacement.” Fig. 4 illustratesthat the angle, , is the absolute angular displacement be-tween the rotating -axis and stationary frame. It is a function ofthe angular speed, , of the rotating axes and the ini-tial value . The angle of the vector relative to -axisherein denotes as the relative angular displacement . Thus the

components for the vector can be expressed as

(11)

(12)

The quantities as given by (11) and (12) contain sinusoidalfunctions of the angular difference between the load bus voltageand -axis.

A. Relative Rotation Speed Theory

For a grid-connected IG, the angular speed of the load busvoltage is identical to that of the -axis. Therefore, the axisand axis components of ( and ) remain steady asthe angular difference between and is steady. How-ever, if the IG loses its connection to the grid, the frequency ofthe load bus will deviate from the -axis. Thus, a time-varyingangular difference between and will be observed asfollows:

(13)

The rate of change in the relative angular displacement asdescribed in (13) yields a relative rotation speed

and may be employed to measure the extent offrequency deviation between two independent power systems,such as a grid-disconnected IG and a grid. This theory can beapplied to force an IG to be operated at a specific frequencyduring islanding condition and also to reach in-phase controlbetween the generator output voltage and grid voltage when re-closing operation is necessary.

Fig. 5. Block diagram of the proposed constant frequency controller.

B. Frequency Controller

The rate of change in the relative angular displacementas described in (13) may be employed to measure the extent offrequency deviation between the stator frequency of theIG and the predefined synchronous frequency ), as set bythe frequency controller.

The relative rotation speed theory leads directly to a rule thatwill provide effective frequency control by a PI controller, asdepicted in Fig. 5. An increment in torque command, ,generated by the frequency controller, will increase the rotorspeed if the stator frequency is smaller than the predefinedsynchronous frequency ).

C. Voltage Controller

Since the system frequency of the isolated IG may deviatefrom the synchronous frequency, the advanced STATCOMshould be designed to stay in synchronous operation with theIG system under all operating conditions. The measurementof the relative angular displacement is effective for theadvanced STATCOM in capturing synchronism at various loadconditions. In addition, the relative angular displacementalso plays an important role to decouple the instantaneousactive power and reactive power control while axis is notcoincident with the load voltage vector as shown in Fig. 4.

Thus, the instantaneous active and reactive power, through acoupling path to the STATCOM, at the load bus can be repre-sented as follows:

(14a)

(14b)

To decouple the STATCOM active and reactive power controlloops, a new reference frame which uses the vector for loadbus voltage as the new -axis (denoted as -axis) is de-fined in Fig. 4. Using the quantities (which is equalto , and in the new reference frame, the active andreactive powers in (14) can be written as

(15a)

(15b)

As (15) shows, the -axis current component, , accounts forthe instantaneous active power and the -axis current compo-nent, , is the instantaneous reactive current. Thus, STATCOMcontrol design is simplified to a great extent with this new ref-erence frame since the load bus voltage regulation is achievablethrough the control of the reactive power which only relatedto the - axis current .

CHEN et al.: STATCOM CONTROLS FOR A SELF-EXCITED INDUCTION GENERATOR FEEDING RANDOM LOADS 2211

To ensure zero steady-state errors for load bus voltageand DC capacitor voltage , two integral terms and

are required in addition to the system state variables.Furthermore, the operating point of the IG changes with windspeed and load and various reference values for andmay be required by the STATCOM to achieve good regulation ofload bus voltage and dc capacitor voltage . With the col-lection of four integral terms ,and as the additional state vector ( , the aug-mented state equations can be represented as follows:

(16)

where (see equation at the bottom of the page).Thus, the controlinputs for the state equations in (16) are

(17)

Note that the mechanical power can be conditioned using a smallsignal torque command in (17) through a controller withIG speed error as the primary stabilizing signal in theoutput vector in (16).

