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Versatile Model for Power Flow Control Using FACTS :DevicesYing Xiao Y.H.SongBrunel Institute of Power SystemsBrunel University, UK

Abstract - Increased loading of power systems, and environmentalrestrictions, combined with a world wide deregulation of the powerindustry, require more effective control means for power flow controland voltage support.From the point view of steady-state analysis, this paper concentrateson building up a versatile model for all types of FACTS devices. Inthis respect, decomposed Power Injection Model (PM) is proposed.Based on the model, a novel framework for power flow control usingFACTS devices, is adopted to derive FACTS control parameters.In order to take full advantage of Optimal Multiplier Newton-Raphson power flow method (OMNR) for power flow withembedded FACTS devices and Interior Point Linear Programming(IPLP) for power flow control, a rectangular - polar hybridformulation is adopted. Numerical results on a modified IEEE 5-nodenetwork are presented to illustrate the proposed approach.Ke yw ord s- Flexible AC Transmission System; power flow control;decomposed power injection model

I. IntroductionIn the last few years, there has been a w orldwide movement ofderegulation to facilitate the development of competitiveelectric energy markets, stipulating the unbundling of powergeneration from transmission and mandating open access totransmission services [2]. The arrival of Flexible ACTransmission System (FAC TS) technology has coincided withmajor restructuring of the Electricity Supply Industry aroundthe world. By use of the power electronics-based controllablecomponents, the possibility of controlling power flow withoutgeneration rescheduling or topological changes is beingrealised. The flexibility, effectiveness and versatility offeredby FACTS, present pressing challenges for power flowoperation and control.In order to investigate the impact of FACTS devices on powersystems effectively, it is essential to formulate a correct andappropriate model for them. Generally speaking, in terms ofsteady-state model of FACTS devices, there are two kinds,one is decoupled model, another one is coupled model. Theformer is proposed in [20] by N abavi-Niaki et.al. in 1995 forpower flow control using the Unified Power Flow Controller(UPFC). The model, while neglecting losses of the UPFC andthe line, decouples the line in which the UPFC installed, andtakes the voltage controlled node as PV node, and the op positenode as PQ node. However, the model cannot be applied todeal with the situation where the reactive capacity of theUPFC is not large enough to support the specified voltage ofthe PV node. Through classifying existing FACTS devices, D.J. Gotham et.a1.[6] improved the model to cover other types ofFACTS devices, and expanded it to the field of power flowcalculation. Nevertheless, for such an ideal model, withfictitious nodes added , it will modify the structure of Jacobianmatrix.In contrast to the decoupled model, generally, coupled modelconsists of two major models: Voltage (-current) SourceModel-(VSM) [ 5 , 12, 231 and Power Injection Model (PIM).For -the UPFC, Static Synchronous Compensator(STATCOM), Thyristor-controlled Phase-Shifter (TCPS),

Y.Z. sur1Department of Electrical EngineeringTsinghua University, ChinaStatic Synchronous Series Compensator (SSSC) and staticvar compensator (SVC), VSM is formulated as series or(and) shunt inserted voltage (-current) source, while as aspecial case of VSM, the thyristor-controlled seriescapacitor (TCSC) is expressed using controllable reactance.The VSMs of the FACTS devices are all formulatedaccording to their principles, thus, have a more intuitiverepresentation of the corresponding FACTS devices. But,the disadvantage of the VSM is that, it destroys thesymmetric characteristics of admittance matrix [3]. Apartfrom that, for power flow operatiojn and control using VSM,trigonometric functions are involved [24]. However,different from normal polar formulation load flowequations where all angles of nodal voltages are supposedto be small, the range of angle of the source is between 0and 2 z , which will inevitably lead to oscillation ofcalculation of power flow control. Derived from the VSM,power injection representation IS nitially proposed byZ.X.Han [3] in 1982 for Phase Shifter (PS). With theconversion of inserted voltage or (and) current source topower injections to related node!;, it enables to keep thesymmetric characteristics of admittance matrix. Justbecause of this advantage, its applications are extended tonearly all kinds of FACTS devices and are wide spread inmost of the literatures of operation and control of powersystems with embedded FACTS devices [4 ,5 ,8 , 121.With the further development of power electronicstechnique, more and more new FACTS devices areexpected to be applied in the near future. It is very clum syand inconvenient to integrate every different kind ofFACTS devices installed in inetwork into advancedapplication programs of energy management system(EMS). Since different type of FACTS devices possessesdifferent models and different control parameters, it isdifficult to cover all types of FACTS devices in the powerflow control methods aforementioned. In 1996, a versatilemodel is proposed by Arabi et.al [18]. It seems that themodel only focuses on the coordination of intemal controlparameters of FAC TS electronics devices themselves, wh leneglecting the consideration of the interface with powersystems. From the view of steady state power system, aversatile model of FAC TS devices for power flow control isstill in demand.In addition to the model of FACTS devices themselves, asfar as the topic of this paper is concemed, application ofeffective methodology to deal with power flow control withembedded FACTS devices is another important issue to beconsidered. In the past, various methods have beenproposed to derive control strategies for FACTS devices.The error-feedback adjustment [3, 41 involves modificationsof a control variable to maintain another functionallydependent variable at a specified value. Generally, thismethod is simple and easy to lbe implemented but theconvergence speed is slow. There are other three methodsbased on this solution with the aim to improve the algorithmconvergence or flexibility, such as sensitivity method [5, 12,

