facts controllers. kalyan k. sen

300 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 18, NO. 1, JANUARY 2003

Comparison of Field Results and Digital SimulationResults of Voltage-Sourced Converter-Based

FACTS ControllersKalyan K. Sen, Senior Member, IEEE,and Albert J. F. Keri, Senior Member, IEEE

Abstract—This paper compares the field results ofVoltage-Sourced Converter(VSC)-based Flexible AlternatingCurrent Transmission Systems(FACTS) Controllers, such asSTATic synchronous COMpensator(STATCOM), Static Syn-chronous Series Compensator(SSSC), andUnified Power FlowController (UPFC) with that obtained from the computer modelsof the FACTS controllers using an Electro-Magnetic TransientsProgram (EMTP) simulation package. The operational results ofthe actual equipment include the control of the line voltage andthe control of the real and reactive power flow in the line. Thecorrelation of the results establishes the validity of the models.The protection scheme of the FACTS controllers during faults andcontingencies is also described. The simulation results can be usedto accurately predict the behavior of an actual controller.

Index Terms—Converters, FACTS, load flow control, power elec-tronics, power system transients, power transmission, power trans-mission control, UPFC.

I. INTRODUCTION

F LEXIBLE Alternating Current Transmission Systems(FACTS) controllers, namelySTATic synchronous COM-

pensator(STATCOM), Static Synchronous Series Compensator(SSSC), and Unified Power Flow Controller (UPFC), are usedto control the bus voltage and/or the power flow throughan electrical transmission line. The UPFC consists of twosolid-state Voltage-Sourced Converters (VSCs), which areconnected through a common dc link capacitor. Each converterhas a coupling transformer with the utility interface. The VSC1,known as STATCOM, injects an almost sinusoidal current, ofvariable magnitude and in quadrature with the line voltage, atthe point of connection. The VSC2, known as SSSC, injectsan almost sinusoidal voltage, of variable magnitude and inquadrature with the line current, in series with the transmissionline. When the STATCOM and the SSSC operate as stand-alonecontrollers with open dc link switch, they exchange almostexclusively reactive power at their terminals. When both VSCsare operating together as a UPFC with closed dc link switch,the injected voltage in series with the transmission line canbe at any angle with respect to the line current; therefore,the exchanged power at the terminals of the SSSC can bereactive as well as real. The real power exchanged by the

Manuscript received February 13, 2002.K. K. Sen is with the Westinghouse Electro-Mechanical Division Technology

Center, Mount Pleasant, PA 15666 USA (e-mail: [email protected]).A. J. F. Keri is with American Electric Power, Gahanna, OH 43230 USA

(e-mail: [email protected]).Digital Object Identifier 10.1109/TPWRD.2002.804012

Fig. 1. Unified Power Flow Controller in a simplified network.

SSSC with the line flows bidirectionally to the line throughthe STATCOM and the common dc link capacitor. In addition,the STATCOM carries a reactive current to regulate the busvoltage independently. The concept of a shared dc link betweena shunt-connected VSC and a series-connected VSC was firstintroduced in theActive Power Line Conditioner(APLC) [1],[2] for distribution power level applications. The same conceptwas implemented in the UPFC [3]–[5] for transmission powerlevel applications.

The objective in this paper is to demonstrate that the simula-tion results of a VSC-based FACTS controller can be used to ac-curately predict the behavior of the controller in the field. Thisdemonstration is substantiated with the comparison of resultsfrom the computer simulation and actual field measurements.The operation of the model is verified with the model connectedto a simple 2-bus network. Although the simulation and field re-sults correlate qualitatively, a proper representation of field datarequires, in general, the simulation of 100 or more buses, whichis out of the scope of this paper.

II. DESCRIPTION OF THEMODEL

A power system network has been set up for simulation inconjunction with the VSC-based FACTS controllers. The net-work is a simplified 2-bus model as shown in Fig. 1. The net-work consists of an equivalent source voltage,, and a sourcereactance, , at the Inez substation bus, INEZ, an equivalentsource voltage, , at the Big Sandy bus, and a line reactance,

, between Big Sandy and the bus, BUS05. This model isused to verify the response of the controller following a stepchange in the reference of the control inputs.