For the linear system described in (16), the linear quadraticstate feedback control which minimizes the performanceindex

(18)

where Q is the weighting matrix of the state variable variationsand R of the control effort, is given by [16]

(19)

To determine the state feedback gain matrix for thelinear quadratic control in (19), a set of weights werechosen for the weighting matrix Q in (18) based on phys-ical reasoning and past experience. In this work, a diagonalweighting matrix with the diagonal elements 10, 0, 0, 1,100, 100, 1, and 1 corresponding to the state variables

, and, respectively, are selected as the initial weighting

matrix.The output feedback control is then given by

(20)

where the output feedback gain matrix can be derived usingand through the pseudo-inverse [14], [17] technique as

follows:

(21)

TABLE ISUMMARY OF SYSTEM EIGENVALUES

After the control design, the open-loop and the closed-loopeigenvalues for the isolated IG system are given inTable I. It is clear that a poorly damped STATCOM mode

presented in the open-loop response can beshifted by an output feedback controller to the new locations

. Moreover, a moderately damped eigen-value pair corresponding to the closed-loopelectromechanical dynamics can be also reached using theproposed controller. Fig. 6 gives the root loci for the electro-mechanical mode and STATCOM current mode as the loadpower factor is varied from 0.1 lagging to 0.98 lagging. As theillustrations show, all eigenvalues are moving to left with theincrease in power factor. In addition, to ensure that load powervariations would not adversely affect the closed-loop systemdynamics, additional root loci over the load power range of

to 1.0 p.u. are performed in Fig. 6.

V. EXPERIMENTAL IMPLEMENTATION

Fig. 7 schematically shows the architecture of the proposedvoltage-frequency controller for an SEIG. To investigate the dy-namic performance of the IG stator voltage and frequency, avariable load is directly connected to the isolated IG. The ad-vanced STATCOM, implemented by an insulated-gate-bipolar-transistor (IGBT)-based inverter, was coupled to the isolated IGthrough an output filter for bus voltage regulation. A harmonicelimination pulse-width-modulation (HEPWM) technique wasdesigned to eliminate the low order harmonics such as the order

( of the fundamental frequency. Therequired switching patterns for the HEPWM inverter were pre-programmed according to various modulation indexes. Noticethat the prime mover, comprising an ac servomotor and a speedgovernor, can be used to reduce the rotor speed as the stator

2212 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 4, OCTOBER 2008

Fig. 6. Root loci of (a) electromechanical mode and (b) STATCOM currentmode for various combinations of load power and power factor.

Fig. 7. Scheme of the experimental bench.

voltage phase leads the reference signal (a positive relative ro-tation speed). The IG stator voltage, rotation speed, inverter dcvoltage and ac current were simultaneously sampled using amultichannel data acquisition system with 12-bit A/D resolu-tion, and all the control and signal processing arithmetic wereperformed on a PC platform. The digital interface circuits such

Fig. 8. Dynamic responses of the isolated IG system following a sudden re-moval of load (from 0.386 pu, pf = 0:798 lagging to 0.0 pu).

as modulo-k counter (for generating time reference), lock-outtime circuit (for inverter blanking time control) and inverter dis-able circuit (for isolating inverter from power system) were im-plemented in a complex programmable logic device (CPLD)through the hardware description language. The HCPL4503 isa diode-transistor photocoupler which provides electrical isola-tion between the digital control system and the inverter powersystem.

VI. RESULTS AND DISCUSSION

A. Dynamic Responses of the Isolated IG Following a StepChange in Load Demand

The load variation effects on dynamic performance were ex-amined using the simulation and an experimental bench. Thefirst disturbance considered was a step removal in load demandthat took place at s. While removing the load demand,the accumulated kinetic energy accelerated the IG to a higherrotation speed, causing the stator frequency to increase imme-diately after the load removal, as shown in Fig. 8(d). However,this increase in stator frequency was soon detected by the fre-quency controller through the rate in change (of relative angular displacement , as depicted in Fig. 8(e). Atorque command was then generated by the frequency controllerto reduce the mechanical power input to the IG until the statorvoltage phase returned to a stable values (

and the stator frequency returned to the preset reference.Fig. 8(c) shows that after the step removal in load demand, thesurplus reactive power in IG system would be absorbed by theadvanced STATCOM through rapid response in inductive cur-rent . The result in Fig. 6 show the stator voltage supported bythe advanced STATCOM settled to the reference value in threecycles after the sudden change in load demand. The DC capac-itor voltage was also well regulated by the proposed controller.

