Operation of the Unified Power Flow

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116 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 11, NO . 6, NOVEMBER 1996

Operation of the Unified Power Flow

Controller as Harmonic IsolatorJohan H. R. Enslin, Senior Member, IEEE, Jian Zhao, and RenC SpCe, Senior Member, IEEE

Abstract-The unified power flow controller (UPFC) is a tool

in the implementation of Flexible AC Transmission Systems

(FACTS). It provides for the equivalent of static VAr compen-

sation and series injection using back-to-back force commutated

converters. This paper ]proposes a control strategy to extend

UPFC operation to allow for the isolation of harmonics due to

nonlinear loads. Simulation results based on the Electromagnetic

Transients Program (EMTP) are used to illustrate device per-

formance in a power system environment. Experimental results

based on a single phase laboratory implementation verify the

proposed control algorithm.

Ih

x,xX ,

LO

L ,CO

c,

Nonlinear load harmonic currents [A].

Transmission line reactance [Q].Equivalent series compensation reactance [RI .

Transformer leakage inductance [HI.

High frequency filter inductance [HI.

Passive harmonic filter inductances [HI.

High frequency filter capacitance [F].

Passive harmonic filter capacitances [F].

NOMENCLATUREFlexible AC Transmission Systems.

High voltage direct current transmission.

Thyristor controlled series compensator.

Unified power flow controller.

Static VAr compensator.

Insulated gatle bipolar transistor.

Injected series voltage [VI.

Sending-end transmission line voltage [VI.

Receiving-end transmission line voltage [VI

{BEFORE Hhrmonic Isolation}.

Isolated receiving-end voltage [VI

{AFTER Harmonic Isolation}.

Load-side receiving-end voltage [VI

{AFTER Harmonic Isolation}.

Injected voltage with series compensation [VI.

Injected voltage with angle compensation [VI.

Transmission line midpoint voltage [VI.

Converter dc bus voltage [VI.

Injected volttage with terminal

voltage compensation [VI.

Transmission power angle ["I .Transmission power angle between V, and V, [" I .Phase shift control angle ["I.Load thyristor firing angle ["I.UPFC shunt current source [A].

Load ac current [A].

Transmission line current [A].Passive filter current [A].

I. INTRODUCTION

ONCEPTS relating to Flexible AC Transmission Sys-tems (FACTS) are gaining popularity internationally for

enhancing steady-state power transfer limits as well as improv-

ing power system dynamic response [1]-[S]. FACTS devices

include solid-state phase shifters [11, 121, thyristor-controlled

series capacitors [ 3 ] , 4], and static VAr devices [SI, [6]. First

generation installations using phase controlled series compen-

sators are currently being commissioned [4], [14]. Recent

efforts have addressed the synthesis of FACTS controllers

using converter-based topologies. The unified power flow

controller (UPFC) [SI-[7] provides for the equivalent of static

VAr compensation and series injection using back-to-back

force-commutated converters [6], [7].With an increasing em-

phasis on power quality [E], [12], harmonic isolation [8]-[111,

and harmonic compensation [12], [13] issues are also being

investigated for high power applications [12], [13].

The present paper discusses the extension of UPFC opera-

tion to include not only fundamental phase shift and reactive

power compensation, but to also provide for harmonic isolation

in the presence of nonlinear loads. This is performed using a

combined harmonic/fundamental control strategy in a single

converter topology. This approach allows for the optimum

use of installed converter kVA with a potentially attractive

cost/performance characteristic. Simulation results using the

electromagnetic transients program (EMTP) illustrate device

performance of a 120 kVA converter implementation. Thisconverter system is suitable for a 1.3 MVA nonlinear load,

implementing both fundamental phase shift and harmonicisolation. Experimental results are presented for a low power,

Manuscript received July 18, 1994; revised June 9, 1996. A version of thispaper was presented at the 1994 Power Electronics Specialists Conference.This work was supported by ESKOM and the FRD.