The description of the FACTS controller models [6]–[8], inEMTP and the application of the models in the AEP networkare given below. The objective for this model is to characterizethe behavior of the UPFC as viewed from the network terminals,especially the dynamic behavior during disturbances and faultson the power system. The model provides the implementation ofthe automatic power flow controller using the series-connected

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SEN AND KERI: COMPARISON OF FIELD RESULTS AND DIGITAL SIMULATION RESULTS 301

Fig. 2. Unified Power Flow Controller.

VSC2 and bus voltage control using the shunt-connected VSC1,including protective limits on series voltage injection, and over-current and overvoltage limits for both VSCs.

The UPFC model in EMTP is shown in Fig. 2. Themodel consists of two multi pulse harmonically neutralizedvoltage-sourced converters,VSC1andVSC2, two magnetic cir-cuits,MC1 andMC2, shunt coupling transformer,XFSHN, andseries coupling transformer,XFSRS, shunt breaker,SHNBRK,series breaker,SRSBRK, dc Link Switch,DCLS, bypass breaker,BYPBRK, line breaker,LBR, current and voltage sensors, and acontrol and protection unit.

The model is a virtual representation of the actual UPFC atits terminals. Two idealized twenty four-pulse three-level VSCs,each of which is rated at 160 MVA, are connected at theirdc link capacitor terminals. The control unit implements basiccontrol and protection schemes. The gating signals for the polevalves are generated “on the fly.” The ideal pole voltages aremathematically combined to produce two three-phase harmoni-cally neutralized converter voltage sets,and . The VSC1 isoperated at a fixed “dead angle” to produce a forty eight-pulsevoltage source and the VSC2 is operated as a twenty four-pulsevariable magnitude voltage source. The coupling transformersinclude a lumped leakage reactance that is an equivalent of allthe leakage reactances of the intermediate magnetic circuits andthe main transformers. The lumped leakage reactance is 15% ofthe converter rating and divided equally on both primary andsecondary sides. The dc link capacitor is modeled by a singlelumped capacitor (14.1F, 190 kV) sized in accordance withother model parameters, including the transformer leakage reac-tance, to preserve the same dynamics in the model as in the realsystem. The voltage, , across the dc link capacitor is main-tained by the instantaneous power balance equation at both ACand dc sides of the two converters. Special effort has been madeto model the rectifier action that charges the dc link capacitor tothe peak of the AC bus voltage when the VSC1 gating is inhib-ited. At that moment, the ideal VSC is replaced by a six-pulserectifier bridge circuit with appropriate snubber circuits. The dclink capacitor overcharges until the VSC1 gating begins, whichdischarges the dc link capacitor to the normal level. The voltagevariation across the dc link capacitor is correctly represented andalso represented is the operation of the voltage clamp that dis-charges the dc link capacitor partially when certain overvoltage

Fig. 3. Modeling structure of a FACTS controller.

threshold is exceeded. The model does represent the behavior ofthe FACTS controller in the presence of negative sequence andharmonic line voltages and currents.

In the model, the bypass of VSC2 is a representative of anelectronic bypass on the secondary side using the VSC2 valves.The behavior of the UPFC during disturbances is in large partdetermined by a number of threshold levels and delay times.These numbers determine the current and voltage thresholds andtiming delays associated with protective action and they are setin the first instance to values that seem reasonable for the ac-tual equipment. This model is well suited for system transientsstudies.

Fig. 3 shows the modeling structure of a FACTS controller.The inputs from the voltage and current measuring units, thebreaker status and the operator—voltage reference, voltageslope (also known as droop) and the real and the reactive powerreferences are fed to the control & protection unit. The outputof the control & protection unit is a set of low voltage opticalsignals. These optical signals are fed to theValve InterfaceCircuit, which converts them into electrical gating signals thatturn ON and OFF the semiconductor switches of the highpower VSC, which is connected to the power system networkthrough its coupling transformer. The output of the control &protection block is also another set of low voltage electricalsignals that operate the interposing relays, which operate thehigh power breakers and disconnect switches that connect theFACTS controller with the power system network.