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131, distribution factors [14] and user defined models [17]. Apartfrom the error-feedback adjustment, another major algorithm isautomatic adjustment method [8, 9, 111, in which the controlparameters are directly considered as independent stateVariables in power flo w calculation. Although the metho d has agood convergence, it is inevitable that the resultant Jacobianmatrix will be enlarged. Moreover, about the methodology ofpower flow con trol using FA CTS devices, there are a number ofimportant issues which need to be addressed further:l)It is widely recognized that conventional power flowalgorithms are convergence failure-prone when applied topower systems with embedded FACTS devices [8, 181. AsFACTS devices are used to better exploit the existing powersystem resource, the system is often pushed to the verge ofoperational ranges. If the pow er flow m ethod for co ntrol isnot flexible enough to sustain such situations, it isimpossible to evaluate their impacts on the system correctly.2)While undertaking power flow control to make the specifiednodal voltage and line flow attain predetermined targets, thestate variables of other nodes, such as node voltages andangles, should be guaranteed to be within the given operatingranges. Otherwise, even though control targets are satisfiedwith control parameters calculated, from the view of thewhole system, the power flow scenario achieved is stillinfeasible. It is obvious that neither error-feedbackadjustment nor automatic adjustment method can deal withthis aspect.3)As is well known, power flow control can also be tackled viaequality constraints in Optim al Power Flow (OP F) efficiently.However, if the capacity of the FACTS devices is not largeenough to support the prescribed target, i.e., the limits of theFACTS device and the equality Constraints for local controltargets can not be fulfilled simultaneously, this will result inthe O PF problem unsolvable.Taken into account of all these factors aforementioned, aversatile model of FACTS devices and a novel framework forderiving FACTS control parameters are presented in thispaper. The new methodology has the following characteristics:1)Applying active and (or) reactive power injections ascontrol variables, it is a versatile approach which is capableof modelling virtually any type of FACTS deviceseffectively. Once the required power injections are derived,they can be easily converted into original control parametersaccording to the type of the FAC TS devices.2)Since in the model built, power injections are treated asindependent variables, which do not vary with the connectednode voltage amplitudes and phases, thereby the Jacobianmatrix needs not to be modified in terms of the changes ofthe power injections of FACTS devices during the powerflow iterations. Thus, it is easy and efficient to implementthe proposed method.3)Taken into consideration the operating limits of statevariables and internal limits of FAC TS devices to ensure the

feasibility of the resultant scenario, optimisation algorithmis applied in this module. With respect to the advantage ofreliability and extraordinary speed, a Primal-dual InteriorPoint Lineal Programming (IPLP) is used to implement thepower flow control. Furthermore, if it is not possible toattain the specified control target within the feasible area ofthe FACTS device, the available nearest target in terms ofthe capacity of the FACTS d evice installed can be achievedand sugg ested to the system operator, if necessary.