J. H. R. Enslin is with the Department of Electrical and Electronic

J . Zhao is with the Department of Electrical and Electronic Engineering,

R. Sp6e is with the Department of Electrical and Computer Engineering,

sing1e phase laboratory prototype' Both and lab-

oratory data show the capability of the proposed combined

control approach.

device an d micro-contro11er de -

velopments make this new principle already applicable to

the multi-megawatt power range. Some first applications to

be considered are controlling power flow and stabilizing

distribution networks in the presence of harmonic generating

Engineering, University of Stellenbosch, 7600, Stellenbosch, South Africa.

University of Stellenbosch, 7600, Stellenbosch, South Africa.

Oregon State University, Corvallis, OR 97331 USA.Publisher Item Identifier S 0885-8993(96)06856-1.

"ITent power

0885-8993/96$05.00 0 996 IEEE

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ENSLIN et al.: OPERATION OF UNIFIED POWER FLOW CONTROLLER

Modes of Operation

1) No Compensation

2) Series Compensation

3) Shunt Compensation

4) Phase ShiftControl

171

Power Controller Output

v, 1

0 0

-jX;I, 0

0 -jllV/X(l-cos 6/2)

*jV, tana No Reactive Current

A s 2

Fig. 1. FACTS with phase shifter.

PS

industrial loads. Transmission system applications to be con-

sidered include the isolation of harmonic power flows and the

stabilization of geographically separate power systems.

11. UNIFIEDPOWER ONTROL ONCEPTS

A . Flexible AC Transmission (FACTS)

Consider power flow over an ac line, in (1) and Fig. 1

V s .Vrp=- .sin S,, .

* S

The power flow depends on transmission angle, S,,, be-

tween the line-end voltages, the sending-end voltage, V,, the

receiving-end voltage, V,, and line impedance, X,. Currently,

only limited high speed control over any one of these parame-

ters is used. In electromechanically controlled power systems,

the operators arrive at the required steady-state power flow

while maintaining voltages and angles within safe tolerable

limits. These levels are well below the peak stability limits

of the power system. The consequences of the lack of fast,

reliable control are stability problems, power flowing through

other than the intended lines, the inability to fully utilize the

transmission resources to their thermal and/or economic limits,

undesirable VAr flows, higher losses, bad voltage regulation,cascade tripping, and long restoration times [l], 121.

Fig. 1 shows a representation of a phase shifter in one

transmission line. This phase shifter can be realized with a high

speed thyristor based converter [11, 151.With this arrangement,one can obtain substantially the same advantages as with an

HVDC line but at a fraction of the cost, since not all power is

processed through the power electronic converter. This phase

controller forms the basis of the UPFC [SI. All the network

parameters in (1) can now be controlled by means of this

equivalent UPFC.

(a)

- a n n+a2

(b)

Equivalent circuit and operating modes of the UPFC [ 6 ] .ig. 2.

B. Basic Principle o UPFC Operation

The basic equivalent circuit of the UPFC is shown in Fig. 2.

A fully controllable voltage source, V,,, is injected in series

with the transmission line, and a controllable shunt current

source, I,, is connected in parallel with the transmission line.

The modes of operation are summarized in Fig. 2(b). For

generalized series (shunt) compensation, the source 1, (V,,)

could be omitted if a sufficient dc energy storage device wascoupled to the controlled voltage (current) source. The UPFC

then operates either as a converter-based series compensator

or static VAr compensator [61, 171.

The different UPFC modes of operation are plotted in

the power flow diagram, depicted in Fig. 2(b), while the

appropriate values for Iq and V,, are shown in Table I [6].

Multiple power flow control functions can be achieved by

adding an appropriate voltage phasor V,, to the terminal volt-

age phasor VO s shown in Fig. 3, which concentrates on the

voltage control aspect and does not show the load dependent

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ENSLIN et al.: OPERATION OF UNIFIED POWER FLOW CONTROLLER

Extended Modes of Operation

5 ) Harmonic Isolation

179

Power Controller Output

"m 4

4.1, 0

(c )

Fig. 4. UPFC as harmonic isolator: (a) equivalent circuit, (b) principle of operation, and (c) power electronics implementation.

