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NIETJOURNALoFENGINEERING&TECHNOLOGY

Winter 2011coNcoL{)

I(J)

NNNZ(j)(j)STUDY AND SIMULATION OF THE

UNIFIED POWER FLOW CONTROLLER(UPFC) IN POWER SYSTEM

Ragini Malviya'

AbstractThe main objectives of Flexible AC TransmissionSystems (FACTS) devices are to increase thetransmission capacity of lines and to control thepowerflow over designated transmission system. FACTSdevices can perform all objectives of reactive powercontrol and voltage control required for transmissionand lines. Several schemes of flexible ACtransmission systems FACTSare in use today. One ofthe most important FACTS devices is the UnifiedPowerFlow Controller (UPFC),which is used for seriesvoltage injection in desired phase as shown in Fig 1.The UPFC is a combination of a static Compensator(STATCOM) and a static synchronous seriescompensator (SSSC), which are coupled via acommon DC link.

'Department of Electrical EngineeringNJET, Gr. Noida

Introduction

The UPFC is a device, which can controlsimultaneously all the three parameters of linepower flow which are line impedance, voltageand phase angle. The UPFC improves terminalvoltage regulation, series capacitorcompensation and transmission angleregulation. This paper explains the controlscheme and comprehensive analysis of aunified power flow controller (UPFC). A MATlASprogram using SIMULI N K/SI M POWERSYSTEMS toolboxes is developed for simulationof UPFC. This developed simulation technique isfound to be very effective and it enables us tostudy and investigate how the UPFC can affectthe transmission system using the series voltageand shunt current injection. It is possible todemonstrate with this simulation that the UPFCcan improve the system characteristics and givethe best transient and dynamic stability. It alsoimproves the power factor. Some cases areinvestigated and studied such as the applicationof the UPFC to control voltage and power flow.These cases are tested for a power system withvarying active and reactive power requirementsof load. In all cases, the performance of thesystem was analyzed, tested and studied toindicate voltages, currents and powerperformance and shown to be satisfactory.

Unified Power Flow Controller (UPFC) is ageneralized Synchronous VoltageSource(SVS), represented at the fundamental(power system) frequency by voltage phasorVpq as shown in fig 2 with controllable """"~ •..••

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TransmissionLine vV"VN"~~~----~~~----~~~~~~~

I I Supply transformer /' Series transformerllJ Convertor 1 Convertor 2 ~

at the DC link [1]. The reactivepower exchanged at the AC ofconverter 2 terminal isgenerated internally by theinverter [2]. Converter 1supplies or absorbs the realpower demanded by converter2 at the common DC link. ThisDC link power is convertedback to AC and coupled to thetransmission line via a shuntconnected transformer.Converter 1 can also generateor absorb controllable reactivepower and can provideindependent shunt reactivecompensation for the liner(voltage control). Converter 2controls the magnitude and the

angle of Vpq which controls voltage at thereceiving end and the power flow. The basicfunctions of the UPFC are terminal voltageregulation, series capacitor compensation andtransmission angle regulation.

The UPFC is an extremely powerful and versatiledevice for power flow control. The capability ofchanging all transmission parameters affectingpower flow simultaneously by UPFC andequation pertaining to Real f(P) and Reactive(Qr) are as follows:

m ~.~~ .• • II. ; )~ ,r .~ ~~ ,r );

-~ ,~~

I~ ac-~-

;Ir •~ ;Ir •~ ,I. ,III. I ,1. • III. • ,1. .11I.

ured j I r I Iles CONTROL Eeter

MeasVariabParamSettingsFig 1./mp/ementation of UPFC by two back-to-back voltagesource convertors

magnitude Vpq (osvpqsvpqmax; and angle2n, in series with the transmission line, asillustrated for the usual elementary two-machinesystem (or for two independent systems with atransmission link intertie). In the UPFC, the realpower exchanged is provided by one of the endbuses(e.g.,the sending-end bus), as indicated inFig. 1 --- ..•••..

v.

Fig 2. Phasor diagramrepresentation of UPFC

II. Unified power flow controller

The proposed implementation of the unified flowcontroller, using two voltage source invertersoperated from a common DC link capacitor, isshown schematically in Fig. 1. This arrangementis actually a practical realization of an AC to ACpower converter which independently controlsinput and output parameters. Converter 2 injectsan AC voltage Vpq with magnitude and angle, viaan insertion transformer. The real power at the ACterminal of the insertion transformer is convertedby the shunt inverter into DC power that appears

r : "Q = v (Vs+Vpq-Vrj*) r r l jX

PlfVPqQ=O__v (Vs-VrJ*- j r r jX

With Vpq *0

( J* •v -v vr pq

P-j"Q =V .s.:»;+-xr r jX - j

Substituting

j8/2 (8"" 8)V, = Ve = V COS"2-1S111"2

NIETJOURNALoFENGINEERING&TECHNOLOGY

v - )'6 12 V ( '6 .. '6)r = Ve = cos2- 1SIn2

and

v = VfXI fXI

)('6/2Xp) {('6 h (8 )}= Vpq cas 2 +P )' )sin "2+P

and

where

andv2

Q (8)=--(1-00;8)or X

These equations can be used to show thevariation P and Qr for different values of Vpq and0=0·, 0=3070=60·& 0=90· is

respectively.