4)In order to take full advantage of OMNR method forpower flow with embedded FACTS devices and IPLP forpower flow control, a rectangular - polar hybridformulation is applied in this method.5)In general, as system becomes more heavily loaded, andwith the utilization of the FA CTS devices, especially theUPFC, there is highly physical P-Q coupling existing inpower system network. Coniparing to P-Q couplednetwork framework, the decoupled model is inaccurateand can lead to poor or even failed convergence duringthe power flow iterations. Therefore, in the paper, thecoupled network model is applied.11. Power Flow Control Model

2.1 Classificationof FACTS DevicesIn order to build a versatile model for all kind of FACTSdevices, whether it is Thyristor Controlled or Converterbased, it is necessary to classify them into several typesaccording to their steady-state characteristics and functions.As stated, applying series compensation, the TCSC, TCPSas well as the SSSC can be effective in active power flowredistribution [2]. For the SVC and the STATCOM, theyare normally operated to regulate the voltage of thetransmission system at a selected terminal [2]. With theseries inverter and the shunt inverter, UPFC offers a uniquecapability of independently regulating the active andreactive power flow on the transmission line, while alsoregulating the local bus voltage [l]. Thus, for power flow>studies,with respect to their capabilities, it is undoubtedlylogical to classify FACTS devices into such three types,which are Series Controller, Shunt Controller and UnifiedController respectively.2.2 Decomposed Power Injection ModelOne of the most important advantages of power injectionrepresentation is that it does not destroy the symmetriccharacteristics of a b t t a n c e matrix [3]. Moreover, it ishighly noticeable that all the FACTS devices can bereformulated to power injection model, which is shown inFig. 1 (b). Thus, active and (or) reactive power injectionsrepresent all the features of the steady-state FACTS m odel. Itis natural to take active and (or) reactive power injections asindependent control variables to formulate a versatileapproach for power flow control. Once the required powerinjections are derived, they can be easily converted intocontrol parameters according to the type of the FACTSdevices.For an N node system, L is the controlled line in which theFACTS device is installed, shown in Fig.1. Normally, inVSM model, the line flow of L is a function of voltage andangle of node Z and J , as well as control parameters ofrelated FAC TS devices, that is,PL = P L (VI VJ,8,,8J,T, T 1,)QL = Q L V i VJ el , e J > v r VT I q)

(1)(2)Essentially, power flow redistribution is caused by anadditional load flow through the related line, which issuperimposed on the nature line flow. Based on thisanalysis, when applying power injections as control

variables, the relevant line flow PL+ jQL should bereformulated as line flow PL O jQLop ertaining to thevoltage and angle of node Zand J which consider the

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effects of the F ACTS device, together w ith active and reactivepower injections of the FACTS device which flow along theline. At the same time, voltage of the target node should bedetermined by reactive power injection direct to the node. Inthis formulation, for active power injection, there is no doubtthat it flows along line L . In terms o f reactive power injection,for Series Controller, the relevant active power injection canonly flow via line L ;while it m ust inject directly to node I forShunt Controller, that is, there is no reactive injection throughline L .For Unified Controller, as a matter of fact, there are tworeactive injec tions, one is for regulating voltage, and another isfor reactive line flow control. When applying conventionalpower injection model, which is shown in Fig. l(c), eventhough the total reactive power injection to node I is known ineach control iteration, there is still no way to determine thesetwo factors separately. As a res ult, the actual line flow PL + j Q Lcan not be calculated without the reactive power injection alongthe line. In light of those analyzed above, a decomposed powerinjection model (DPIM) is proposed in this paper, as shown inFig. l(c). In DPIM, obviously, Qr,,) is decomposed into twoparts, one is reactive power injection of FACT S devices QrL(lnJ)through the line L . Another is direct reactive power injectionQII(rn,)o the node I , which only affects the voltage of nodeI .That is, @(tnj) = &(inj) + QIL(mnj) .The detailed power injections of DPIM are shown in Fig. l(c).It is no doubt that the DPIM is not only clearer and moreintuitive than the co nventional one. The m ost important point is,it is the key to the versa tile model for pow er flow control usingFAC TS devices, which can not carry out using the conventionalPIM without the decomposition of the reactive power injectionand further definition of the exact paths and affects of the powerinjections.When considering no loss in FACTS devices, active powerinjection to the second node PJ(,n,,s equal to that to the firstone PI(mn,),hile the directions are opposite to each othe r. Andfor the UPFC, since line flow control target is set as the line

