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POWER FLOW CONTROL AND LOSS MINIMIZATION WITHUNIFIED POWER FLOW CONTROLLER (UPFC)Alireza Farhangfar", S . Javad Sajjadib , Saeed Afsharniafarhang@ece.ualberta.ca,s.i.saiiadi@,ece.ut.ac.ir, safshar@ut.ac.irUniversity ofAlberta, University of Tehran , University o Tehran

AbstractIn this paper the injection model of UPFC ir used toinvestigate its effect on load flow and loss redu ction inpower system. In this study the New ton-Raphson algorithmis modijied to consider the benefirs of having UPFC in thepowe r system.Simulations have been implemented in M A T U B and theIEEE 14-bus system has been used asa case study. Simula-tions investigate the effect of injecting series voltage withvarious magnitude and phase characteristicson the pawerflow of the system.It also considers the effect ofparallel branch on the volt-age of the related bus. Finally the optimum place or install-ing VP FC in order to have minimum loss in the system issuggested based on the simulation results and the structureof the power system.Keywords: Power system, FACTS devices, U PFC , loadflow , loss reduction

1. INTRODUCTIONThe concept of Flexible AC transmission systems(FACTS) was introduced first by Hingorani in 1988 [I].Since then, different kinds of FACTS devices have beenproposed. One of the most comprehensive FACTS d evicesis UPFC,whicb has been introduced by Gyugiy in 1991The UPFC consists of two series and shunt converterswith AC transmission systems. It is a combination ofSSSC and STATC OM (Fig.1). To investigate d e effect ofthe UPFC on the steady state condition of the system andload flow, different models have been introduced. Thesemodels are usually based on modification of traditional

load flow m ethods [3-81. It is sho wn that by m odeling theUPFC in the form of bus-voltage dependent loads and us-ing the resultant equations in load flow, there is no need tochange the elements o f the Jacobian Matrix [8].

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2. OPERATING PRINCIPLES OF UPFC

The UPFC consists of two voltage-source converters,which are connected back-to-back through a DC capacitor(Fig.1). It injects an AC series voltage into the transmis-sion line and regulates the power flow by controlling theamplitude and phase of the injected voltage. The seriesvoltage converter is connected to d e ransmission line bymeans of a series transformer. The shunt converter can ex-change active and reactive powers with the system, whichenables the system to do shunt compensation independ-entlv.

Con". I1Fig.1: Single line diagram of UPFC.3. MODEL OF UPFC

In this paper the configuration shown in Fig.2a is used tomodel the UPFC [2]. This model has a wide range of ap-plications for investigating the effect of the UPFC on thesystem.

Fig.2a: UPFC model consisting of the series voltage sotme andshunt current source.

Fig.2.b: Vector diagram of the circuit

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In this model, the active and reactive currents of shuntbranch are shown by lp and Iq respectively. The imped-ance o f the series transformer is introduced by Z , .3.1 The M odel of Series Branch

In UPFC, both the amplitude and phase of the seriesvoltage source are controllable. Assume that the voltagesou rce is connected in series to the line betw een the i and jbuses. This series branch can be modeled as an ideal volt-age source V , in series with a reactance X , [8].In Fig. Za, V , represents an ideal voltage source and V )

represents the voltage behind the series reactance. Thevector diagram of the equivalent circuit is shown inFig.2.b.

1Fig. 3: Equivalent Nunun circuit fu r series branch

To obtain the model for the series branch, the voltagesource V , is replaced by the current source 7, = - j b , Y , inparallel with A [ 8 ] .

--This current source injects S an d sj, powers into the i

an d j buses, respectively, where :

These powers are shown as follows:S = V i [ j b s r V i e j 7 ] ' = - b, r V ? sin y - j b , r V ; c o s yAssuming 0, =e.-si, e have_ _ - (3 )- -V j - j b , r Ke ' Y] '= b s r F V j sin(Oy+ y)+ b , r F V j COS(S,+ y ) (4)In the above equations, v, s defined as v, r cwhere o < r s i .According to equations (1) and (2), the series voltagesource is considered as tw o bus-voltage dependent loads.3.2 UPFC O verall Model

The shunt branch is used to supply the active power,which is injected into the system. Hence, this amount ofactive power must be add ed to the equatio n (3).The reactive power of the shunt converter can be con-trolled independently and modeled as a controllable sbunt-reactive power source. Therefore, to obtain the overallmodel, the injected reactive power to the bus i should be

added to the series branch model (3). The overall model isshown in Fig.4.