The second disturbance considered was a step increase in loaddemand that took place at s. Fig. 9(a) and (b) showthat the stator voltage dropped rapidly after the sudden increasein load demand until the inductive current was controlledtoward capacitive region. An observation of the response curvein Fig. 9(f) indicates that the rotor would decelerate as result of

CHEN et al.: STATCOM CONTROLS FOR A SELF-EXCITED INDUCTION GENERATOR FEEDING RANDOM LOADS 2213

Fig. 9. Dynamic responses of the isolated IG system following a step increasein load demand (from 0.386 pu, pf = 0:798 lagging to 0.535 pu, pf = 0:807

lagging).

Fig. 10. Transient performances of the isolated IG system. (ch1: stator voltage,ch2: STATCOM dc voltage, and ch3: load current).

part of kinetic energy was used to make up the additional loadpower. It is also observed from Fig. 9(e) that the relative angulardisplacement can be stabilized to new equilibrium point by thecontrol of the speed governor. Thus, the constant frequency canbe reached since there is no relative rotation speed between thesystem frequency and the preset reference frequency (the ratechange of the relative angular displacement (is close to zero).

Fig. 10 shows the transient responses of the stator voltage andinverter dc bus voltage at the sudden increase in load demand.

Fig. 11. Dynamic responses of the isolated IG system following a step changein stator frequency setting.

It is clear that the stator voltage dropped instantaneously butsettled to the reference value within 3 cycles.

As the illustration shows, it is clear that the frequency fluc-tuation due to various load variations has no apparent effect onthe dynamic performance of the advanced STATCOM since itsoutput frequency can be dynamically controlled to synchronizewith the system frequency. A close agreement between simu-lation and experimental results is observed, thus validating thatthe simultaneous voltage and frequency controller is effectivefor an SEIG system.

B. Performance Test for the Isolated IG Operated at DifferentStator Frequency Setting

Employing the proposed simultaneous voltage and frequencycontroller, the isolated IG can be controlled to operate at arbi-trary stator frequency setting under constant stator voltage. Toevaluate this, additional experiment was conducted through a10-Hz step change in stator frequency setting ( is reducedfrom 60 Hz to 50 Hz).

It is observed in Fig. 11 that, immediately following the stepchange, the frequency controller detected the sudden changein the relative angular displacement and generated a torquecommand to reduce the mechanical power input to the IG. Asa result, causing a decrease in the kinetic energy and rotation

2214 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 4, OCTOBER 2008

Fig. 12. Steady-state waveforms of the isolated IG system. (ch1: stator voltage,ch2: STATCOM dc voltage, and ch3: load current)

speed of the IG. In addition, the frequency-depended constantload impedance would consume more power from the isolatedIG while the system frequency was reduced. The increased loadpower and the decreased mechanical power during the tran-sient period would deteriorate the stator voltage response, asillustrated in Fig. 11(a). Meanwhile, starting with the reactivecurrent control, tight stator voltage regulation under the syn-chronous frequency of 50 Hz was achieved [Fig. 11(c)].

Fig. 12(a) and (b) depicts the steady-state stator voltage andcurrent waveforms recorded from the oscilloscope before andafter the step change in the reference frequency setting .Fig. 12 shows that the stator frequency follows the refer-ence frequency setting very closely in both cases. This ex-periment validated that both constant voltage and constant fre-quency control can be reached for the isolated IG under differentreference frequency setting.