Fig. 4 shows the control block diagram of the UPFC, whichcan be divided into two parts: the control of the VSC1 as a shuntcompensator and the control of the VSC2 as a series compen-sator. An instantaneous 3-phase set of transmission line volt-ages, , at BUS 1 is used to calculate the reference angle,,which is phase-locked to the phaseof the bus voltage, .An instantaneous 3-phase set of measured VSC1 currents,, isdecomposed into its real or direct component,, and reactiveor quadrature component, , respectively. An instantaneous3-phase set of measured line currents,, is decomposed into itsreal or direct component, , and reactive or quadrature compo-nent, , respectively. An instantaneous 3-phase set of measuredline voltages, , at BUS 1 is decomposed into its direct com-ponent, , and quadrature component, , respectively. Themagnitude of the BUS 1 voltage, , is calculated.

The control for the VSC1 works in such a way that the desiredbus voltage magnitude reference, , (adjusted by the droop



Fig. 4. Control block diagram of a Unified Power Flow Controller.

factor, ) is compared with the BUS 1 voltage magni-tude, , using an outer voltage control loop and the erroris passed through an error amplifier, which produces the ref-erence quadrature component, of the VSC1 current. Thedroop factor, , is defined as the allowable voltage errorat the rated reactive current flow through the VSC1. The refer-ence quadrature component,, is compared with the measuredquadrature component, , of the VSC1 current using an innerreactive current control loop and the error is passed through anerror amplifier, which produces a relative angle,, of the VSC1voltage with respect to the BUS 1 voltage. The phase angle,,of the VSC1 voltage is calculated by adding the relative angle,

, of the VSC1 voltage and the phase-locked-loop angle,.This is called “Voltage Control” mode of operation. The dc linkcapacitor voltage, , is dynamically adjusted in relationshipwith the VSC1’s AC terminal voltage. Note that for a particularbus voltage magnitude demand,, if the reference quadraturecomponent, , exceeds the rated current of the VSC thenis limited to 1 pu and the bus voltage is regulated to an inter-mediate value. The controllable range of the bus voltage caneasily be determined by operating the VSC1 with the inner re-active current control loop and varying from 1 pu to 1 pu.The reference quadrature component,, of the converter cur-rent is defined to be either positive if the VSC1 is emulatingan inductive reactance or negative if it is emulating a capacitivereactance. This is called “Reactive Current Control” mode ofoperation. In this case, there is no need for the use of the outervoltage control loop.

The control for the VSC2 works in such a way that the de-sired real and reactive power, and , are compared withthe measured real and reactive power,and , using an au-tomatic power flow control algorithm and the errors are passedthrough an error amplifier, which produces, with respect to thebus voltage, the direct and the quadrature components of the se-ries injection voltage, and , respectively. Next, the mag-nitude of the voltage, , at the output of the VSC2 and its rel-ative angle, , with respect to the reference phase-locked-loopangle are calculated. The phase angle,, of the VSC2 voltageis calculated by adding the relative angle,, of the VSC2 voltageand the phase-locked-loop angle,. This is called “AutomaticPower Flow Control” mode of operation. Note that for a partic-ular real and reactive power, and , demand, if the magni-

Fig. 5. Protection scheme of a Unified Power Flow Controller.

tude of the voltage, , exceeds the rated voltage of the VSCthen is limited to 1 pu and the real and reactive power floware regulated to an intermediate value. The controllable range ofthe real and reactive power flow can easily be determined withthe open loop voltage injection by injecting the rated withinits entire 360 range. This is called “Voltage Injection” mode ofoperation. In this case, there is no need for the use of the auto-matic power flow control algorithm.

