Bhim Singh Sir

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006 1437

STATCOM-Based Voltage Regulator for Self-ExcitedInduction Generator Feeding Nonlinear Loads

Bhim Singh, Senior Member, IEEE, S. S. Murthy, Senior Member, IEEE, and Sushma Gupta

AbstractThis paper deals with the performance analysis ofa static compensator (STATCOM)-based voltage regulator forself-excited induction generators (SEIGs) supplying nonlinearloads. In practice, a number of loads are nonlinear in nature,and therefore, they inject harmonics in the generating systems.The SEIGs performance, being a weak isolated system, is verymuch affected by these harmonics. The additional drawbacks ofthe SEIG are poor voltage regulation and that it requires anadjustable reactive power source with varying loads to maintaina constant terminal voltage. A three-phase insulated-gate-bipolar-transistor-based current-controlled voltage source inverter work-ing as STATCOM is used for harmonic elimination, and it providesthe required reactive power for the SEIG, with varying loads tomaintain a constant terminal voltage. A dynamic model of theSEIGSTATCOM feeding nonlinear loads using stationary dqaxes reference frame is developed for predicting the behaviorof the system under transient conditions. The simulated resultsshow that SEIG terminal voltage is maintained constant, evenwith nonlinear balanced and unbalanced loads, and free fromharmonics using STATCOM-based voltage regulator.

Index TermsHarmonic elimination, load balancing, nonlinearloads, self-excited induction generator (SEIG), static compensator(STATCOM).

I. INTRODUCTION

IN REMOTE areas, plenty of nonconventional energy

sources are available. These nonconventional energy

sources are identified as potential prime movers for the gener-

ating systems. An externally driven induction machine operates

as a self-excited induction generator (SEIG), with its excitation

requirements being met by a capacitor bank connected across

its terminals. The SEIG has advantages like simplicity, main-

tenance free, absence of dc, brushless, etc., as compared to the

conventional synchronous generator.

Considerable reported literature exists on steady state and

transient analysis of SEIG under balanced/unbalanced resistive,

reactive, and motor loads. In [1][3], d

q axes modeling is

reported for the transient analysis of SEIG. Wang and Deng[4] have presented the transient performance of the SEIG under

unbalanced excitation system. Jain et al. [5] have given a

generalized model for the transient analysis of SEIG under

symmetrical and unsymmetrical conditions.

Manuscript received November 25, 2004; revised April 26, 2005. Abstractpublished on the Internet July 14, 2006. Presented and published in IECON2003 USA, pp. 709714, Nov. 24, 2003.

The authors are with the Department of Electrical Engineering, Indian In-stitute of Technology, New Delhi 110016, India (e-mail: [email protected];[email protected]; [email protected]).

Digital Object Identifier 10.1109/TIE.2006.882008

A major disadvantage of SEIG is its poor voltage regulation

requiring a variable capacitance bank to maintain a constant

terminal voltage under varying loads. Attempts have been made

to maintain a constant terminal voltage by fixed capacitor and

thyristor-controlled inductor (SVC) [6], saturable-core reactor

[7], and short-shunt connections of capacitors [8]. However,

voltage regulation provided by these schemes is of a discrete

type and injects harmonics in the generating system. However,

due to the invention of solid-state self-commutating devices, it

is now possible to make a static noiseless voltage regulator,

which can provide a continuously variable reactive power to the

SEIG with a varying load to keep the terminal voltage constant.

This system, called a static compensator (STATCOM), has spe-

cific benefits compared to SVC [9]. Schauder and Mehta [10]

have derived governing equations of the STATCOM to deter-

mine the response of the STATCOM.

Singh and Shilpakar [11] have proposed an analysis of

a solid-state voltage regulator for SEIG with a static load.

Wekhande and Agrawal [12] have proposed a controller to reg-

ulate the three-phase ac output voltage of the SEIG with vary-

ing rotor speed, transient load conditions, and reactive loads.

Miranda et al. [13] have proposed a static volt ampere reactive

(VAR) compensator for an electrical pumping system drivenby induction generator. Kuo and Wang [14], [15] have de-

scribed a method of voltage control of SEIG under unbalanced/

nonlinear load using a current-controlled voltage source

inverter (VSI).

However, due to extensive use of solid-state devices, energy

can be saved by employing adjustable speed drives used

in pump, compressor, air-conditioner, and other domestic

appliances such as TV, computer, switch-mode power supply

(SMPS), uninterruptible power supply (UPS), etc. Three-phase

and single-phase rectifiers are the front-end converter of the

aforementioned systems, which are nonlinear in nature. These

nonlinear loads draw nonsinusoidal currents (fundamentalalong with harmonics) from the generating system; therefore,

they inject the harmonics in the system. The SEIG is an isolated

system, which is small in size and where injected harmonics

may pollute the generated voltage. The STATCOM eliminates

the harmonics, provides load balancing, and supplies the

reactive power to the load and generator. In this paper, a simple

mathematical modeling is presented for the transient analysis

of the SEIGSTATCOM system under balanced/unbalanced

three-phase and single-phase nonlinear loads (uncontrolled and

controlled rectifiers), and the simulated results show that the

SEIGSTATCOM system behaves like an ideal supply under

these unbalanced nonlinear loads.