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Gi = rb,?Vj sin(8, + y )Qsi = rbsc2cos y+Qshunt

Gj = r b p j sin(Oy+y )Q, = - r b s KVj cos(@y+ y)

Fig. 4: Complete model u fUP F C4. SIMULATION RESULTS

In previous section, the UPF C m odel was introduced forload flow studies. In this section, the model is used to con-sider the effects of the UPFC on load flow and powerlosses. The simu lated system is an IEEE 14-bus system asshown in Fig.5 [ 9 ] .

4.1 Effect of UPFC on Load Flow and VoltageSettingTo simulate the effect of the UPFC on power flow, theUPFC was installed between bus es 5 an d 1, and the shuntbranch was connected to bus 5. The effect of the seriesbranch of the UPFC on the transmission power betweenbuses 5 and 1 is sho wn in Fig.6. It can he seen that bychanging the series voltage coeficien t from FO to 1-0.08

and its phase from 0 to 36 0 degrees, the active and reac-tive powers of the line can be co ntrolled by 30 M W and 35WAR, espectively [8].Fig.7 show s the changes in active and reactive power inthe lines between buses 5 and 1 versus series voltage an-gle, y . From the figure 7, it can he seen that at point A,the active power has the minimum value. However atpoint C, the amount of the active power transmitted

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through the lme is maximum in consequence to the factthat the injected series voltage is in alignment with the linecurrent.

01 I~ ) I ~ L S W O S ~ B SFig. 6: The effectof UPFC on the transmission power betweenbuses 5 and 1

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G a m l m lFig.7: Reactive power variations in line between buses 4 an d 5 ,versus angle y .In the case that the series voltage is perpendic ular to theline current @oints B and D), there is no change compar-ing to the non-compensation mode. In this case, the varia-tion of the reactive power is either minimum or maximum.

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30--

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F i g . 8 Variation of voltage in bus 5 with series and shunt com-pensation.The effect of the reactive power injected by shunt con-verter on the voltage of bus 5 is shown in Fig.8. In this

figure, the variation of the bus voltage versus injectedvoltage source is shown for the following modes. Each linerepresents the voltage in a known angle ( y), where Tis

from 0 to 90' degrees. As it is evident in Fig.8, in non-shunt compensation mode when the phase of the seriesvoltage changes from 0 to 90 degrees, the bus voltage willchange from 0.932 P.u.o 0.950 P.u..The voltage variation caused by series compensation isfixed regardless of the amount of shunt compensation.This proves that the shunt compensation is independentfrom series compensation.4.2 The Effect Of UPFC On Power Loss Re-duction

Qrhuni = o =15MYor an d = - " f 7 ' r

One of the capabilities of UPFC is the power loss reduc-tion. In this study, the UPFC has been installed in differentlines to investigate its effect on the power loss reduction.The effect of UPFC, installed on the line between buses5 and 1, is shown in Fig.9. Its effects on the other lines aredemonstrated in Table 1.According to Tab lel, the connection of shunt branch tobuses 1 and 2, which are generating buses, does not haveany effect on the power losses; however, this connectiondoes reduce the reactive power produced by the genera-tors.Comparing the ang le of the injected series voltage andline impedance, it is inferred that by decreasing the busvoltage through injecting inverse series voltage into highimpedance lines, the power transmission decreases. There-fore, the power loss will also decrease. Vice versa, in lowimpedance lines, with injecting series voltage in phasewith the bus voltage, the transfe rring power in the line willbe increased; therefore the power loss will d ecrease.Table 1: Effects of UPFC in the power loss reduction when it isinstalled in different lines with maximum 8% series voltage in-

I lo 2 I 0.019+j0.059 I 15 I 8.3(8.3serics+Oshunl)Note that there are two generators in this system thatproduce 275 MW of active and 164 War of reactivepower together. Also, the load of the system i s 25 9 M W