j I , Ir...................................................... !DEVICESFig l(a ). V oltage (-current) source model of the FACTS device

nPJoll + jQJ,,,

Fig l(b). Conventional PIM of the FACTS deviceU

V , &,+jQLo GU ~ B u VJ,iQ1i,lnl,Fig l(c). The DPIM of the FACTS device

Fig 1 . Formulationof the DPIM of the FACTS device

flow PL+ j Q L fiom the node I side along line L shown inFig. 1, even though Q.,(,,,,, has impact on the operational stateof power system, it is still not put into the list of controlparameters. Based on the assumptions stated above, the lineflow control vector of the Controllers and the correspondingavailable control target are listed in Tab .1.Thus, based on the decomposed model, eqs.(l) and (2)Should be converted to (3) and (4 ) accordingly:'f. = (1 > vJ > ' 1 > - I ( t"J) (3)Q L = QL o (v,v, e,,e, - ILr,nJ, (4)2.3 Application of Interior Point Linear Programmingand OMNR Power Flow A lgorithmRegarding the third shortcoming of the current methods forpower flow control, a further advancement in the proposedtechnique is the consideration of unfeasible results becauseof state variables of some other nodes beyond the operatinglimits. Additionally, in this paper, in order to ensure thefeasibility of the resultant control scenario, internal physicallimits of the FACTS devices have been taken intoconsideration. For Shunt controller, only limit for currentthrough the shunt transformer or converter I, should beconsidered. For Series controlbar, in VSM, three limitsshould be checked, such as series inserted voltage angle pr ,current I,, through the series transformer limit, and activepower Pdc transferred through thyristor switches orconverters limit, while for TCSC, the limits is only forreactance x, . It is undoubted that for Unified Controller, allof the five limits should be taken into considered [4]. It isvery convenient for optimisation programming to considerit by using relevant constraints.It is widely recognized that, Linear Programming (LP) hasthe advantages of reliability and high speed. As stated, thecurrent Interior Point M ethod [(IPM ) has su rpassed theconventional simplex method for large scale LP problemsin the solution speed by a factor of 10-100[27]. Therefore,in this module, a Primal-dual Interior Point LP is used tosolve the problem of FACTS devices control.Aimed at overcoming the problem of divergence o f powerflow with embedded FACTS devices, OMNR power flowalgorithm in terms of the rectangular form is adopted inpower flow as it offers a number of advantages in handlingill-conditioned power system successfully [4,15,16].However, normally, the control parameters of FACTSdevices are given in polar form which is more intuitivebecause the state variables are voltage magnitudes andangles. In this respect, a rectangular - polar hybridformulation is employed in this method. That is, in order totake full advantage of OMNR method, rectangular form isutilized in power flow, while in ]power flow control, polarformulation is adopted. The cost is both of the two form ofJacobian matrices have to b e developed, and another job is totransform the rectangular formulation power flow result topolar one before forming the linear programming problemduring every iteration. Nevertheless, as one form can betransformed to another easily, and OMNR method hasadvantage of good convergence and much fewer iterations, itis still more progressive and effective when compared withconventional methods.

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111. Problem FormulationEssentially, the o ptimization problem is based on minimizing anobjective function with regard to the control vector U . Fromwhat have been analyzed, minimizing the mismatch of controltarget is set as the optimization objective for the problem ofpower flow control using FACTS devices, whch is shown in(5). The repre sentation of the objective function depen ds on thechosen target. According to different types of controllerclassified, the model ca n be transformed into three expressionsthat represent the three different types of FACTS devicesrespectively, by suitably setting the coefficient pI ,p l ,p3according to eqs.(6), (7), (8). Power flow equations are used asequality constraints. Apart from the normal inequalityconstraints of operating limits of voltage of PQ n odes and noda langle, there are also several additional constraints whichdescribe the limitsof interna l limits of the FACTS device.The power flow control of FACTS devices with powerinjections as c ontrol variables, can be formu lated as follows:3.1 ObjectiveFunction:prescribed control target, w hich is represented byFor power flow control of the Series Controller:For voltage control of the Shun t Controller:For the U nified Controller:PI =PZ =p3When applying the decomposed model, according to eqs.(3),(4), the objective can be reformulated as