0278-0046/$20.00 2006 IEEE


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1438 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006

II. SYSTEM CONFIGURATION AND CONTROL SCHEME

The schematic diagrams of SEIG with excitation capacitor,STATCOM, load, and control scheme are shown in Fig. 1(a)and (b). Excitation capacitors are selected to generate the ratedvoltage of SEIG at no load. The additional demand of reactivepower is fulfilled using the STATCOM under varying loads.

The STATCOM acts as a source of lagging or leading currentto maintain a constant terminal voltage with varying loads.The STATCOM consists of a three-phase IGBT-based current-controlled VSI, dc bus capacitor, and ac inductors. The outputof the inverter is connected through the ac filtering inductors tothe SEIG terminals. The dc bus capacitor is used as an energystorage device and provides self-supporting dc bus.

The control scheme to regulate the terminal voltage of theSEIG is based on the control of source currents (which havetwo components in phase and quadrature with ac voltage). Thein-phase unit vectors (ua, ub, and uc) are three-phase sinusoidalfunctions, computed by dividing the ac voltages va, vb, and vcby their amplitude V

t. Another set of quadrature unit vectors

(wa, wb, and wc) is a sinusoidal function obtained from in-phase vectors (ua, ub, and uc). To regulate the ac terminalvoltage, the amplitude of the terminal voltage (Vt) is computedfrom sensed instantaneous ac terminal voltages (va, vb, andvc). This amplitude of ac voltage (Vt) is compared with thereference voltage (Vtref), and the voltage error is processedin the proportional integral (PI) controller. The output of thePI controller (Ismq) for ac voltage control loop decides theamplitude of reactive current to be generated by the STATCOM.Multiplication of quadrature unit vectors (wa, wb, and wc) withthe output of PI-based ac voltage controller (Ismq) yields thequadrature component of the reference source currents (isaq,

isbq, and iscq). To provide a self-supporting dc bus of STAT-COM, its dc bus voltage (Vdc) is sensed and compared with thedc reference voltage (Vdcref). The error voltage is processedin another PI controller. The output of the PI controller (Ismd)decides the amplitude of the active power component of thesource current. Multiplication of in-phase unit vectors (ua, ub,and uc) with output of PI controller (I

smd) yields the in-phase

component of the reference source currents (isad, isbd, and

iscd). The sum of the quadrature (isaq, i

sbq, and i

scq) and in-

phase (isad, isbd, and i

scd) current components is the reference

source currents (isa, isb, and i

sc), which are compared with the

sensed source line currents (isa, isb, and isc) in a pulsewidthmodulation (PWM) current controller to generate switching

signals for the devices of the STATCOM.Nonlinear loads draw nonsinusoidal currents (fundamental

as well as harmonics) due to which harmonics produced areinjected in the generating system, resulting in a distortion in theterminal voltage. Under unbalanced loading conditions, SEIGcurrents may be unbalanced (produce positive and negativesequences) due to which the machine is to be derated. TheSTATCOM is able to filter out the harmonics and balances theunbalanced load, resulting in balanced currents and voltages ofthe SEIG.

III. MODELING OF SEIGSTATCOM SYSTEM

Mathematical model of SEIGSTATCOM system consists ofthe modeling of SEIG and STATCOM.

A. Modeling of Control Scheme of STATCOM

Different components of SEIGSTATCOM system are

shown in Fig. 1(a), and its control scheme is shown in Fig. 1(b);

these are modeled as follows. Three-phase voltages at the SEIG

terminals (va, vb, and vc) are sensed, which are consideredsinusoidal; hence, their amplitude is computed as

Vt =

(2/3)

v2a + v2b + v

2c

1/2. (1)

The unit vectors in phase with va, vb, and vc are derived as

ua = va/Vt; ub = vb/Vt; uc = vc/Vt. (2)

The unit vectors in quadrature with va, vb, and vc may bederived using a quadrature transformation of the in-phase unit

vectors ua, ub, and uc [11] as

wa = ub/

3 + uc/

3 (3)

wb =3ua/2 + (ub uc)/23 (4)wc =

3ua/2 + (ub uc)/2

3. (5)

1) Quadrature Component of Reference Source Currents:

The ac voltage error Ver at the nth sampling instant is

Ver(n) = Vtref Vt(n) (6)where Vtref is the amplitude of the reference ac terminal voltageand Vt(n) is the amplitude of the sensed three-phase ac voltagesat the SEIG terminals at nth instant. The output of the PI con-troller (Ismq(n)) for maintaining ac terminal voltage constant atthe nth sampling instant is expressed as

Ismq(n) = Ismq(n1) + Kpa

Ver(n) Ver(n1)

+ KiaVer(n)

(7)

where Kpa and Kia are the proportional and integral gainconstants of the PI controller, Ver(n) and Ver(n1) are thevoltage errors in nth and (n 1)th instant, and Ismq(n1) is theamplitude of the quadrature component of the reference source

current at (n 1)th instant. The quadrature components of thereference source currents are estimated as

isaq = Ismqwa; i

sbq = I

smqwb; i

scq = I

smqwc. (8)