The general protection scheme of a UPFC is shown in Fig. 5.When the VSCs are operated as stand-alone compensators, thebasic protection scheme is as follows. The VSC1 stops gatingwhen the instantaneous overcurrent through it exceeds a setvalue or the bus voltage drops below a set value. The VSC2goes into an electronic bypass mode when the instantaneousovercurrent through it exceeds a set value. When both VSCsare operated together as a UPFC, the VSC2 is bypassed whenthe VSC1 stops gating for the following reason. Since the realpower exchanged by the VSC2 with the line flows bidirection-ally through the dc link capacitor and the VSC1, VSC2 must bebypassed when the VSC1 is stopped. The dc link capacitor isprevented from being excessively charged by switching on a re-sistive clamp circuit when the dc voltage rises above a set valueand by switching off when the dc voltage falls below a set value.

III. RESULTS

For the comparison of simulation and field results, stepchanges in the reference of the control inputs and the sub-sequent response have been used. The natural real and thereactive power flow on the Big Sandy to Inez line (representedwith a reactance ) at the bus, BUS05, flowing toward theUPFC are 312 MW and 68 Mvar, respectively. The voltage atthe Inez bus, INEZ, is 0.97 pu.

Fig. 6 shows the changes in (a) Inez bus voltage, (b) line realand reactive power, and (c) shunt converter reactive power dueto the operation of the 160 MVA, 138 kV rated STATCOMonly while the SSSC is injecting no voltage in series with theline. The STATCOM is operated in “Reactive Current Control”mode with a step reference of 1 pu capacitive at 50 ms, 1 puinductive at 175 ms, and zero reactive current at 300 ms. Thevoltage at the Inez substation bus can be varied between 0.91 pu



(a)

(b)

(c)

(d)

Fig. 6. (a), (b), and (c) STATCOM simulation results—controlling voltageat Inez bus—(series converter not operating). (a) Inez bus voltage, (b) linereal power and line reactive power, (c) shunt converter reactive power and(d) STATCOM test results—controlling voltage at Inez bus—(series converternot operating).

and 1.02 pu. The real and the reactive power flow on the BigSandy to Inez line at the Inez substation bus can be varied from293 MW to 325 MW and 0 to 132 Mvar, respectively. When theSTATCOM is operated at 1 pu capacitive current control mode,it delivers 163 Mvar reactive power to the Inez bus. When theSTATCOM is operated at 1 pu inductive current control mode,it absorbs 145 Mvar reactive power from the Inez bus. Fig. 6(d)shows the corresponding test results. These test results verifythe computer simulation results on a qualitative basis.

Fig. 7 shows the changes in (a) Inez bus voltage, (b) line realand reactive power, and (c) shunt converter reactive power dueto the operation of the 160 MVA, 138 kV rated STATCOMand the 160 MVA, 13.33 kV rated SSSC as a UPFC. TheSTATCOM is operated in “Voltage Control” mode to hold theInez bus voltage at 1 pu level at 50 ms. The real and the reactivepower flow on the Big Sandy to Inez line at the bus, BUS05,change from 312 MW to 316 MW and from 68 Mvar to 101Mvar, respectively. At 200 ms, the reactive power flow in theline is brought to zero. While holding unity power factor load onthe line and bus voltage at 1 pu level, the real power flow in theline was varied between 266 and 366 MW. The STATCOM ex-changes an appropriate amount of reactive power with the lineto hold the bus voltage at 1 pu level. Fig. 7(d) shows the cor-responding test results. These test results verify the simulationresults on a qualitative basis.

Fig. 8 shows the changes in (a) Inez bus voltage, (b) line realand reactive power, and (c) shunt converter reactive power dueto the operation of the 160 MVA, 138 kV rated STATCOMand the 160 MVA, 13.33 kV rated SSSC as a UPFC. TheSTATCOM is operated in “Voltage Control” mode to hold theInez bus voltage at 1 pu level at 50 ms. The real and the reactivepower flow on the Big Sandy to Inez line at the bus, BUS05,change from 312 MW to 316 MW and 68 Mvar to 101 Mvar,respectively. While holding the reactive power flow in the lineat 101 Mvar and bus voltage at 1 pu level, the real power flowin the line is varied between 266 and 366 MW. The STATCOMexchanges appropriate amount of reactive power with the lineto hold the bus voltage at 1 pu level. Fig. 8(d) shows the cor-responding test results. These test results verify the simulationresults on a qualitative basis.

