Operating modes and control interaction analysis ofunified power flow controllers

H.F. Wang, M. Jazaeri and Y.J. Cao

Abstract: In order to simplify the description of the operating status at a unified power flowcontroller (UPFC) the authors proposs that the UPFC has three operating modes: (1) anuncontrolled mode; (II) an inductive mode; and (III) a capacitive mode. An application exampledemonstrates that the effect in the changes in the UPFC operations on the power system stabilitycan be described more systematically and explained more clearly when they are represented bychanges in these UPFC operating modes. It is shown that the linkage pattern of the UPFC seriesand shunt parts decides whether or not the control functions implemented by the UPFC series andshunt parts conflict with one another. This linkage pattern can be described by the flow of activepower through the UPFC at steady-state operation of the power system. Hence, the direction andamount of active power flow through the internal link of the UPFC series and shunt parts atsteady-state operation of the power system is suggested to be an indicator adds to predict thepossibility of UPFC control interactions. This proposed interaction indicator is calculated from apower system load flow solution without having to run simulations on the power system withUPFC controllers installed. An application example to demonstrate the analytical results on UPFCcontrol interactions is presented.

1 Introduction

To deal only one prototype unified power flow controller(UPFC) has been trialed in a real power system [1],however, there continues to be considerable researchactivity in this area following on from the pioneeringwork of [2]. The literature can be divided into threemain areas: (i) UPFC modelling in power systems; (ii)UPFC integration into power flow solutions; and(III) UPFC dynamic control [3–12]. We intend toinvestigate two areas: (i) the effect of UPFC operatingstatus on the power system stability; and (ii) an analysis ofUPFC control interactions.

The major control functions of a UPFC are: (i) activepower regulation; (ii) reactive power regulation; and (iii)voltage regulation. To date, these control functions havebeen considered under steady-state operation of the powersystem. The important and fundamental issue of howUPFC operation, implemented via its major controlfunctions (and not the supplementary stability control),affects the power system dynamic performance, for examplethe system stability, has not been addressed. This may bedue to the lack of research to link UPFC steady-stateoperation and UPFC dynamic control, for which differentUPFCmodels are used. The UPFCmajor control functionsare set for power system steady-state operation by giving

these control functions pre-set control references, i.e. thelevel of active and reactive power transfer along thetransmission line where the UPFC is installed and alsothe level of the UPFC bus bar voltage. These pre-set controlreferences, in fact, decide the operating status of the UPFC.Changes in the UPFC operating status affect the powersystem is dynamic performance, as will be demonstrated inthis study, which can only be assessed in dynamic operation,of a power system. Therefore this work will propose thatvariations in the UPFC operating status, identified by thepre-set control references of its active power, reactive powerand voltage control, define three operating modes of theUPFC: (i) an uncontrolled mode; (ii) an inductive mode:and (iii) a capacitive mode. This proposed concept ofUPFC operating modes will be able to systema-tically represent the UPFC operating status and signifi-cantly simplify investigations on the effect of UPFC steady-state operating status on the power system dynamicperformance.

As far as UPFC control interactions are concerned,results in [14] and [15] have shown that individual UPFCcontrollers, that have been designed satisfactorily, canfail to operate with the UPFCs because of their interac-tions. However, the reasons for the failure of jointoperation of UPFC multiple control functions are notexplored in [13] and [14] and no physical explanation fromthe viewpoint of power system operation is given. Theinteractions have been observed only in power systemsimulations when multiple UPFC controllers are designedand installed in the power system [13, 14]. However, thisresult cannot tell whether the interactions are due to theinternal connections among the UPFC multiple controlfunctions or caused by badly set parameters of theUPFC controllers.

This study will focus on investigating the interactionsbetween UPFC multiple control functions.