Transm. Line Ind L,

UPFC Filter Ind. Lo

Trans. 2 Leakage

Load RL *

Load Ind. L,

UPFC Filter Cap. CO

Trans. 2 Wind. Ratio0 pH

82-84"

* RL s adjusted to keep load current, ILdoconstant (83 Adc)

filters, Cf and L f . It is assumed that the dc link voltage inthe UPFC circuit is kept constant at 400 V by Inverter 1.

To simulate the circuit under the same defined conditions, the

load current IL, filter current I F ,and line current I , are kept

constant. The sending-end voltage, V,, is also kept constant

at 6.35 kV (rms). The load current is the current produced

by the phase controlled rectifier circuit. A clean sinusoidal

voltage at V, is assumed on the sending-end power bus. The

main parameters of the simulations are shown in Table 111.

Case 1-Small Phase Compens ation and Harm onic Isola-

tion: No fundamental phase shift between VTi and v! s

implemented in the controller of the UPFC for the firstsimulation case. Thus, the UPFC is operating mainly asharmonic isolator. The EMTP output is plotted in Fig. 5. For

this case, the power rating of the UPFC (Inverter 2) is very

small compared to the transmission rating. The waveformof V,, still has a small fundamental component, which is

included to compensate for the internal leakage reactance of

the injecting transformer, Tr2. The load voltage V, before

harmonic isolation (resulting from the six-pulse load converter

and network impedance X , ) exhibits the well-known distortion

of fifth, seventh, eleventh, thirteenth, etc., harmonics.




780

Description

Load Apparent

Power [S,]

Load ActivePowe r [P,]

Transmission

Apparent Pow er

UPFC Inv. 2

Power Rating

UPFC Inv. 1

Power Rating

Passive Filters

Eqv. Impd. Z,

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL 11 , NO . 6, NOVEMBER 1996

No (i) Harmonic (ii) Phase Contr .

Compensation Isolation & Harm. Iso.

1.27 MVA 1.28 MVA 1.04 MV A

140 kW 173 kW 104 kW

1.27MVA 602 kVA 602 kVA

0 7 kVA 120 kVA (Rect.)

0 0 36 kVA (Inv.)

0 770 kVA 625 kVA

X, 1.88 Q X, 1.88 Q Z, = 1 2 j 3 8 Q

TABLE IVRELATIVE OWE RCA LCU LA TIO N S

t - --I0 10 20 30 40

f(ms)

Fig. 5 . EMTP simulation of UPFC as harmonic isolator: (a) I:: sourcesending-end voltage and reference {6.35 kV} , (b) 1’; : receiving-end loadvoltage without compensation {6.22 kV}, (c ) VrZ: eceiving voltage withharmonic isolation 16.31 kV}, (d) I);!: load voltage with harmonic isolation{ 6.28 kV} , (e) injected harmonic isolation voltage (7 0 V} , (f) I, : sourcecurrent through UPFC and line after compensation { 32 A } , and (g) I L : currentthrough nonlinear load {68 A]. (For Fig. 5 : RL = 25 n: I L ~ < 8 3 A:

RJ,= 20 C without harmonic isolation.)

After the unified converter has injected the voltage Vpq,he

voltage harmonics are isolated as shown in the V,,waveform.The small fundamenlal component to cancel the leakage

impedance voltage drop is clearly visible in the injected V,,

[Fig. 5(e)] waveform. For this case, no phase shift exists

between V,, nd V:.A passive filter is also added to keep the

power rating of the UPFC small compared to the transmission

system. The passive filter compensates the load current, IL , o

the transmission line current , I s ,as c,hown in Fig. 5(f) and (g).

In the process, the load voltage is affected as shown in V:.

Case 2-Phase Com pensation with Harm onic Isolation:

Operation of the UPFC as a phase shifter and harmonic

‘ 0 10 20 30 40

tfms)

Fig. 6. EMTP simulation of UPFC as harmonic isolator and phase shiftcompensator: (a) IT,: ource sending-end voltage and reference { 6.3.5 kV},(b ) V,: receiving-end load voltage without compensation (6.22 kV}, (c ) VrZ:receiving voltage with harmonic isolation { 6.304 kV}, (d ) V load voltagewith phase shift and harmonic isolation (5.08 kV} , (e) injected phaseshift and harmonic isolation voltage { 1.25 kV}, (f) I,? source current throughUPFC and line after compensation (3 2 A} , an d ( 8 ) I L : current throughnonlinear load before compensation (68 A } . (RL = 1 5 0 : 1 ~ ~ 1 ~83