The objective is to minimize the deviations from theF = PIldpl + PZ dQl+ ~3 ldvl ( 5 )PI = Lp2 = p3 = 0 (6 )pi = P z = o , P ~ = 1 (7)

(8 )

=pl(f?O e(,,")E T y +/%(a&!nj) -eTy+P3(& (9)3.2 Operating and Control Constraints:These cons traints are catego rized as follows:3.2.1 Equality ConstraintsFor PV node i : # I , i f J , = l ; . . ,N IPtc - ;r - ViF o r P Q n o d e i : i # I , i + J , i = N l + l , - . . , N - l

N-J V j GU os 8i,j+ BU sin 6i,j =0

& - & - K ~ V j ( G U c o s 8 1 : , j + B U s i n e i , j ) = 0 (1 1)

(10)j= J

N - 1

j d

Qic - Q L - K y V j ( G U s i n B i . j - B , c o se i ,j ) = O (12)

P , ~eL p , ~ , , , ~ ~cos^^,^ cos^^,^ +B,s ine , , j )=o (13)j= JFor node I:

I- )

For node J

Nod al voltage limits: (for PQ no de i )Series inserted voltage magnitude limitSeries inserted voltage angle rangeCurrent through the shunt transformer or (and) converterlimitCurrent through the series transformer limit

V i , m m I K I V ; , , , , i = N l + I , - . . , N - l (17)PIVT PIVT" (18)0 p i n PI27r (19)

~ 3 I qm 5 ~ 3 I q 3 I q a (20)(21)I Ise P@ +V T - < ) ( G I J + J ~ I J ) ~ ~ ~ i ~ s e m

Active power transferred through thyristor switches orconverters limitP~L ,~P& = P , R ~ ~ ( ( ~ + ~ - ~ ) ( G ~ , + ~ B , , ~ ) . ] I P / ~ ~22)WhereN : the total number of nodes;N I the number of PV nodes;V , : the voltage of node i ;e, the voltage angle of node i ;G, + j B , : he impedance of line i,& ,QIG: the active and reactive power generations of nodeeL ,IL: he active and reactive load of node i ;PLT QLT : the control target of active and reactive line flowV , : he co ntrol target of voltage of node I .Subscript min and max in each equation represents theminimum and maximum limits of the variable respectively.

IV. ImplementationFrom the model built above, it is obvious that in theversatile model, for d ifferent type FACTS devices, Jacobianmatrix and control framework need not be reconstructed,even though they have different physical forms anddifferent control parameters. Essentially, it is a versatilemodel in terms of the process of the iterations of powerflow calculation and power flow control. Meanwhile, itshould be noted that, power injection model and VSM areinseparable. Actually, DPIM is only a transitional model,since in power flow calculation, the known variables ofFACTS devices are magnitudes and angles of voltage and(or) current sources. Besides that, in pow er flow control, theknown limits are only for the initial control parameters.And the ultimate control scenario should also be providedin the form of their original control parameters.For the IPLP-application in this paper, the non-linear termof the objective must be piecewise linearized. And eachsegment is treated as a separated linear programming. Thus,an iterative procedure is needed. Actually, each iterationconsists of power flow part and control part. The outline forthe implementation of the methodology for power flowcontrol using FACTS devices is given below:1. Calculate conventional power flow without FACTS

e,,,= e , e ,i ;

along line L ;

device;

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2. Set the initial power injections U0 = 0 .Accordingly, set theoriginal control parameters of the corresponding FACTSdevices as 0, which are all listed in Tab. 1 respectively;3. Calculate AU , et U +! =U + AU ;4. If the FAC TS device is a Series Controller, according to thepresent U t , determine the original control parameter using

Newton m ethod. Then calculate e(,,, QIL(,,,,) and Q,,,",,5.For Unified Controller, according to the present Ut ,

determine the corresponding original parameters usingNewton method. Then Obtain e,,,*, ;6. Solve power flow w ith the current power injections;7.Accord ing to a given threshold value, check if the differencebetween the current objective value and the value of theprevious iteration is less than the value or not. (For the firstiteration, check if the current objective value is less than the

Classifr-cation

ControllerSeries

ShuntControllerUn jiedController

threshold value or not.) If no, go back to step 3, andfollow the iterations described above;8.According to the present U and system state variables,derive the original control parameters of the FACTSdevice using Newton m ethod.If the objective value is less than the given threshold, itmeans hat according to the resultant control scenario, theprescribed target can be fulfilled. If the objective value'islarger than the given threshold value, the available nearestcontrol target under the current operating limits of FACT Sdevices can also be obtained.