H.F. Wang is with the University of Bath, Claverton Down, Bath, BA2 7AR,UK

M. Jazaeri is with the Semnan University, Semnan, Iran

Y.J. Cao is with the Zhejiang University, Hangzhou, China

r IEE, 2005

IEE Proceedings online no. 20041220

doi:10.1049/ip-gtd:20041220

Paper first received 18th December 2003 and in revised form 6th August 2004.Originally published online: 24th January 2005

264 IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005

2 Definition of UPFC Operating Modes and anApplication Example

Figure 1 shows a UPFC installed on a transmission linebetween nodes 1 and 2 in a power system. UPFC operationin the power system is implemented by controlling the activepower flow, PIE¼PB2, and reactive power flow, QB2 (orQIE), along the transmission line between nodes 1 and 2,and regulating the voltage at the UPFC shunt busbar E,VEt. Hence, the UPFC operating status or operatingconditions can be identified by the pre-set values ofPIE¼PB2, QB2 (or QIE) and VEt as control references ofthe UPFC active power, reactive power and voltage controlfunctions. These UPFC operating conditions, together withgenerator operating conditions and system load conditions,decide the overall operating condition of the power system.

It is well known that the power system dynamicperformance, for example the system stability, is affectedby the system’s operating conditions under steady-stateoperation. Hence, UPFC operating conditions, i.e. the pre-set values of PIE¼PB2, QB2 and VEt for the UPFC dynamiccontrol functions, should also affect the power systemdynamic performance. However, the variations in theUPFC operating conditions are due to different combina-tions of three pre-set values of PIE¼PB2, QB2 and and VEt

of the UPFC control functions. Representation of thevariations occurs in a three-dimensional parameter space,characterised by three pre-set values of PIE¼PB2, QB2 andVEt. It will be very complicated and difficult to effectivelyillustrate how the changes in UPFC operating conditionsaffect the power system dynamic performance, unless theUPFC operating status can be described by a simplerepresentation in a reduced-dimensional parameter space.This, point underpass the proposal of the following threeoperating modes for the UPFC to systematically representthe variations in UPFC operating conditions.

1. Uncontrolled operating mode of the UPFC. As shown inFig. 2, without the installation of the UPFC between nodes1 and 2 in the power system, the power flow through busbarE (or B) is PU+jQu and the voltage at busbar E is VU. Ifthe pre-set values of the UPFC control functions arePIE¼PB2¼PU, QB2¼QU and VEt¼VU, then the UPFC issaid to be operating in the uncontrolled operating mode.This is the case in which the UPFC active power, reactivepower and voltage controllers are all opened and the powerflow along the transmission line between nodes 1 and 2 issubject to no forced control from the UPFC at all, and wehave that QIE¼QB2.

2. Inductive operating mode or capacitive operating modeof the UPFC. Variations in PIE¼PB2 and VEt change thereactive power input to the UPFC, QIE. Hence, we can

define DQ¼QIE–QB2. No matter how PIE¼PB2, QB2 andVEt may vary, if DQ¼QIE–QB240, then the UPFC absorbsreactive power from the power system and we define this asthe operation of the UPFC in the inductive mode; ifDQ¼QIE–QB2o0, then the UPFC provides reactive powerto the power system and we define this as the operation ofthe UPFC in the capacitive mode. DQ¼QIE–QB2¼ 0 is theuncontrolled operating mode of the UPFC.

With the introduction of these three operating modes forthe UPFC, any variations in the UPFC operatingconditions, characterised by the pre-set values of PIE¼PB2,QB2 and VEt, result in changes in the UPFC operatingmode. The changes in the UPFC operating mode are in aone-dimensional parameter space, in the direction of eithera more inductive or a more capacitive mode, separated inthe uncontrolled operating mode as shown in Fig. 3. Forexample, for an increase in the active power delivery,PIE¼PB2, along the transmission line between nodes 1 and2 in Fig. 1, the UPFC operation should move in thedirection of a more capacitive mode. On the other hand, ahigher pre-set value of VEt means a less reactive power inputto the UPFC at the busbar E and hence the UPFC operatesin a more capacitive mode. Of course an increase in QB2

requires that the UPFC produces more reactive power andthus the UPFC should again move towards amore capacitivemode. Therefore, the changes in operating status of theUPFC can be presented more simply and systematically in