2) In-Phase Component of Reference Source Currents: Thedc bus voltage error Vdcer at nth sampling instant is

Vdcer(n) = Vdcref Vdc(n) (9)where Vdcref is the reference dc voltage and Vdc(n) is thesensed dc link voltage of the STATCOM. The output of the PI

controller for maintaining the dc bus voltage of the STATCOM

at the nth sampling instant is expressed as

Ismd(n) = Ismd(n1) + Kpd

Vdcer(n) Vdcer(n1)

+ KidVdcer(n). (10)

Ismd(n) is considered as the amplitude of the active powercomponent of the source current. Kpd and Kid are the


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SINGH et al.: STATCOM-BASED VOLTAGE REGULATOR FOR SEIG FEEDING NONLINEAR LOADS 1439

Fig. 1. (a) Schematic diagram of proposed scheme of SEIGSTATCOM system. (b) Control scheme of SEIGSTATCOM system.


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proportional and integral gain constants of the dc bus PI voltage

controller, respectively. In-phase components of the reference

source currents are estimated as

isad = Ismdua; i

sbd = I

smdub; i

scd = I

smduc. (11)

3) Total Reference Source Currents: Total reference source

currents are sum of the in-phase and quadrature components ofthe reference source currents as

isa = isaq + i

sad (12)

isb = isbq + i

sbd (13)

isc = iscq + i

scd. (14)

4) PWM Current Controller: The reference source currents

(isa, isb, and i

sc) are compared with the sensed source currents

(isa, isb, and isc). The ON/OFF switching patterns of the gatedrive signals to the IGBTs are generated from the PWM current

controller. The current errors are computed as

isaerr = isa isa; isberr = isb isb; iscerr = isc isc.

(15)

These current error signals are amplified and then compared

with the triangular carrier wave. If the amplified current error

corresponding to phase a (isaerr) signal is greater than thetriangular carrier wave signal, the switch S4 (lower device) of

the phase a leg of VSI is ON, switch S1 (upper device) of

the phase a leg of VSI is OFF, and the value of the switching

function SA is set to zero. If the amplified current error signal

corresponding to isaerr is less than the triangular carrier wavesignal, switch S1 is ON, switch S4 is OFF, and the value of SA

is set to one. Similar logic applies to other two phases b andc, respectively.

B. Modeling of STATCOM

The STATCOM is a current-controlled VSI and modeled as

follows. The derivative of its dc bus voltage is defined as

pvdc = (icaSA + icbSB + iccSC)/Cdc (16)

where SA, SB, and SC are the switching functions for the

ON/OFF positions of the VSI bridge switches S1S6.The dc bus voltage reflects at the output of the inverter in

the form of the three-phase PWM ac line voltage ea, eb, and ecwhich are expressed as

ea = vdc(SA-SB) (17)

eb = vdc(SB-SC) (18)

ec = vdc(SC-SA). (19)

The voltcurrent equations of the output of VSI of STATCOM

are

va = Rfica + Lfpica + ea Rficb Lfpicb (20)

vb = Rficb + Lfpicb + eb Rficc Lfpicc (21)ica + icb + icc = 0. (22)

The value of icc from (22) is substituted into (21) whichresults in

vb= Rficb + Lfpicb + eb + rfica + Lfpica + Rficb + Lfpicb.(23)

By rearranging (20) and (23), these result in

Lfpica Lfpicb = va ea Rfica + Rficb (24)Lfpica + 2Lfpicb = vb eb Rfica 2Rficb. (25)

Hence, the STATCOM current derivatives are obtained by

solving (24) and (25) as

pica = {(vb eb) + 2(va ea) 3Rfica} /(3Lf) (26)picb = {(vb eb) (va ea) 3Rfica} /(3Lf). (27)

C. Modeling of SEIG

The dynamic model of the three-phase SEIG is developedusing stationary dq axes reference frames, whose voltagecurrent equations are [11]

[v] = [R][i] + [L]p[i] + Wg[G][i]. (28)

From above, current derivatives can be expressed as

p[i] = [L]1 {[v] [R][i] Wg[G][i]} (29)where

[v] = [vds vqs vdr vqr]T [i] = [ids iqs idr iqr]

T

[R] = diag [Rs Rs Rr Rr]

[L] =

Ls + Lm 0 Lm 00 Ls + Lm 0 Lm

Lm 0 Lr + Lm 00 Lm 0 Lr + Lm

[G] =

0 0 0 00 0 0 00 Lm 0 Lr + Lm

Lm 0 Lr + Lm 0

. (30)

The electromagnetic torque balance equation of SEIG is

defined as

Tshaft = Te + J(2/P)pWg. (31)

The derivative of the rotor speed of the SEIG from (31) is

pWg = {P/(2J)} (Tshaft Te) (32)where the developed electromagnetic torque of the SEIG (Te)is expressed as [11]

Te = (3P/4)Lm(iqsidr idsiqr) (33)and the shaft torque of the prime mover is considered as a

function of speed and denoted as

Tshaft = (A BWg) (34)