A: {RL 2 0 s2 without c ompensation.)

isolator is considered in Fig. 6, using EMTP. The load current

is the same as in the previous simulations. The original phase

shift between V, and V,, due to X , , is now completely

compensated by means of the injected voltage Vpq.The

receiving-end voltage, Vi, s also controlled to be lower than

in Case 1 (5.08 kV vls 6.28 kV}. Energy is thus withdrawn

from the transmission system through V,, and injected back

by Inverter 1, using Iq .

For this case, there is active and reactive power flow through

the UPFC. This is also evident from the large fundamental




ENSLIN et al.: OPERATION OF UNIFlED POWER FLOW CONTROLLER

Transm. Line Ind L, 11.9m H

UPFC Filter Ind Lo

Trans. 2 Leakage L,

56 pH

12 pH

Nom. Load Current I, 5 A

3rd Passive Filter 42 pF

27 mH

781

Load Ind. LL 28 mH

UPFC Filter Cap CO 800pF

Trans. 2 Turn Ratio 1:2.13

Nominal Voltage V, 22 0 v

5th Passive Filter 7.1 pF

57 mH

Fig. 7. Simplified circuit of UPFC as harmonic isolator (dc energy added at dc bus, Vd).

IGBT Current Limit 10 A Load Firing Angle 95"

7th Passive Filter 3.56 pF 11 DC Bus V, I50 Vdc

15 7 m~

Load RL' 110-1662 11'Adjusted to keep I, constant

+Vd

T2&- T ,,O o I 7 $ ~i 2

IGBTInv.2S0

- 4l VPq

Fig. 8. Block diagram of simpli stic harmonic isolator and UPFC.

frequency voltage { 1.25 kV} injected at Vpq.The UPFC

operates simultaneously in modes 2, 4, and 5.

B. Power Flow and Rating Calculations

When considering the power ratings of the different com-

ponents in the simulation cases, the power rating of the UPFC

is mainly a function of the phase shift effort required in the

power flow control strategy. The power requirements for the

UPFC in the harmonic isolating mode is only a fraction of

the total power system requirements when passive filters are

also integrated. The minnmum dynamic response of the UPFC,

however, is determined by the harmonic spectrum of the load




782 IEEE

to be isolated. Table IV shows the power flows and rating

requirements for Invertiers 1 and 2 in the different simulation

cases. In Case ii, the eiquivalent network impedance, Z,,as

a resistive and capacitive portion, which indicates the amount

of active power removed from the system at point V.,, and

injected back by means of I q .

v. EXPEIRIMENTAL VERIFICATION

A. Power Electronic System f o r Experimental Verification

To evaluate the concept of harmonic isolation using a unified

power controller experimentally, a small single phase exper-

imental UPFC has been developed using an IGBT inverter.

The basic experimental converter system is shown in Fig. 7

and represents a portion of the system shown in Fig. 4(c). In

Fig. 7, energy is added at the dc bus to maintain a constant

link voltage, V d .

The parameters of the experimental system are included

in Table V and refer to Figs. 7 and 4(c). Only Inverter

2, reference to Fig. 4(c), is integrated using IGBT devices

with associated gate-drives and controllers. The active poweris directly supplied to1 the dc bus V d , from a separate dc

power supply. Due to the harmonic isolator topology and

series transformer, a half-bridge power electronic converter

implementation was adequate for this application. The IGBT

inverter is rated for 100 V, 10 A. Controller inputs are the

desired angle, S , and Ithe instantaneous values of sending-end

voltage, v, ( t ) , nd receiving-end voltage, vT z t ) . he passive

filters are designed as tuned harmonic traps for the third, fifth,

and seventh current harmonics and are connected on the load

voltage bus Vi.The single phase implementation also requires

a third harmonic passive filter.