C

V1

E2

j

j

x1

1E

2VP1E PB2

Q1E QB2

BT

EtV

BtV

Em � �E B

ET

UPFC

E BE-VSC B-VSC x

j

I

x2

mB

Fig. 1 Single-line diagram of a transmission line installed with a UPFC

1V 2V

E

j x2jx1UV

PU

Q U

Fig. 2 Uncontrolled operating mode

2BPE1P =

2BQ

V

uncontrolledoperating mode

inductive mode capacitive mode

towards the more capacitivedirection

towards the more inductivedirectionEt

variations of UPFCoperating conditions in three-

dimensional parameterspace

variations of UPFCoperating modes in one-

dimensional status space

Fig. 3 Transformation of the three-dimensional UPFC operatingconditions to the one-dimensional UPFC operating modes

IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005 265

terms of changes in its operating modes rather than itsoperating conditions. Furthermore, it will be demonstratedin the following example that the effect of changes is theUPFC operating status on power system stability can beexplained more clearly when represented by the proposedUPFC operating modes.

The application example is the New England test powersystem (NETPS) shown in Fig. 4 where a UPFC is installedbetween buses 1 and 2 so as to control the line active andreactive power flows and also regulate the voltage at thebusbar where the UPFC is installed. Figure 5 presents theloci of a pair of system electromechanical modes as theoperating mode of the UPFC moves from the inductivemode towards the capacitive operating mode. From Fig. 5we can see that the more capacitive is the UPFC, then theless damping of the system oscillation occurs. Hence, nomatter how the three UPFC control references change, ifthey result in the UPFC moving in a more capacitivedirection then the power system will become more unstableas far as system oscillation stability is concerned. Thisconclusion act as a guideline when setting up UPFCoperation in cases where power system oscillation stability isa major concern.

Figure 6 shows the changes in the rotor angle of machine1 as the UPFC operating mode varies. Again this can besimply explained by the point that as the UPFC becomesincreasingly capacitative then the electrical distance frommachine 1 to the rest of the power system becomesincreasingly shorter then and hence the more stable thepower system becomes as far as system transient stability isconcerned. This also indicates that the system transient

stability is improved when the UPFC operates in a morecapacitive mode. This conclusion is confirmed by thesimulation results presented in Figs. 7–9 which show thesystem critical clearance time subject to a three-phase to-earth short circuit on one of transmission lines increaseswhen the UPFC operates in the capacitive mode,uncontrolled mode and the inductive mode. FromFigs. 7–9 we can see that the power system is more stablein terms of system transient stability when the UPFCoperates in a more capacitive mode.

10

8

6

7 4

5

3

2

2

1

39

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29 2826

12 19

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9

8

32

Fig. 4 Single-line diagram of the NETPS

−0.89

−0.74 −0.73 −0.72 −0.71

−0.88

−0.87

−0.86

0.86

0.87

0.88

0.89

j�

�

more capacitive

more inductive

Fig. 5 Loci of a pair of system electromechanical oscillation modeswhen the UPFC operating modes change

0.800 0.900 1.0 1.176 1.300 1.400 1.500

line active power flow, pu

13.00

14.00

15.00

16.00

12.50

13.50

14.55

15.50

16.50

roto

r an

gle,

deg

.

more capacitivemode

uncontrolledoperating mode

more inductive mode

Fig. 6 Changes in the rotor angle of machine 1 when the UPFCoperating modes are varied

0 1 2 3 4 5−50

0

50

100

150

200

250

time, s

roto

r an

gle,

deg

.

the fault is clearedafter 748 ms

the fault is clearedat 747 ms

Fig. 7 Simulation when the UPFC operates in a capacitive mode

0 1 2 3 4 5

time, s

−50

50

100

150

200

250

roto

r an

gle,

deg

.