VII. CONCLUSION

A method that enables STATCOM operation in a frequency-perturbed system was presented. An SEIG system feeding thebalanced three-phase loads with constant voltage and frequencyis thus promised. The major features of the proposed controllerare summarized as follows.

i) An important conclusion obtained from the steady-stateanalysis results for an SEIG is that the stator frequencyis proportional to the rotation speed at constant statorvoltage. The advantage is that, through the action of thespeed governor, the stator frequency can be precisely reg-ulated.

ii) The relative rotation speed theory leads directly to a rulethat will provide effective frequency control.

iii) The measurement of the relative angular displacementis effective for the advanced STATCOM in cap-

turing synchronism at various load conditions. As a re-sult, constant voltage can be attained by the advancedSTATCOM when the system frequency is apt to change.

iv) To make the controller more robust, the root locustechnique is employed to investigate closed-loop perfor-mances of the IG system under various load conditions.

v) Experimental results on a laboratory machine agree quitewell with the proposed theory and indicate that an SEIGis a viable option for interfacing the diesel engine systemto standby generation applications.

As feeding the balanced three-phase loads, such asthree-phase induction motors and three-phase rectifiers,constant voltage and frequency control for an SEIG systemby means of the proposed strategies is feasible. However, theissue about feeding unbalanced three-phase loads is needed tobe taken into consideration when designing the simultaneousvoltage-frequency controller for comprehensive applications.This issue needs more detailed studies and the results will bereported in a future publication.

APPENDIX

A. System Parameters

B. Fixed Shunt Capacitor Model

C. Load Model

CHEN et al.: STATCOM CONTROLS FOR A SELF-EXCITED INDUCTION GENERATOR FEEDING RANDOM LOADS 2215

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Woei-Luen Chen (S’02–M’05) received theM.S.E.E. and Ph.D. degrees from National TaiwanUniversity, Taipei, Taiwan.R.O.C., in 1997 and 2006,respectively.

From 1999 to 2002, he was an Electrical Engineerfor China Engineering Consultants, Inc. (CECI),working on the High Speed Rail Project. In 2001,he joined the faculty of the Electronic EngineeringDepartment, Hwa Hsia Institute of Technology, firstas an Instructor and later as an Assistant Professor.Since 2006, he has been with the Electrical En-

gineering Department, Chang Gung University, Tao-Yuan, Taiwan, R.O.C.,where he is currently an Assistant Professor. His research interests includepower-electronic applications, reactive power compensation systems, and windenergy conversion systems.

Yung-Hsiang Lin received the B.S.E.E. degree fromKun Shan University, Tainan, Taiwan, R.O.C., in2004 and is currently pursuing the M.Sc. degree inelectrical engineering at Chang Gung University,Tao-Yuan, Taiwan.

His areas of research include power-electronic ap-plications and wind energy conversion systems.

Hrong-Sheng Gau received the B.S.E.E. degreefrom National Taipei Institute of Technology, Taipei,Taiwan, R.O.C., in 1991 and is currently pursuingthe M.Sc. degree in electrical engineering at ChangGung University, Tao-Yuan, Taiwan.

From 1993 to 1995, he was an Electrical Engineerwith Chung-Hsin Electric and Machinery Manufac-turing Corporation, working on power-electronic cir-cuit design. In 1995, he joined the Nan Ya PlasticsCorporation as a Chief of the Department of Gen-eration working on cogeneration system design. His

main research interests are power system stability and reactive power compen-sation systems.

Chia-Hung Yu received the B.S.E.E. degree fromVanung University, Tao-Yuan, Taiwan, R.O.C., in2005 and is currently pursuing the M.Sc. degree inelectrical engineering at Chang Gung University,Tao-Yuan, Taiwan.

From 2000 to 2003, he was an Electrical Engineerwith Chunghwa Picture Tubes Corporation workingon power-electronic circuit design. In 2003, he joinedthe Chunghwa Telecom Corporation as a SeniorEngineer working on uninterruptible power-supplysystem design. His main research interests are

power–electronic applications, harmonics, and power quality.