Control~~~e~ Control Variables FACTS Devices Parameterslements ofDPIMteady-state functionsTCPS Pr

CB sssc VTactive line flow control p I ( i n j ) 3 p J ( i n j ) P L P l r i n j ) Tc TCSC x c

voltage regulation Q w i n j ) ,VI Qw u) CB STATCOM 1,

voltage regulation Q / ~ . ( t n j , 7 Q,,(iM) 3 vi ' Q i ( w , CB UPFC I ,

TC svcV r SP rctive line flow control TCreactive line flow control p l / i a j ) PJ ( in J ) 3 PL Q L P l ( i n j ) Q a ( i M) 3

Table 1. Classifications of all of the F ACTS Devices, their control variables for the available control target and original controlparameters of different devices

V. Test Results and Case StudiesIn order to investigate the feasibility of the proposed technique, alarge number of power systems of different sizes under differentsystem conditions have been tested. All the results indicate goodperformance and high accuracy achieved by the proposedmethod.In this section, a modified IEEE 5-bus system is presented tonumerically demonstrate its performance. Several of the mainaspects of power flow control via FACTS devices, includingobtaining a specified operating condition, alleviating heavyburden of line flow, improving voltage profile, enhancingavailable transmission capability etc., are illustrated using thenetwork.In order to assess the c ontro l performances fully, in addition tothe base case which is under normal operation pattern as case1,an alternative pattern of th e same test system is em ployed. Inthis constructed case (case 2), the generation and load at eachlocation is raised by up to 600% from the base pattern, whichresults in an increase of power transfer along all the linessignificantly. Various schemes of controlling node voltage andline transfer power have been studied to testify the proposedmethod, which are analysed as follows respectively.5.1 Control Performance Analysis5.1.1 Unined ControllerThe performances of a typical d i e d controller-UPFC on thetest system are evaluated, which are shown in Tab. 2. The databeyond the o perational limits are all marked using grey blocksfor clarification.

From the data of case 2 in Tab. 2, it is noticeable that underheavy load, voltage of node 2 is lower than their normal limits,which is set as 95% - 105% of the nominal voltage. Meanwhile,since thermal limit of line 2-5 arid line 1-2 is 25.000 p.u. and30.000 p.u respectively, power sharing between the lines isunbalanced and there is enough capacity left for line 2-5 totransfer power without exceeding thermal limits. Apparently,the transfer capability of line 2-5 and 1-2 is limited byviolations of thermal limits of line 1-2.In order to alleviate the situations, a UPFC is installed alongline 2-5, near the side of no de 2. 'The co ntr ol target of line flowis pre-assigned as -2O.OOO-j2.000. And the target of voltage ofnode 2 is set as 0.95. The controlded scenario is also sh own inTab. 2, in which the mismatch for the real and reactive lineflow is only 1.0200E-3 and 3.1240E-3, at the same time, thedeviation of controlled voltage is 0.0087E-3, after 10iterations. The exact control parameters of the UPFC are alsoshown in the table. The iteration of d p ,d Q ,d V an d theobjective of power flow control using the UPFC aredemonstrated in Fig.2 and Fig.3. hdeanwhile, iteration processesof original control parameters of the UPFC are also set out inFig.4.It is clear that due to the reactive pow er injection of shunt partof the UPFC, the voltage of nodlc 2 is raised to be within theoperational range. Additionally, power injections of the seriespart of the UPFC force more pow er flow throu gh line 2-5, thusalleviate the stress of line 1-2. It seems very effective andconvenient to improv e the available transmission apability viaredistribution of load sharing between connected lines usingFACTS devices. Therefore, it is I O doubt that UPFC c an playan important role in enhancing th e system from these aspects.