0

the fault is clearedafter 735 ms

the fault is clearedat 734 ms

Fig. 8 Simulation when the UPFC operates in the uncontrolledmode

266 IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005

3 Analysis of UPFC Control Interactions

In order for the UPFC installed in the power system ofFig. 1 to operate normally, a controller is needed to regulatethe DC voltage across the capacitor inside the UPFC. Theconventional selection of a control signal for the UPFC ofthe four UPFC controllers is: (i) for the AC voltageregulator: the modulation ratio of the UPFC shuntconverter, mE; (ii) for the DC voltage regulator: themodulation phase of the UPFC shunt converter, dE; (iii)for the reactive power regulator: the modulation ratio of theUPFC series converter, mB; (i) and (iv) for the active powerregulator: the modulation phase of the UPFC seriesconverter, dB. Figure 10 shows the detailed arrangementsof these four UPFC controllers, which are all proportional-integral (PI) controllers.

For the four SISO (single-input single-output) UPFCcontrollers in Fig. 10, interactions can lead to systeminstability as reported in [13] and [14]. To demonstrate that

the interactions are from the control conflict between theUPFC shunt and series parts, Fig. 11 shows a simplifiedillustration of the UPFC configuration given in Figs. 1 and10, where the series and shunt parts of the UPFC arerepresented simply by the series voltage source �VVB and theshunt voltage source �VVE respectively. The step-down shunttransformer is modelled by a reactance jxE. At steady-stateoperation, an active power input to the UPFC, PIE, arrivesat the UPFC series busbar B through two channels, thedirect channel and the internal channel, as shown in Fig. 11,i.e. PIE¼Pdirect+PInternal. The amount of active power flowthrough the UPFC internal channel, PInternal, decides theelectrical connection between the series and shunt parts ofthe UPFC. When PInternal¼ 0, the series part of the UPFCneither absorbs active power from, nor supplies activepower to, the power system and the phase between �VVB andthe line current-�II2 is 901. This is the case where the seriespart of the UPFC operates independently from the shuntpart as a reactive power source, because it does not needactive power support from the shunt part. Electrically, inthis case, the UPFC series and shunt parts are two

0 1 2 3 4 5

time, s

−50

50

100

150

200

250

roto

r an

gle,

deg

.

0

the fault is clearedafter 727 ms

the fault is clearedat 726 ms

Fig. 9 Simulation when the UPFC operates in the inductive mode

t

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PI

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C

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PI

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2

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+

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+

+ +

mE E �

�

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= 2BBEt IVS =

BBV

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+

+

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AC voltage

DC voltage

reactive power

regulator

regulator

E

B

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regulator

regulator

active power

Shunt part of the UPFC

series part of the UPFC

Fig. 10 The arrangements for the four UPFC controllers

V

EV

BV

j xE

shunt busbar E series busbar B

BB

EtV

internal link of series and shunt part

UPFC

P1E PB2directP

direct channel

internal channel

PinternalDCP

Fig. 11 Simplified representation of the UPFC configuration

IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005 267

independently operating units, supplying reactive power to,or absorbing reactive power from, the power system.Hence, we should expect that this is the case when thereexist no interactions between the control functions of theUPFC series and shunt parts.

On the one hand, PInternal decides whether or not theseries and shunt parts of the UPFC are directly electricallyconnected as discussed above. On the other hand, thedirection of PInternal determines whether or not the controlfunctions of the UPFC series and shunt parts conflict withone another, because inside the UPFC, there exists anotheractive power flow. This second active power flow, PDC asshown in Fig. 11, is due to the DC voltage controllerimplemented by the UPFC shunt part to charge thecapacitor in order to maintain the normal operation of theUPFC. Since PInternal is regulated by the UPFC activepower controller, the flow direction of PInternal and PDC

indicates whether or not the control functions of the UPFCseries and shunt parts conflict with one another. If PInternal isin the same direction as PDC from the UPFC shunt part tothe series part then the control functions of the UPFC seriesand shunt parts electrically pose no conflict requirement.Hence, we should not expect a high probability forinteractions between the UPFC series and shunt partscontrol functions. We define this is the case for a positivePInternal. On the other hand, however, if PInternal flows in theopposite direction to PDC from the series part to the shuntpart, we will observe conflicts between the different controlfunctions, resulting in possible control interactions leadingto a poor control performance. This is the case in whichPIntervalo0.