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where A and B are the coefficients of the torquespeed char-acteristic of the prime mover. The values of these coefficients

are given in C. The SEIG operates in the saturation region, and

its magnetizing characteristic is nonlinear in nature. Therefore,

the magnetizing current should be calculated in each step of

integration, in terms of stator and rotor currents as

Im =

(ids + idr)2 + (iqs + iqr)

2

/2. (35)The magnetizing inductance is calculated from the magnetiz-

ing characteristic between Lm and Im. A relation between Lmand Im is obtained by a synchronous speed test [11] and can bewritten as

Lm = A1 + A2Im + A3I2m + A4I

3m. (36)

The coefficients A1, A2, A3, and A4 are given in C.

D. AC Line Voltage at Point of Common Coupling

The direct and quadrature stator axis currents of the

SEIG (ids and iqs) are converted in to three-phase statorcurrents of SEIG (iga, igb, and igc). From these stator phasecurrents of SEIG (iga, igb, and igc), the line currents (ia, ib,and ic) of SEIG are computed. The derivative of the ac terminalvoltage of the SEIG is defined as

pva = {(ia i1a ica) (ib i1b icb)} /(3C) (37)pvb = {(ia i1c ica) + 2(ib i1b icb)} /(3C) (38)va + vb + vc = 0 (39)

where ia

, ib

, and ic

are the SEIG stator line currents, ila

, ilb

,

and ilc are the three-phase load currents, and ica, icb, and iccare the three-phase STATCOM currents. C is the per-phase noload excitation capacitor value connected parallel to SEIG, as

shown in Fig. 1(a).

E. Modeling of Nonlinear Loads

The mathematical modeling of nonlinear loads has been

carried out into the following four categories.

1) Three-Phase Diode Rectifier With Resistive Load: A cir-

cuit diagram of a three-phase diode rectifier with resistive

load is shown in Fig. 2(a). A general case of three-phase

uncontrolled diode bridge rectifier with resistive load (RRL) istaken as a balanced nonlinear load, in which the voltage across

the dc load (vs) would be the maximum line voltage of SEIG(va, vb, vc, va, vb, and vc) and the rectifier dc load currentis obtained as

id = iRL = vs/RRL. (40)

Rectifier input ac currents are defined in Table I at a resistive

load with a dc bus current id, which is considered as the loadcurrents of an SEIG system.

2) Single-Phase Diode Rectifier With Resistive Load:

Fig. 2(b) illustrates the circuit diagram of single-phase diode

rectifier with a resistive load. A single-phase uncontrolleddiode bridge rectifier with feeding resistive load is taken as an

unbalanced load. In this case, load voltage is the maximum line

voltage vs (va, va), and the rectifier dc current is defined as

id = vs/RRL. (41)

Rectifier ac currents will be the same with those defined in

Table I, and it will be zero in nonconducting phase, namelyphase C.

3) Three-Phase Diode Rectifier With Capacitive Filter and

Resistive Load: The circuit diagram of three-phase diode rec-

tifier with capacitive filter (CRL) and resistive load (RRL) isshown in Fig. 2(c). However, the practical uncontrolled diode

bridge rectifier has a notional value source impedance, and a

dc capacitor is used in filtering of the output ripple dc of the

rectifier. The three-phase uncontrolled diode bridge rectifier has

two operating modes, i.e., conducting and nonconducting of

diodes. When the diodes are in conduction, lineline voltage

of an ac source (va, vb, and vc) is connected to the load, and dcside basic equation is given by

vs = 2RSLid + 2LSLpid + vd. (42)

In the state space derivative form, the previous equation can be

expressed as

pid = (vs vd 2RSLid)/(2LSL). (43)

The ac load currents in all three phases (ila, ilb, and ilc) of athree-phase diode rectifier are achieved by using the magnitude

ofid and direction (sign) corresponding to conduction pairs ofdiodes, which are the same with those shown in Table I. RSLand LSL are the notational value of the source resistance andinductance of the ac supply system.

Moreover, charging/discharging equation of the dc load ca-

pacitor is expressed as

pvd = (id ir)/CRL (44)

where CRL is the filter capacitance on the dc side, vs is themaximum line voltage of the generator (va,vb,vc, va, vb,and vc), vd is the instantaneous voltage across the capacitor,and ir is the resistive load current (vd/RRL). When none of thediode pair is conducting, then the charged capacitor would be

discharged through load resistance (RRL), and id is zero.

4) Single-Phase Diode Rectifier With Capacitive Filter andResistive Load: Fig. 2(d) shows the circuit diagram of a single-

phase diode rectifier with capacitive filter (CRL) and resistiveload (RRL). The single-phase rectifier with capacitive filterand resistive load has two operating modes, i.e., conducting

and nonconducting of diodes. When diodes are conducting, the

ac source is connected to the load, and the basic equation is

expressed as

vs = RSLid + LSLpid + vd. (45)

In the state-space derivative form, the previous equation can be

expressed as

pid = (vs vd RSLid)/(LSL) (46)


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Fig. 2. Circuit diagrams of nonlinear loads. (a) Three-phase diode rectifier with resistive load. (b) Single-phase diode rectifier with resistive load. (c) Three-phasediode rectifier with capacitive filter and resistive load. (d) Single-phase diode rectifier with capacitive filter and resistive load. (e) Three-phase thyristorized rectifierwith resistive load.