Fig. 9 shows the changes in (a) Inez bus voltage, (b) line realand reactive power, and (c) shunt converter reactive power dueto the operation of the 160 MVA, 138 kV rated STATCOMand the 160 MVA, 13.33 kV rated SSSC as a UPFC. TheSTATCOM is operated in “Voltage Control” mode to hold theInez bus voltage at 1 pu level at 50 ms. The real and the reactivepower flow on the Big Sandy to Inez line at the bus, BUS05,change from 312 MW to 316 MW and 68 Mvar to 101 Mvar,respectively. While holding the real power flow in the line at316 MW and bus voltage at 1 pu level, the reactive power flowin the line is varied between 51 and 151 Mvar. The STATCOMexchanges appropriate amount of reactive power with the lineto hold the bus voltage at 1 pu level. Fig. 9(d) shows the cor-responding test results. These test results verify the simulationresults on a qualitative basis.

Fig. 10 shows the changes in (a) Inez bus voltage and (b) linereal and reactive power due to the operation of the160 MVA,13.33 kV rated SSSC. The SSSC is operated in “Reactance



(a)

(b)

(c)

(d)

Fig. 7. (a), (b), and (c) UPFC simulation results—holding unity powerfactor while changing line power. (a) Inez bus voltage, (b) line real powerand line reactive power, (c) shunt converter reactive power, and (d) UPFC testresults—holding unity power factor while changing line power.

Control” mode [7]. First, a 0.5 pu voltage at 50 ms and an ad-ditional 0.5 pu voltage at 175 ms are injected in quadrature and

(a)

(b)

(c)

(d)

Fig. 8. (a), (b), and (c) UPFC simulation results—controlling real power onBig Sandy—Inez line. (a) Inez bus voltage, (b) line real power and line reactivepower, (c) shunt converter reactive power and (d) UPFC test results—controllingreal power on Big Sandy—Inez line.

lagging the line current so that a capacitive reactance is emulatedin series with the line. The real and the reactive power flow onthe Big Sandy to Inez line increase. At 300 ms, a 0.5 pu voltageis injected in quadrature and leading the line current so that an



(a)

(b)

(c)

(d)

Fig. 9. (a), (b), and (c) UPFC simulation results—controlling reactivepower on Big Sandy—Inez line. (a) Inez bus voltage, (b) line real power andline reactive power, (c) shunt converter reactive power and (d) UPFC testresults—controlling reactive power on Big Sandy—Inez line.

inductive reactance is emulated in series with the line. The realand the reactive power flow on the line decrease. Since, the

(a)

(b)

(c)

Fig. 10. (a) and (b) SSSC simulation results—changing line real and reactivepower—(shunt converter not operating). (a) Inez bus voltage (unregulated),(b) line real power and line reactive power, and (c) SSSC test results—changingline real and reactive power—(shunt converter not operating).

STATCOM is not operating, the bus voltage is not regulated.Fig. 10(c) shows the test results corresponding to a capacitivereactance emulation. These test results verify the simulation re-sults on a qualitative basis.

IV. CONCLUSION

FACTS controllers—STATCOM, SSSC, and UPFC, havebeen modeled using an EMTP simulation package. The UPFCconsists of two voltage-sourced converters—one injects analmost sinusoidal current at the point of connection and theother injects an almost sinusoidal voltage in series with thetransmission line. In the UPFC operation, the dc link switchis closed. The injected voltage in series with the line can be atany angle with the prevailing line current, thereby emulatingan impedance in series with the line. The shunt-connectedcurrent source has two components. First, the real component,which is in phase with the bus voltage, carries real power that isexchanged by the series-connected voltage source and losses inthe UPFC. Second, the reactive component, which is in quadra-ture with the bus voltage, emulates an inductive reactance ora capacitive reactance at the point of connection. When the



STATCOM and the SSSC are independently operated, the dclink switch is open. A STATCOM regulates the bus voltageand, in turn, regulates the reactive current flow through it. AnSSSC injects a voltage in series with the transmission line andin quadrature with the line current. The operation of the modelis verified with the model connected to a simple 2-bus network.Although the simulation and field results correlate qualitatively,a proper representation of field data requires, in general, thesimulation of many more buses than the 2-bus network.