III. Control modes of UPFC

Basically, the three modes of control areSTATCOM, SSSC and UPFC

• Unified power flow controller (UPFC) mode

When the shunt and series converters areinterconnected through the DC bus and whenthe switches between the DC buses of the shuntand series converter are opened, two additionalmodes are available:

• Shunt converter operating as a Static

Synchronous Compensator STATCOM

controlling voltaqe :

A static synchronous generator operated asshunt-connected static var compensator whosecapacitive and inductive output current can becontrolled independent ofthe ac system voltage

• Series converter operating as a Static

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Synchronous Series Capacitor (SSSC)

controlling injected voltage, while keepinginjected voltage in quadrature with current. TheSSSC is one of the most recent FACTS devicesfor power transmission series compensation. Itcan be considered as a synchronous voltagesource as it can inject an almost sinusoidalvoltage of variable and controllable amplitudeand phase angle, in series with a transmissionline.

Q p

Controllable region

p

1.5

.P"",(5)

I

(a)6=0"

Q. PControllable region

Fig3:Control region of the powerP and Or with UPFC

(),. p

Controllable region

,=Q.¥BI-------: -~I_~=~~'~'-----I Ei~J1LI_65km 111

doublecircuit 899 MW 28 Mvar

VI I I Irnu 796 MW 15 Mvar

Trl: 1000 MVA uk"230kV/500kV "1"'~ Shun1500 kV 100 MVA

" Q Series 100 MVA10'(, Injection Tr2 800 MVA [~~ IUPFC 230 kV/500 kV "1"'.

" 0Bypass rr------. Trip m III 587 MW.27 Mvar ~w - 1279 MW

1l---~IBypass 687 MW·27 Mvar 1277 MW

:11>-;"~-=:~::;;UPFC A2 il-....IF -I:-a~1====1: :~: ::U :B31Tr~ B5=~B2 L2_SOkm B_UPFC '- __ -' 500 kVrwo r 15000 MVA, OOMW

o

Controllable region(d) 0=90·

Fig3:Control region of the powerP and Or with UPFC

500MW500MW

95MW197MW

500 kV

589MW690MW

Natural power flows(Bypass breaker closed)are shown In red notes.

Power flows with UPFC(Pref=687MW. Qref=-27 Mvar)are shown in blue notes. Qref(pu)

P Pref (pu)

UPFC 0 Oref(pu)

Measurements .V:;:coo::,:v="_ma::,,9C'!(p::!.U),..--,----.Vconv..P/lase(<leg.)

UPFC

EJpowerguide

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v. SIMULATION OF UPFC

A UPFC is used to control the power flow in a 500kV /230 kV transmission system as shown byFig.4. The system, connected in a loopconfiguration, consists essentially of five buses(81 to 85) interconnected through threetransmission lines (L1, L2, L3) and two 500kV/230 kV transformer banks Tr1 and Tr2. Twopower plants located on the 230 kV systemgenerate a total of 1500 MW which is transmittedto a 500 kV, 15000 MVA equivalent and to a 200MW load connected at bus 83. Each plant modelincludes a speed regulator, an excitation systemas well as a power system stabilizer (PSS). Innormal operation, most of the 1200 MWgeneration capacity of power plant #2 isexported to the 500 kV equivalent through two400 MVA transformers connected betweenbuses 84 and 85. For this demo we areconsidering a contingency case where only twotransformers out of three are available (Tr2=2*400 MVA = 800 MVA). The load flow showsthat most of the power generated by plant #2 istransmitted through the 800 MVA transformer

loooMWtooo uw

230 kV

196.6 I689.1 I

681 I796 Ivpos. seq. 81 82 B3 B4 as

12/1 IVPQMeasurements

P B1 82 B3 B4 as (MW)

a 818283 B4 B5 (Mvar)l..__ J"-"-'-::::"::::'==:.:::L---:v'PQ LinesActive Powers (MW)

BI B2B3B485

UPFC (Phasor Model)Control of Power Flow using a Unified Power Flow Controller (UPFC)

info

NIETJOURNALoFENGINEERING&TECHNOLOGY

bank (899 MW out of 1000 MW) and that 96 MWis circulating in the loop. Transformer Tr2 istherefore overloaded by 99 MVA. The exampleillustrates the capability of UPFC to relieve thispower congestion. The UPFC located at theright end of line L2 is used to control the activeand reactive powers at the 500 kV bus B3, aswell as the voltage at bus B_UPFC. The UPFCconsists of two 100 MVA, IGBT-based,converters (one shunt converter and one seriesconverter interconnected through a DC bus).The series converter can inject a maximum of10% of nominal line-to-ground voltage (28.87kV) in series with line L2.