TABLE ITHREE-PHASE NONLINEAR LOAD CURRENT


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Fig. 4. Waveform of three-phase SEIGSTATCOM systemsupplying diode rectifier with resistive load change from no load, to three-phase(22 kW), to one-phase(15 kW), to three-phase (22 kW) loads, and to no load.

three-phase ac terminal voltages (vabc), SEIG line currents(iabc), STATCOM currents (icabc), amplitude of ac terminalvoltage and its reference value (Vt and Vtref), dc bus voltageand its reference value (Vdc and Vdcref), and generator speed(Wg). To generate the rated voltage of 400 V (565-V peak)at no load, a delta-connected capacitor bank of 62.5 F perphase is connected across the SEIG. Initially, a dc bus capacitor

of the current-controlled VSI of STATCOM charges to 565 V

(peak of ac voltage) during voltage build up through the an-tiparallel diodes of VSI (without control action). At 2.6 s,

gate pulses are given to the IGBTs, and the control action

of the current-controlled VSI of STATCOM is activated. The

STATCOM behaves as a source of the reactive power and

draws active power from the generator to charge its dc bus

capacitor at reference voltage (700 V). There is a small oscilla-

tion at the switching in STATCOM, but it damps out within

a few cycles. For a chosen value of the moment of inertia

(0.305 Kg/m2), there is a small change in speed (0.6%), which

recovers quickly to the normal value due to the control actionof STATCOM.


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Fig. 5. Waveform of three-phase SEIGSTATCOM system supplying diode rectifier with capacitive filter and resistive load change from no load, to three-phase(15 kW), to one-phase (24 kW), to three-phase (15 kW) loads, and to no load.

B. Performance of SEIGSTATCOM System Feeding

Three-Phase Diode Rectifier With Resistive Load

Fig. 4 shows the transient waveforms of the three-phase

SEIG voltages (vabc), SEIG line currents (iabc), three-phaseSTATCOM currents (ica, icb, and icc), three-phase rectifier acload currents (ila, ilb, and ilc), amplitude of SEIG terminalvoltage and its reference value (Vt and Vtref), dc bus volt-

age and its reference value (Vdc and Vdcref), and generatorspeed (Wg), demonstrating the response of the STATCOM in

regulating the SEIG terminal voltage supplying the rectifier

with a pure resistive load (22 kW). At 6.4 s, a three-phase

diode rectifier with a resistive load (22 kW) is applied on the

SEIGSTATCOM system. Under the three-phase diode rectifier

with resistive load, the SEIG voltages remain constant, but

SEIG and STATCOM currents increase to supply the active

and reactive power to the load, respectively. A small dip in

the dc bus voltage of STATCOM is observed at the application

of the load, but it recovers within a few cycles. At 6.6 s,a three-phase diode rectifier with resistive load is changed to


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Fig. 6. Waveforms of three-phase SEIGSTATCOM system supplying diode rectifier with capacitive filter and resistive load change from no load, to three-phase(15 kW), to three-phase (22 kW), to three-phase (15 kW) loads, and to no load.

a single-phase rectifier load with a generated power of 15 kW.

It is observed that the STATCOM is able to regulate the SEIG

terminal voltage without any transient. When a single-phase

rectifier load is connected, generator currents decrease to show

the lesser power generation by the SEIG. An overshoot in

the dc bus voltage of STATCOM is observed when a single-phase rectifier load is connected. Similarly, a small undershoot

in the dc bus voltage of STATCOM is observed at the appli-

cation of three-phase loads. Charging and discharging of the

dc bus capacitor of STATCOM is clearly observed under a

single-phase rectifier load, which also shows the load-balancing

feature of STATCOM. At 7 s, a three-phase diode rectifier load

is disconnected from the SEIG. A small overshoot in the dc busvoltage of STATCOM is observed at the removal of the load.


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Fig. 7. Waveforms of three-phase SEIGSTATCOM system supplying thyristorized rectifier with resistive load change from no load, to three-phase (18 kW) at60 firing angle, to no load.

C. Performance of SEIGSTATCOM Feeding Three-Phase

Diode Rectifier With Capacitive Filter and Resistive Load

Fig. 5 shows the transient waveforms of all the performance

variables of the three-phase SEIGSTATCOM supplying a

diode rectifier with a dc capacitive filter (CRL) and resistiveload (RRL). At 6.4 s, a three-phase diode rectifier withcapacitive filter and resistive load (15 kW) is applied at

the SEIGSTATCOM system. Under this load, SEIG andSTATCOM currents increase to supply active and reactive

powers to the load. The SEIG voltage remains constant and

sinusoidal. At 6.65 s, the load is changed from a three-phase

load (15 kW) to a single-phase diode rectifier load (24 kW).