From the above analysis we can propose that PInternal isused as an interaction indicator. If PInternal40, we expect alow probability for serious interaction problems; IfPInternalo0, we expect a high probability for interactions;PInternal¼ 0 indicates that there will be no interaction amongthe UPFC multiple control functions. Obviously, thecalculation of PInternal does not need the UPFC controllersto be designed and installed. Hence, this proposedinteraction indicator only identifies the interactions due to

the electrical coupling of the UPFC control. It successfullyexcludes the impact of UPFC controllers’ parameters on thecontrol interactions. In the next application example, weshall demonstrate the results of this proposed interactionindicator being used in the NETPS of Fig. 4, where aUPFC is installed on the transmission line between nodes14 and 15 at the position with a 20% line length to busbar14. The UPFC regulates the active and reactive powerdelivered along the transmission line and controls thevoltage at the shunt busbar of the UPFC. Four SISOcontrollers are assigned to the UPFC to perform fourcontrol functions: (i) of active power regulation; (ii) reactivepower regulation; (iii) AC voltage regulation; and (iv) DCvoltage regulation.

In order to investigate the potential problem of controlinteractions created by the UPFC multiple control func-tions, the UPFC interaction indicator proposed in thepreceeding Section was calculated at three different operat-ing conditions of the UPFC, and these results are listed inTable 1

From Table 1 we can see that the calculated orinteraction indicators predict a problem with UPFCinteractions under operating condition 1, becausePInternal¼�0.063o0. At the other two UPFC operatingconditions, there is little likelihood of interactions betweenthe UPFC multiple control functions.

In order to verify this prediction from the calculated orinteraction indicator, four individual UPFC SISO con-trollers were designed to operate under operating condition

Table 1: Calculation of UPFC interaction indicator

UPFC operatingcondition

PEt, p.u. QEt, p.u. VEt, p.u. PInternal, p.u.

Condition 1 0.6342 0.3086 0.9959 �0.063

Condition 2 0.3342 0.3068 0.9959 0.0

Condition 3 0.1342 0.4068 0.9909 0.008

0 1 2 3 4 5 6 0 0.5 1.5 2.52 31

0 5 10 15 20 25 300 5 10 15 20 25 30

−0.6−0.4−0.2

0

0.20.4

0.60.81.01.2

DC

vol

tage

, pu

0.99480.9950

0.99520.99540.9956

0.99580.99600.9962

0.99640.9966

AC

col

tage

mag

nitu

de, p

u

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0.44

line

reac

tive

pow

er fl

ow, p

u

0.20.4

0.60.81.0

1.2

1.41.61.82.0

line

real

pow

er fl

ow, p

u

a b

dc

time, s time, s

time, stime, s

Fig. 12 Simulation to assess the joint operation of UPFC controllers when the interaction indicator predicted possible interactions

268 IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005

1, and they achieved a satisfactory control performance.Then their joint operation was assessed as shown by thesimulation results of Fig. 12. From Fig. 12 we can see thatas is predicted by the calculated of interaction indicator,significant interactions leading to system instability wereobserved. Furthermore, this design of four UPFC con-trollers was tested under UPFC operating conditions 2 and3, and Fig. 13 and 14 show the simulation results. FromFigs. 13 and 14 we can see that exactly as it had beenpredicted by the calculation of interaction indicator, nointeractions were observed.