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and 147.5 MVar so the active power loss of the system is16 MW, which accounts to 6.1% of the total active power.In this study, the losses are considered as(&C& where f,zs3 is the system loss without UPF C.&

Fig.9 Effects of the UPFC in line between buses 5 to 1 on thereduction ofthe total power loss.a) With series compensationh) With series and shunt compensation

It can he seen from Fig.9a that when the series compen-sation coefficient is equal to 0.08, the total power loss isabout 4% less than that in non-compensation mode. InFig.9b the effect of the series com pensation plus 15Warshunt reactive power com pensation is shown. It can he in-ferred that the pow er losses have been decreased ahout 8%compared to non-compensation case.5. CONCLUSION

In this paper, the proposed model is used to simulate theeffect of UFFC on the power flow in power system. HereUPFC is modeled as variable loads, which are the functionof the bus voltage. Newton-Raphsons algorithm is used tocalculate the power flow equations. Modeling the UPFCas variable loads does not change the Newton-Raphsonsalgorithm much; therefore its implementation is easy.While in other methods, in order to consider the effect ofUFFC, the elements of Jacobian matrix should hechanged. Simulation results show that the hest place for

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UPFC for loss reduction is in the main transmission lines.The loss reduction ca n be achieved by two methods. Thefrst method is to increase the transition power in lines,which have low impedance. This can he implemented byinserting series voltage in phase with the sending end bu svoltage. In this method, because of the increased trans-mission power, the size of the UPFC is greater, whichmakes it expensive. Another method is to decrease thetransmission power in lines with high impedance. Hence,inverse series voltage with respect to the voltage of thesending end bus sho uld he inserted to the line. Finally inorder to decrease the loss in the power system, it is moreeconomical to install UPFC in the lines with high imped-ances that are usually long lines.AcknowledgementsThe first author would like to acknowledge the financialsupport from IEEE northern Canada and Mary Louise Im-ne Graduate Student Aw ard. Also we appreciate Dr. K.B.

Brown, A.M. Rahiei and R. Mohammadi for their guide-lines.References

[ I] N.G. Hingorani, High Power Electronics and flexible ACTransmission System, IEEE Power Engineering Review,Volume: 8 Issue: 7, pp. 3 -4, July, 1988[21 L. Gyugyi, A unified flow control concept for flexible ACtransmission systems, AC and DC Power Transmission,1991,International Conferenceon, pp. 19 -26,1991[31 C. Fuerte-Esquivel, E. Acha, A Newton type algorithm forthe control of power flow in electrical power networks,IEEE Transactions on power systems, Volume: 12, No.4,pp. 1474-1480,November, 1997[4] D. J. Gotham, G.T. Heydt, Power flow control and powerflow studies for systems with FACTS devices, IEEETransactions on power systems, vol. 13, NO.l, pp. 60-65,February 1998[5] F. Dazhong, D. Liangying; T.S. Chung, Power flow analysisof power system with UPFC using commercial power flowsoftware, Power Engineering Society Winter Meeting,2000. IEEE, Volume: 4, pp. 2922 -2925,200 0[6] H. Sun; D.C.Yu, C. Luo, A novel method of power flowanalysis with unified power flow controller (IJPFC), PowerEngineering Society Winter Meeting, 2000 IEEE, Volume:4, pp. 2800- 2805,2000[7] A. J. F. Ken, A. S. Mehrahan, X. Lombard, A. A Edris, Vni-fied Power Flow Controller (WF C) : Modeling and analy-sis, IEEE Transactions on power de l i vev , vol. 14, N0.2,pp. 648-654, April 1999Use of UPFC for optimal po wer flo w control, IEEETransactions on Po wer De livery, Volume: 12. Issue: 4. DD.

[8] M. Noroozian, L. Angquist, M. G handhari,G. Andersson,~. .. .I629-1634,0ct.1997.191 S.K.M. Kodsi. C.A. Canizares. Modelline and simulation of. IEEE 14 bus system with FACTS control& Technical Report 2003-3, University of Waterloo, Waterloo, March 2003.