In the case of the dc capacitive filter and resistive load, on a

single-phase rectifier, the rectifier ac load current is highly

discontinuous. The dc link capacitor of the load diode rectifier

tries to keep the voltage constant across the rectifier dc load;

therefore, the STATCOM and generator currents increase tofulfill the increased reactive and active power requirements,


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Fig. 8. Steady-state waveforms and harmonic spectrum SEIGSTATCOM system supplying three-phase diode rectifier with capacitive filter and 15-kW resistiveload. (a) SEIG voltage (Va). (b) SEIG current (Ia). (c) Load current (Ila).

respectively. The speed of SEIG also drops down, which

shows that the generator is heavily loaded. At 6.9 s, a three-

phase diode rectifier with capacitive filter and resistive load is

connected again to the SEIGSTATCOM system. The system

recovers steady-state conditions within a couple of cycles. At

7.1 s, the load is completely removed, thus resulting in a

decrease in SEIG and STATCOM currents.

D. Performance of SEIGSTATCOM System With Load

Change on Three-Phase Diode Rectifier With DC Capacitive

Filter and Resistive Load

The simulated waveforms of SEIG voltages, currents, STAT-

COM currents, load currents, amplitude of ac terminal voltage,

dc bus voltage, and generator speed are illustrated in Fig. 6

for a three-phase SEIGSTATCOM system with load change

in the three-phase diode rectifier with capacitive filter and

resistive load. At 6.4 s, a three-phase nonlinear load of 15 kW is

switched on. At 6.6 s, load is increased on the three-phase diode

rectifier load from 15 to 22 kW on the SEIG. Consequently, the

generator, STATCOM, and load currents increase to providethe increased demand of reactive and active powers to the

load. As the load on the rectifier increases, an undershoot

in the dc bus of STATCOM is observed, which supplies the

reactive power to the rectifier load and generator, and the

dc bus voltage of STATCOM settles at the reference voltage

within a few cycles. At 6.8 s, load is decreased from 22 to

15 kW on the SEIGSTATCOM system. This instantaneously

generated surplus power is absorbed by the dc bus capacitor

of STATCOM; as a result, an overshoot in the dc bus voltageof STATCOM is observed, which settles down to the reference

voltage within few cycles due to the action of the PI voltage

controller. The speed of the SEIG also drops down by 6%

when the generated power is increased to 22 kW, due to small

drooping characteristics of the prime mover.

E. Performance of SEIGSTATCOM System Feeding

Three-Phase Thyristor Rectifier With Resistive Load

Fig. 7 illustrates the transient waveforms of SEIG voltages

and SEIG currents, load currents, STATCOM currents, its dc

bus voltage and its reference value, ac voltage of SEIG and its

reference value, and the speed of the SEIG under the three-phase thyristorized rectifier with resistive load at a 60 firing


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Fig. 9. Steady-state waveforms and harmonic spectrum SEIGSTATCOM system supplying single-phase diode rectifier with capacitive filter and 24-kW resistiveload. (a) SEIG voltage (Va). (b) SEIG current (Ia). (c) Load current (Ila).

angle of thyristors. At 6.4 s, an 18-kW load is switched on

the SEIGSTATCOM system. Similarly, at 6.6 s, the controlled

rectifier load of 18 kW is removed from the SEIG system. This

controlled rectifier load draws harmonic currents and funda-

mental active and reactive currents which are fed from SEIG

STATCOM system. Under this type of nonlinear load, SEIG

voltage remains constant and sinusoidal, which shows that the

STATCOM acts as a voltage regulator and harmonic eliminator.

F. Power Quality Issues

Figs. 810 illustrate the steady-state waveforms and har-

monic spectrum of SEIG voltage (Va), SEIG current (Ia), andload currents (Ila) under a three-phase diode rectifier withcapacitive filter and resistive load, single-phase diode recti-

fier with capacitive filter and resistive load, and three-phase

thyristorized rectifier with resistive load, respectively. Table II

shows the total harmonic distortion (THD) of SEIG voltage

(Va), SEIG current (Ia), and load currents (Ila) under a three-phase diode rectifier with capacitive filter and resistive load,

single-phase diode rectifier with capacitive filter and resistiveload, and three-phase thyristorized rectifier with resistive load,

respectively. Under the three-phase diode rectifier with capaci-

tive filter and resistive load, load current THD is 80.56%, but

SEIG voltage and current THDs are only 1.37% and 3.2%,

which shows that STATCOM has eliminated the harmonics

from the SEIG voltage and current. Under a single-phase diode

rectifier load, the load current has a THD of 76.95%, but SEIG

voltage and current have only THDs of 0.87% and 0.73%.

Under a three-phase thyristorized rectifier with resistive load,

SEIG voltage has a THD of 1.24%, and SEIG current has

3.44%, in spite of the percent THD load at 44.82%. From

these results, it can be observed that the SEIG voltage and

current remain sinusoidal even under nonlinear loads. The

STATCOM eliminates the harmonics from the SEIG system. It

is observed from these results that the STATCOM improves the

power quality in an SEIG system and thus avoids its derating

due to harmonic injection, unbalancing, and reactive power

requirements of nonlinear loads.