REFERENCES

[1] E. J. Stacey and M. B. Brennen, “Active Power Conditioner System,”U.S. Patent 4 651 265, 1987.

[2] M. B. Brennen, “Low cost, high performance active power line condi-tioners,” inThird Int. Conf. Power Quality: End-Use Applicat. Perspec-tives, EPRI, Amsterdam, The Netherlands, Oct. 24–27, 1994.

[3] L. Gyugyi, “A unified power flow control concept for flexible ac trans-mission systems,”Proc. Inst. Elect. Eng. C, vol. 139, no. 4, July 1992.

[4] N. G. Hingorani and L. Gyugyi,Understanding FACTS—Concept andTechnology of Flexible AC Transmission Systems. New York: IEEEPress, 2000.

[5] B. A. Renzet al., “AEP unified power flow controller performance,”IEEE Trans. Power Delivery, vol. 14, pp. 1374–1381, Oct. 1999.

[6] K. K. Sen, “STATCOM—STATic synchronous COMpensator: Theory,modeling, and applications,” inProc. IEEE Power Eng. Soc. WinterMeeting, 1999, pp. 1177–1183.

[7] , “SSSC—static synchronous series compensator: Theory, mod-eling, and applications,”IEEE Trans. Power Delivery, vol. 13, pp.241–246, Jan. 1998.

[8] K. K. Sen and E. J. Stacey, “UPFC—Unified power flow controller:Theory, modeling, and applications,”IEEE Trans. Power Delivery, vol.13, pp. 1453–1460, Oct. 1998.

Kalyan K. Sen (S’83–M’87–SM’01) was born inBankura, WB, India. He received the B.E.E. degree(with first-class honors), the M.S.E.E. degree, andthe Ph.D. degree from Jadavpur University, Calcutta,WB, India, Tuskegee University, Tuskegee, AL, andWorcester Polytechnic Institute, Worcester, MA, allin electrical engineering, in 1982, 1983, and 1987,respectively.

He is currently a Fellow Engineer with theWestinghouse Electro-Mechanical Division Tech-nology Center, Mount Pleasant, PA. He spent three

years as an Assistant Professor at Prairie View A&M University, PrairieView, TX, before joining Westinghouse Electric Corporation’s Science andTechnology Center, as a Senior Engineer, where he was a member of theFACTS development team for nine years. From 1999 to 2001, he worked atABB Power Systems, Västerås, Sweden, and at the Corporate Research Center,Västerås, Sweden. He is the coinventor of the “Sen” Transformer for FACTSapplications. He is also the cofounder of SEN Engineering Solutions, where hepursues his interests in affordable power flow controllers. His interests are inpower converters, electrical machines, control, and power system simulationsand studies.

Dr. Sen is an editor of the IEEE TRANSACTIONS ONPOWERDELIVERY and anIEEE Distinguished Lecturer from the Power Engineering Society.

Albert J. F. Keri (SM’80) received the Ph.D. degreein electrical engineering from University of Missouri,Columbia, in 1972, and the M.B.A. degree from OhioUniversity, Athens, in 1985.

He joined American Electric Power, Gahanna,OH, in 1972 and was involved with protectionand relaying for the first two years. He transferredto the Research Section in 1974 where he hasbeen involved with EMTP, insulation coordinationstudies, system harmonics investigation, field tests,single-phase switching techniques, loss reduction

techniques, and equipment failure analysis. In 1999, he transferred to SystemDynamics Analysis where he has been also involved with planning and stabilitycalculations. He has taught for 14 years a variety of graduate and undergraduatecourses on a part-time basis. He has been a Consultant to power companies inthe U.S., Venezuela, Korea, Brazil, etc. He has authored or co-authored manytechnical papers and patents.

Dr. Keri holds a P. E. License and is the chairman of the IEEE General Sys-tems Subcommittee.


facts controllers. kalyan k. sen

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