B. Controller fo r UPFC as Harmonic Isolator

A simple analog controller was implemented to derive the

reference signal for ' u ~ ( ~t ) . he block diagram for this simple

controller is shown in Fig. 8. Inputs to the controller are the

angle reference S:,, the receiving-end voltage, v,(t), and the

sending-end voltage, v s ( t ) .The control function is shown in

(2).

vpq( t ) A . sin(& + &,) - B . v.(t)

A;B = Constants. (2)

The inner control loops shown in Fig. 8 force the reference

voltage and current to the desired values.

Case I-UPFC as Harmonic Isolator: No fundamentalphase shift = 0) is implemented to illustrate the UPFC

as a harmonic isolator. There is therefore no compensation of

the line impedance X , , and the experimental results in Fig. 9

show the operation of the UPFC mainly as harmonic isolator.

In most cases the frequency spectra (in dB) of the waveforms

are also included to indicate the effect of the harmonic isolator

on the waveforms. For this case, the power rating of the UPFC

is very small compareld to the rating of the transmission line

since mainly harmonic isolation is considered. This operating

mode corresponds to the defined mode 5 in Table 11, and

TRANSACTIONS ON POWER ELECTRONICS, VOL. 11 , NO . 6, NOVEMBER 1996

-

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..................... ................................ . .

. . . . . . . > . . . . . . . . . . . . . . . . : . . . . . .i . . . . . . . . . . . . .. ...................

(d )

L t , s

( e ) (f)

Fig 9 Expenmental results of UPFC as harmonic isolator (zero phasecompensation) (a) V, top}, V,,{bottom} - 100 V/div, 5 mddiv,

dB, 10 dB/dlv; 120 Hz/dlv, (c) FFT{VrZ};A = 32 dB, 10 dB/dlv;120 Hz/div, (d) V,, - 2 V/div, 5 ms/div; (Vpq(rms) = 1 76 V) , ( e )

FFT{V,,}, A = 1 5 dB, 10 dB/div, 120 Hz/div, and (f) V,,{top},

V,'{bottom} - 0 0 V/div, 5 ms/div, V,,,,,,) = 2 13 4 V; Vi(rms)= 2 1 1 7

V,,,,,) = 2 01 4 V , VT/7L(rrllb)21 3 4 V , 5:4 O , (b) F F T{V T} ,A = 21

v, 6 E oo

simulation Case 1. No fundamental component is visible in

Fig. 9(d).

The effect of the harmonic isolation is evident from thesuppression of the third voltage harmonic from 21 dB to 32

dB in V,, shown in Fig. 9(a)-(c).

The voltages at the isolation point V,,, and receiving end,

Vi, after harmonic isolation have no noticeable phase shift,

while the root mean square receiving end voltage has been

raised to 213 V from the value of 201 V before harmonic

isolation. The system currents and associated spectra are

plotted in Fig. 9(h)-(k). The passive filter current is mainly

responsible for the large reduction of the transmission line

current from 4.8 A to 3.23 A.




ENSLIN et al.: OPERATION OF UNIFIED POWER FLOW CONTROLLER 783

(k)

Fig. 9. (Continued.) Experimental results of UPFC as harmonic isolator(zero phase compensation): (g) FFT{V'r}; 6 = 29 dB ; 10 dB/div;120 Hz/div, (h) I L {top}; I , {center}; I~{bottom}-5 Ndiv; 5 ms/div;IL( rms )4.8 A ; I,(,,,,,) = 3.23 A; IF(,,n,) = 4 . 3 1 A; CYL= 95', (i)

FFT{IL}; A = 7. 8 dB; 10 dB/div; 120 Hz/div, (i) FFT(IA}; A = 1 2 . 5dB; 10 dB/div; 1 20 Hz/div, and (k) FFT(I&}; A = 6.25 dB ; 10 dB/div;

120 Hz/div.

While the concept of harmonic isolation is clearly demon-

strated by the results in Fig. 9, it should be noted that the

dynamic range of V,, suffers from a low dc bus voltage during

the experiment. Thus, appropriate selection of V d , and filter

components Lo and CO , ill allow for further improvement of

waveform quality on the isolated bus, VTt.Case 2-UPFC as Harmonic Isolator and Fundamental

Phase Shifter: Fundamental phase shift (S:, = 4") is added

to the injected voltage, Vpq,o illustrate the operation of the

UPFC as harmonic isolator and fundamental phase shifting

device. In this case, line impedance, X , , is fully compensated,

leaving V , in phase with V:. The experimental results of

Fig. 10 show the operation of the UPFC as harmonic isolator

and phase shifter integrated into one power electronic device.