V. Results and discussionsIn this control mode as shown in Fig 5 thevoltage generated by the series inverter iscontrolled by two external signals Vd, Vqmultiplexed at the "Vdqrei'input and generated inthe Vdqrefmagenta block. For the first fiveseconds, the Bypass breaker stays closed, sothat the PO trajectory stays at the (- 27Mvar, 587MW) point. Then when the breaker opens, themagnitude of the injected series voltage isramped, from 0.0094 to 0.1 pu. At 10 s, the angleof the injected voltage starts varying at a rate of45 deg./s. The voltage profile with UPFC isshown by Fig 6

UPFC Controllable Region

-0.262 ... . ... .... ....

-0.264

-0.266:§;a -0.268

-0.27 [

-0.272 f .

-0.274 L- '-- ~ __ ____'

5.5 6 6.5P (pu)

Fig 5: Controllable regionVpos, seq. 81 8283 8485

7

r-- -r-- - ---t------· -~-.- ----.

, ,0.995 ...•....... ;.. - ....f· ...., ... ---'r::"

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Vpos. seq. 81 82 83 84 85 (MW)

1500

500-' _." --!

oVpos. seq. 81 82838485 (MW)

o------; . _. -_. r-- _. -- -.~.--50

-100

_·_·t--- ---i

.•. j

-150 L--.i__ ~......i.._...L..____'__...L-.----' _ _'_:____:'::__::

o 2 4 6 8 10 12 14 16 18 20

Fig. 6: Voltage profile with UPFC

VI. Conclusions

The present paper deals with the study ofsteady-state behaviour of electrical networksequipped with UPFC. Compared to simplifiedmodels, the model considered for the UPFCrepresents improvement in systemperformance. The simulation results are close tothe real behaviour of such a system. This modelhas permitted us to investigate the power qualityand the performances of the UPFC. Theinteraction between the UPFC and thetransmission line is analyzed and carried out onthe model of power system.

DO

References1. N. G. Hingorani and L. Gyugyi,

Understanding FACTS, Concepts andTechnology of Flexible AC TransmissionSystems. Piscataway, NJ: IEEE Press,2000Gyuygi L.,"Unified power flow controller

Concept for Flexible AC TransmissionSystems", IEEE Proceeding, vol.-139, no.-4,pp.323-331, July 1992.

Keri AJ.F., Mehraban AS., Elriachy A.,Lombard X. and Edris A , "A Unified PowerFlow Controller (UPFC): Modelling andAnalysis", IEEE Transactions on PowerDelivery, vol.-14, no.-2, pp. 648-654, April

Sun G.,Wei Z., "Power system state estimationwith unified power flow controller", Third

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3.

4.

5.

International Conference on Electric UtilityDeregulation and Restructuring and PowerTechnologies, pp.775 -778, April 2008.

Schoder K., Hasanovic A., Feliachi A. and AzraH., "Load-Flow and Dynamic Model of UPFCwithin the Power System Toolbox (PST)", 43rdIEEE Midwest Symposium on Circuits andSystems, vol. 2, pp. 634-637, August 2002,

Guo J., Crow M.L., Sarangapani J., "AnImproved UPFC Control for OscillationDamping", IEEE Transactions on PowerSystems, vol.-24, no.-1, pp.288-296, Feb.2009.

6.

7. Zarghami M., Crow M.L.,Sarangapani J.,Atcitty S., "A Novel Approach to InterareaOscillation Damping by Unified Power FlowControllers Utilizing Ultracapacitors", IEEETransactions on Power Systems, vol.-25, no.-1,ppA04-412, Feb 201 O.

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8. Papic I., Zunko P.and Povh 0, "Basic Control ofUnified Power Flow Controller", IEEETransactions on Power Systems, vol.-12, no.-4, pp.1734-1739, August 1997.

Mihalic R., Povh D. and Zunko P, "Improvementof Transient Stability using UPFC", IEEETransactions on power delivery, vol.-11, no.-1,pp. 485-492, August 1996.

Padiyar K.R. and Kulkarni A.M.,"ControlDesign and Simulation of UPFC", IEEETransactions on Power Delivery, vol.-13, nO.-4,pp. 1348-1354, Oct. 1998.

9.

10.

11. L. Gyugyi, "Dynamic compensation of actransmission lines by solid-state synchronousvoltage sources," IEEE Trans. Power Del., vol.9, no. 2, pp. 904-911, Apr. 1994.

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