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Table 2 Power flow control andvoltage control ofthe unified controllerVoltage of ' Line (2-5) Line (1-2)Node2 1 5 Q Q

Scenario Line Active Line Flowinitial 1 target I controlled2 2-5 -2.003 I -5.000 1 -5.001

I I V r L & 2 0 . 2 2 8 1 1 .4 03 , I, =0.920 I

VT4.736

1 &Delta-P +Delta-Q +Objective I12 I

+IQ

- 4 ' I t e r at i o n T i m e sFi g2 The iteration of AP, Q and the objective of powerflow control using the UPFC0.000 * . . - - - . .w , " , - , - ' "5 6 7 8 9 10-0.002 1 2 3 f l-0.004

i t e r a ti o n T i m e s 1~Fig.3. The itera tion of A V of power flow control using the

1PFC

-0.10- 0 . 1 5-0.20-0.25 I t e ra t lo n T lm e s1

.50

i terat ion' T i m e s6 ' 7 ' 8 ' 9 ' I O ' (

Fig.4 Iteration processes of the original control parametersof the UPFC- 87 3 -

f

5.1.2 Series ControllerThe test about power flow control o f a se ries controller is basedon the base case. The results are listed in Tab.3 as scenario 2. Inscenario 2, a series controller, for example TCPS, is applied toregulate the in itial line flow to the target o ne after 5 iterations. Itis very clear that with series controller, he power flow of the lineconcerned can be redistributed directly and effectively.Table 3 Power flow control of he series controller

.--cDelta-P4.00 7 1

-1.00 I ' 7 3 A r;i t e r at i o n T i m e sFig.5 The iteration of dp of power flow control using the TCPS

5.1.3 Shunt ControllerFrom case 2, it is apparent that under heavy load, the voltage ofnode 2 exceeds the lower limit. In order to support it, a shuntcontroller, such as SVC or STATCOM, is installed on thecorresponding node to provide direct reactive power injection.The voltage of node 2 attained the target after 5 iterations andthe results are shown in Tab.4 in details.Table 4 Voltage control of the shunt controller

Voltage I I. 1

VI. ConclusionDecomposed-PIM based versatile control algorithm is proposedto obtain the FACTS control strategy in order to achieve agiven target for different types of FACTS devices, whether inthe form of specified power flow or bus voltage magnitude. Inthe method, the controllable active and reactive powerinjections are taken as control variables, which allows forstudying the control of all types of FACTS devices effectively.Meanwhile, the OMNR power flow method for ill-conditionedsystem and a Primal-dual Interior Point Linear Programmingare integrated to implement the proposed control model. Theeffectiveness of the proposed control method has beendemonstrated on a modified IEEE 5-bus system. The presentedmethod not only shows its satisfactory abiiity of tracidg controlobjectives, but also derives FACTS control parameters directlyand simply without needing any initial values.

V'II. References[l] Colin Schauser, The unified power jlo w controller - a concept becomesrea& Colloquium of Flexible AC Transmission Systems - the FACTS,23. Nov. 1998[2] Laszlo Gyugyi, Converter-based FACTS controllers, Colloquium ofFlexible AC Transmission Systems -th e FACTS, 23. Nov. 1998I31Han,Z.X., Phase Shifferand Power Flow Control,IEEE Trans.on PAS, Vol.[4] Y.H.S ong, J.Y.Liu, Pow erjlow control and voltage support by using UPFCconstrained by internal limits,200-03, CE RE , June 1999[5] M.Noroozian, Ghdersson, Power jlo w control by use of controllableseries components, IEEE PES 1992 Summer Meeting, Seattle, WA, July

101, NO. 10, 1982, pp.3790-3795

12-16, 1992

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MII. AcknowledgementThe work is partly supported by the Chinese National KeyBasic Research Special Fund CNO.Gl998020310) and theOverseas Youth Cooperation Grant of Natural ScienceFoundation, China.

IX. BiographiesYing Xiao was born in Beijing, China, in 1972. She receivedthe degrees of BEng and MSc in China in 1993 and 1996respectively. Currently, she is a PhD student at BrunelUniversity in UK. Her main research areas of interest areFACTS, operation and planning of power system, andapplication of stochastic and fuzzy set method.Y.H. Song is Professor of Electrical Energy S ystems at BmnelUniversity where he also holds the Royal Academy ofEngineeringDTuclear Electric/Siemens Chair of PowerSystems.Y.Z. Sun is Professor of Power Systems at TsinghuaUniversity, Beijing, China.

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