Frequency spectra (in dB) for some waveforms are also

included to indicate clearly the effect of the harmonic isolator

(e )

Fig. 10. Experimental results of UPFC as harmonic isolator and phase shifter(full X , compensation). (a) Kt - 100 V/div; 5 ms/div; V,,(,,,) = 217.1

V ; 6,,, = 3.4', (b) FFT{VTi}) ;A = 31 dB; 10 dB/div; 120 Hz/div, (c)

FFT{V,,}; A = 1 0 dB; 10 dB/div; 120 Hz/div, (d) V, - 10 V/div; 5ms/div; l'p/ps(rms)12 .4 V, and ( e ) V,{top}; VT/,l{bottom}- 00 V/div; 5

ms/div; V,(,,,,,, = 215.4 V; V:,rIr,s, = 2 1 9 . 3 V ; 6,, % 0'.

on the waveforms. The power rating of the UPFC is still small

(but larger than in Case 1) compared to the rating of the

transmission line, since harmonic isolation and phase shifting

modes are integrated.

The large fundamental component in V,, is visible in

Fig. 10(d). This fundamental component is necessary to con-

trol the required phase shift between V, and Vi. As shown in

Fig. 10(e), the phase shift between V, and Vi has been reduced

to nearly zero. The large fundamental component is shown in

the FFT spectrum of Fig. 10(d). A 31 dB reduction (from the




184 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 11 , NO . 6, NOVEMBER 1996

original 21 dB) of the voltage harmonics is still maintained in

this mode of operation while performing fundamental phase

shift control. System currents are maintained at the same levels

as in the previous test case of Fig. 9.

VI. SUMMARY AND RECOMMENDATIONS

The UPFC provides; for excellent control flexibility in ac

transmission systems by allowing for static VAr compensation,

series compensation, and phase shift using the same installed

power electronic hard,ware. The present paper has extended

UPFC operation to provide for harmonic isolation in the pres-

ence of nonlinear loads. EMTP studies outline the principle

of operation for pure isolation purposes and mixed-mode op-

eration. The latter mode incorporates harmonic isolation with

the traditional UPFC modes of operation. Experimental results

are provided for a low power laboratory prototype. While the

practical results shown, are far from optimum, they serve well

to illustrate the concept developed. Future work will address

the optimization of isolation performance by improving theUPFC dynamic range. Three phase implementations at more

realistic power ratings will also be investigated.

As illustrated in the paper, the UPFC does not require a

substantial increase in converter kVA to isolate significant

harmonic loads when used in conjunction with appropriate

passive filters. For example, a converter rating of 120 kVA

was shown to be suitable for harmonic isolation of a 1.3 MVA

load while providing for fundamental phase shift and voltage

control of 0.2 p.u. Thus, UPFC operation including harmonic

isolation provides for the optimum use of installed converter

kVA and offers potentially attractive cosVperformance ratios.

Present day power device limitations will initially limit the

UPFC harmonic isolator concept to several MVA to achieve

the desired switching frequencies of several kHz. There are

numerous applications at the distribution level as well as

for industrial loads, however, where this concept can already

be implemented. Eventually, converter implementations seem

feasible for high power applications, such as isolation of

harmonics between two power systems while providing for

fundamental power flow control. Future work will address

application specific design advantages and tradeoffs for the

UPFC when compared to other, more conventional FACTS

devices, such as the TCSC. For example, the absence of

capacitor based subs,ynchronous resonance may make the

UPFC with harmonic: isolation capability a very attractive

candidate for systems with high levels oE thermal generation

in the presence of high power nonlinear loads, such as arc-

furnaces.

ACKNOWLEDGMENT

The authors gratefully acknowledge the contributions of J .

Beukes.