V. CONCLUSION

It has been observed that the developed mathematical modelof a three-phase SEIGSTATCOM is capable of simulating


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Fig. 10. Steady-state waveforms and harmonic spectrum SEIGSTATCOM system supplying three-phase thyristorized rectifier with capacitive filter and 12-kWresistive load. (a) SEIG voltage (Va). (b) SEIG current (Ia). (c) Load current (Ila).

TABLE IIRMS VALUES AND PERCENT THD OF SEIG VOLTAGE (Va), SEIG CURRENT (Ia), AN D AC LOAD CURRENT (Ila)

its performance while feeding nonlinear loads under transient

conditions. From the simulated results, it has been found that

the SEIG terminal voltage remains constant, with the sinusoidal

feeding of the three-phase or single-phase rectifiers with re-

sistive and with dc capacitive filter and resistive loads. When

a single-phase rectifier load is connected, the STATCOM bal-ances the unbalanced load currents, and the generator currents

and voltage remain balanced and sinusoidal; therefore, the

STATCOM acts as a load balancer. The rectifier-based non-

linear load generates the harmonics, which are also eliminated

by STATCOM. Therefore, it is concluded that STATCOM acts

as voltage regulator, load balancer, and harmonic eliminator,

resulting in an SEIG system that is an ideal ac power-generatingsystem.


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APPENDIX

A. STATCOM Control Parameters

Lf = 1.2 mH, Rf = 0.045 , and Cdc = 4000 F.AC voltage PI controller: Kpa = 0.1 and Kia = 0.01.DC bus voltage PI controller: Kpd = 0.44 and Kid = 0.05.

Carrier frequency = 20 kHz.

B. Parameters of Nonlinear Loads

1) Three-Phase Diode Rectifier With Resistive Load:

RRL = 32 .

For a single-phase diode rectifier with resistive load, one

phase (say phase C) of the three-phase rectifier is disconnected

from the system.


Resistive Load: Lf = 0.1 mH, Rf = 0.7 , Cdc = 470 F, andRRL = 100 .

For a single-phase diode rectifier with capacitive filter and

resistive load, one phase (say phase C) of the three-phase

rectifier is disconnected from the system.


Change in Resistive Load: Lf = 0.1 mH, Rf = 0.7 , Cdc =470 F, RRL = 80 (at 15-kW load), and RRL = 32 (at22-kW load).

4) Three-Phase Thyristorized Rectifier With Resistive Load:

RRL = 15 , and the firing angle of the thyristor is 60.

C. Machines Parameters

1) SEIG Parameters: A 22-kW 400-V 40-A 50-Hz six-pole

delta-connected machine is taken as SEIG. The parameters of

the SEIG are

Rs = 0.58

Rr = 0.81

Ls = Lr = 5 mH

J = 0.305 kg/m2.

2) Magnetizing Characteristic Coefficients: The coeffi-

cients of the magnetizing characteristic of SEIG are given

below:

A1 = 0.2417 A2 =0.0112 A3 =0.0003 A4 = 0.00003.

3) Prime Mover Coefficients: The coefficients of the

torquespeed characteristic of prime mover are

A = 3370, B = 10.

REFERENCES

[1] C. Grantham, D. Sutanto, and B. Mismail, Steady state and transientanalysis of self-excited induction generator, Proc. Inst. Electr. Eng.,vol. 136, no. 2, pp. 6168, Mar. 1989.

[2] K. E. Hallenius, P. Vas, and J. E. Brown, The analysis of saturated self-excited asynchronous generator, IEEE Trans. Energy Convers., vol. 6,no. 2, pp. 336341, Jun. 1991.

[3] M. H. Salama and P. G. Holmes, Transient and steady-state load per-formance of a stand-alone self-excited induction generator, Proc. Inst.

Electr. Eng.Electr. Power Appl., vol. 143, no. 1, pp. 5058, Jan. 1996.[4] L. Wang and R. Y. Deng, Transient performance of an isolated induction

generator under unbalanced excitation capacitors, IEEE Trans. EnergyConvers., vol. 14, no. 4, pp. 887893, Dec. 1999.

[5] S. K. Jain, J. D. Sharma, and S. P. Singh, Transient performance of three-phase self-excited induction generator during balanced and unbalanced

faults, Proc. Inst. Electr. Eng.Generation Transmiss. Distrib., vol. 149,no. 1, pp. 5057, Jan. 2002.[6] M. B. Brennen and A. Abbondati, Static exciter for induction generator,

IEEE Trans. Ind. Appl., vol. IA-13, no. 5, pp. 422428, Sep./Oct. 1977.[7] R. K. Mishra, B. Singh, and M. K. Vasantha, Voltage regulator for an

isolated self-excited cage induction generator, Electr. Power Syst. Res.,vol. 24, no. 1, pp. 7583, Jul. 1992.

[8] L. Shridhar, B. Singh, and C. S. Jha, Transient analysis of the self-regulated short shunt self-excited induction generator, IEEE Trans.