In order to verify that the interactions observed in Fig. 12were due to conflicts between the PInternal and PDC flowsas suggested in the preceeding Section, we connected byUPFC DC voltage regulator to a battery instead of tothe power system. Then joint operation of the fourUPFC controllers was examined. The simulation resultsare demonstrated in Fig. 15. From Fig. 15 we can seethat the interactions observed in Fig. 12 disappear. Thisconfirms that the interactions are indeed due to theconflict between the PInternal and PDC flows. The simulationsin Figs. 11, 12, 13, 14 and 15 are the results of a step

0 5 10 15 200.98

1.021.04

1.06

1.08

1.10

1.12

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1.00

DC

vol

tage

, pu

0 5 10 15 20 25 300.9958

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0.9962

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0.9970

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vol

tage

mag

nitu

de, p

u

0 1 2 3 4 5 6 7 80.30

0.32

0.34

0.36

0.38

0.40

0.42

Line

rel

ativ

e po

wer

flow

, pu

0 2 4 6 8 100.330

0.332

0.334

0.336

0.338

0.340

Line

act

ive

pow

er fl

ow, p

u

time, s time, s

time, s time, s

a b

dc

Fig. 13 Simulation to assess the joint operation of UPFC controllers when the interaction indicator predicted low possibility of interactions

0 5 10 15 20 25 300 5 10 15 20 25 300.40

0.42

0.44

0.46

0.48

0.50

0.52

line

reac

tive

pow

er fl

ow, p

u

0.10

0.15

0.20

0.25

0.30

0.35

line

activ

e po

wer

flow

, pu

0 5 10 150.98

1.021.041.061.081.101.121.141.16

1.00

DC

vol

tage

, pu

0 5 10 15 20 25 300.990

0.991

0.992

0.993

0.994

0.995

0.996

0.997

0.998

AC

vol

tage

mag

nitu

de, p

u

time, s time, s

time, s time, s

a b

dc

Fig. 14 Simulation to assess the joint operation of UPFC controllers when the interaction indicator predicted no interactions

IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005 269

response that is the standard test to examine a controller’sperformance.

4 Conclusions

In order to simplify the description of the operating statusof a UPFC we have proposed that the UPFC has threeoperating modes: (i) an uncontrolled mode; (ii) an inductivemode; and (iii) a capacitive mode. An application examplehas been used to demonstrate that the effect of changes inthe UPFC operations on the power supply system stabilitycan be described more systematically and explained moreclearly when they are represented by changes in theseoperating modes.

Based on an analysis of UPFC operation principles, wehave concluded that the observed control interactions aredue to conflicts between the series and shunt parts of theUPFC, which are connected through the internal commoncapacitor inside the UPFC.We have shown that the linkagepattern of the UPFC series and shunt parts decides whetheror not the control functions implemented by the UPFCseries and shunt parts conflict with one another. Thislinkage pattern of the UPFC series and shunt parts can bedescribed by the flow of active power through the UPFC atsteady-state operation of the power system. Hence thisprovides a physical explanation for the interactions betweenthe UPFC multiple control functions in terms of powersystem operation. In order to predict the possible existenceof interactions between UPFC multiple control functionswithout having to first design UPFC controllers and thenrun power system simulations we have proposed aninteraction indicator. The proposed indicator is the direc-tion and amount of active power flow through the internallink of the UPFC series and shunt part at steady-stateoperation of the power system. It can be calculated from thepower system load flow solution without having to runsimulations of the power system with UPFC controllersinstalled. By using the indicator, the interactions amongmultiple control functions of the UPFC possibly caused by

badly set controller’s parameters are excluded. Hence, itonly identifies the possible existence of inherent controlconflicts between UPFC multiple control functions. Twoapplication examples of a ten-generator 39-bus NETPShave been presented.

5 References

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12 Larsen, E.V., Sanchez-Gasca, J.J., and Chow, J.H.: ‘Concepts forDesign of FACTS Controllers to Damp Power Swings’, IEEE Trans.,Power syst., 1995, 4, (10), pp. 948–956

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14 Wang, H.F., Jazaeri, M., and Johns, A.T.: ‘Investigation into thedynamic interactions of multiple multi-functional Unified Power FlowControllers’, IEEE Power Eng. Lett., 2000, 20, (7), pp. 45–48

0 5 10 15 20 25 300 5 10 15 200.30

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nitu

de, p

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time, stime, s

a b

c d

Fig. 15 Simulation to assess the joint operation of UPFC controllers when DC voltage regulator is connected to a battery to avoid interactions

270 IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 2, March 2005