REFERENCES

N. G. Hingora ni, “FACTS-Flexible AC transm ission system,” in P roc.

Fifth 1991 IE E Int. Conf AC an d D C Power Transmission, London,U.K., Sept. 17-20, 1991, pp. 1-7.__ , “FACTS-Flexible AC transmission system,” presented at EPRIWorkshop on FACTS, Cincinnati, OH, Nov. 14-16, 1990..I. rbanek, R. J. Piwko, E. V. Larsen, B. L. Damsky, B. C. Furumasu,

W. Mittelstadt, and J. Eden, “Thyristor controlled series compensationprototype installation at the slatt 500 kV substation,” IEEE Trans. Power

Delivery, pp. 1460-1469, July 1993.W. A. Mittelstadt, “Considerations in planning use of FACTS devices ona utility system,” presented at EPRI Workshop on FACTS, Cincinnati,OH , Nov. 14-16, 1990.L. Gyugyi, “Unified power-flow control concept fo r flexible AC trans-mission systems,” Proc. Inst. Elec. Eng., vol. 139, pt. C, no. 4, pp.323-331, July 1992.~, “Dynamic compensation of AC transmission lines by solid-state synchronous voltage sources,” presented at the 1 993 IEEE Pow er

Engineering Summer Meeting, Vancouver, Canada, July 1993, no. 93SM 434-1 PWRD.~, “Solid state control of AC power transmission,” presented at theEPRI Workshop on FACTS, Cincinnati, OH , Nov. 14-16, 1990.F. 2. Peng, H. Akagi, and A. Nabae, “Compensation characteristics ofthe combined system of shunt and series active filters,” IEEE Trans. Ind.

Applicar., vol. 29, no. I , pp . 144-152, JadFeb. 1993.N. alboa, D. Sella, R. Penzo, B . Bisiach, D. Cappellieri, L. Malesani,

and A. Zuccato, “Hybrid active filter for parallel harmonic compensa-tion,” in Proc. European Power Electronics Con$, 1993, pp. 133-138.S. Bhattacharya, D. M. Divan, and B. B. Banerjee, “Synchronous frameharmonic isolator using active series filter,” in Proc. EPE-91 Con$,

Firenze, Italy, Oct. 1991.-, “Control and reduction of terminal voltage total harmonicdistortion (THD) in a hybrid series active and parallel passive filtersystem,” in Proc. PESC -93 Con$, Seattle, WA, June 20-24, 199 3.J. H. R. Enslin, J . D. Van Wyk, and M. NaudC, “Adaptive, closed-loopcontrol of dynamic pow er filters as fictitious power com pensators,” IEEE

Trans. hd . lectron., vol. 37, no. 3, pp. 203-211, June 1990.G. L. Van Harmelen and J. H. R. Enslin, “Real-time, dynamic controlof dynamic power filters i n supplies with high contamination,” IEEE

Trans. Power Electron., vol. 8 , no. 3, pp. 301-308, July 1993.Y . Wang, R. R. Mohler, R. SpCe, and W. Mittelstadt, “Variable structureFACTS controllers for power system transient stability,” IEEE Trans.

Power Syst. , vol. 7, no. I , pp. 307-313, 1992.

Johan H. R. Enslin (M’85-SM’92), fo r a photograph and biography, see p.697 of the September I996 issue of th is TRANSA CTIONS.

Jian Zhao received the M.S. degree from the Insti-tute of Special Electrical Machines at the ShenyangPolytechnic University, Shenyang, P.R. China, in1988.

From 1988 to 1991 he was a Lecturer at theDalian Institute of Technology, Dalian, P.R.C. Dur-ing 1991, he was a Research Assistant at the Univer-sity of Catania in Italy. In 1992 he was a ResearchAssistant at the University of Cape Town in SouthAfrica. Since 19 92, he has been a Ph .D. student at

the University of Stellenbosch in South Africa. Hisresearch interests include PM m otors, linear induction motors, DSP control ofsynchronous reluctance drives, and flexible ac transmission systems.

Ren6 Sp6e (S’84-M’%-SM’92), for a photograph and biography, see p. 697of the September 1996 issue of th is TR ANSACTIONS.

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Operation of the Unified Power Flow