Energy Convers., vol. 10, no. 2, pp. 261267, Jun. 1995.[9] E. Larsen, N. Miller, S. Nilsson, and S. Lindgren, Benefits of GTO-

based compensation systems for electric utility applications, IEEE Trans.Power Del., vol. 7, no. 4, pp. 20552064, Oct. 1997.

[10] C. Schauder and H. Mehta, Vector analysis and control of advancedstatic VAR compensator, Proc. Inst. Electr. Eng.C, vol. 140, no. 4,pp. 299306, Jul. 1993.

[11] B. Singh and L. B. Shilpakar, Analysis of a novel solid state voltage

regulator for a self-excited induction generator, Proc. Inst. Electr. Eng.Generation Transmiss. Distrib., vol. 145, no. 6, pp. 647655, Nov. 1998.

[12] S. Wekhande and V. Agrawal, Simple control for a wind-driven in-duction generator, IEEE Ind. Appl. Mag., vol. 7, no. 2, pp. 4453,Mar./Apr. 2001.

[13] M. S. Miranda, R. O. C. Lyra, and S. R. Silva, An alternative isolatedwind electric pumping system using induction machines, IEEE Trans.

Energy Convers., vol. 14, no. 4, pp. 16111616, Dec. 1999.[14] S. C. Kuo and L. Wang, Analysis of voltage control for a self-excited

induction generator using a current-controlled voltage source inverter(CC-VSI), Proc. Inst. Electr. Eng.Generation Transmiss. Distrib.,vol. 148, no. 5, pp. 431438, Sep. 2001.

[15] , Analysis of isolated self-excited induction generator feeding arectifier load, Proc. Inst. Electr. Eng.Generation Transmiss. Distrib.,vol. 149, no. 1, pp. 9097, Jan. 2002.

Bhim Singh (SM99) was born in Rahamapur,U. P., India, in 1956. He received the B.E. (electrical)degree from University of Roorkee, Roorkee, India,in 1977 and the M.Tech. and Ph.D. degrees fromthe Indian Institute of Technology (IIT), New Delhi,India, in 1979, and 1983, respectively.

In 1983, he joined as a Lecturer and, in 1988,became a Reader with the Department of ElectricalEngineering, University of Roorkee. In December1990, he joined as an Assistant Professor, becamean Associate Professor in 1994, and full Professor

in 1997 with the Department of Electrical Engineering, IIT Delhi. His field ofinterest includes power electronics, electrical machines and drives, active filters,static VAR compensator, and analysis and digital control of electrical machines.

Dr. Singh is a Fellow of the Indian National Academy of Engineering(INAE), Institution of Engineers (India) [IE (I)], and Institution of Electronics

and Telecommunication Engineers (IETE); and a Life Member of the IndianSociety for Technical Education (ISTE), System Society of India (SSI), andNational Institution of Quality and Reliability (NIQR).


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S. S. Murthy (SM87) was born in Karnataka, India,in 1946. He received the B.E. degree from Banga-lore University, Bangalore, India, the M.Tech. degreefrom Indian Institute of Technology (IIT) Bombay,Bombay, India, and the Ph.D. degree from IIT Delhi,New Delhi, India.

He has been with IIT Delhi since 1970 and was theChairman of the Department of Electrical Engineer-

ing from 1998 to 2001. He has held assignments withthe University of New Castle, U.K.; University ofCalgary, Canada; ERDA Barodra; Kirloskar Electric,

Bangalore, and Director, NIT Suratkal, Karnataka. He holds four patents on theSEIG, Micro Hydel Applications, and a novel-braking scheme. He has alsotransferred technology of self-excited and grid-connected induction generatorsto industry for low- and medium-power generation under stand-alone or grid-connected mode. He has completed several industry sponsored research andconstancy projects dealing with electrical machines, drives, and energy systems.Recently, he was instrumental in establishing state-of-the-art energy audit andenergy conservation facilities at IIT under World Bank funding. His research in-terests include electric machines, drives, special machines, power electronic ap-plications, renewable energy systems, and energy efficiency and conservation.

Dr. Murthy has received many awards including ISTE/Maharashtra Govern-ment Award for outstanding research and IETE/Bimal Bose Award for contribu-tion in power electronics. He has made significant contributions to professionalsocieties, including being the General Chair of the first IEEE International

Conference on Power Electronics, Drive and Energy Systems (PEDES 96),held in January 1996 in New Delhi.He is a Fellow of theInstitutionof ElectricalEngineer (IEE), Life Fellow of the Institution of Engineers (India), and LifeMember of the Indian Society for Technical Education (ISTE).

Sushma Gupta was born in Lalitpur, U. P., India, in1971. She received the B.E. and M.E. degrees fromBarakatullah University, Bhopal, India, in 1993 and1999, respectively.

She was a Lecturer with the Electronic Engineer-ing Department, Government Engineering College,Rewa, M. P., India. She is currently a ResearchScholar with the Department of Electrical Engineer-

ing, Indian Institute of Technology, Delhi, India.Her research interests include power electronics, ma-chines, digital electronics microprocessors, and self-

excited induction generators.

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Bhim Singh Sir