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1. Flexible AC Transmission
Systems

2. What is FACTS?
FlexibleACTransmissionSystem(FACTS):
Alternatingcurrenttransmissionsystemsincorporatingpower
electronic-basedandotherstaticcontrollerstoenhance
controllabilityandincreasepowertransfercapability.
FACTS Controller:
Apowerelectronic-basedsystemandotherstaticequipment
thatprovidecontrolofoneormoreACtransmissionsystem
parameters.
General symbol of FACTS controller

3. Basic types of FACTS Controllers
Based on the connection, generally FACTS controller can be classified
as follows:
Seriescontrollers
Shunt controllers
Combined series-series controllers
Combined series-shunt controllers

4. Basic types of FACTS Controllers
Seriescontrollers
Theseriescontrollercouldbeavariableimpedanceora
variablesource,botharepowerelectronicsbaseddevicestoserve
thedesiredneeds.Inprinciple,allseriescontrollersinjectvoltagein
serieswiththeline.
General symbol of Series controller

5. Shuntcontrollers
 Theshuntcontrollersmaybevariableimpedance,variable
sourcesorcombinationofthese.Inprinciple,allshuntControllersinject
currentintothesystematthepointofconnection.Aslongastheinjected
currentisinphasequadraturewiththelinevoltage,theshuntControlleronly
suppliesorconsumesvariablereactivepower.
General symbol of Shunt controller

6. Combinedseries-seriescontrollers:
 Thecombinationcouldbeseparateseriescontrollersorunified
series-seriescontroller.SeriesControllersprovideindependentseries
reactivecompensationforeachlinebutalsotransferrealpoweramong
thelinesviathepowerlink-InterlinePowerFlowController.
Symbol of Series-Series controller

7. Combinedseries-shuntcontrollers:
 Thecombinationcouldbeseparatedseriesandshunt
controllersoraunifiedpowerflowcontroller.Inprinciple,combined
shuntandseriesControllersinjectcurrentintothesystemwiththe
shuntpartoftheControllerandvoltageinseriesinthelinewiththe
seriespartoftheController.
Symbol of Series-Shunt controller

8. Brief Description and Definitions of FACTS
controllers
Shunt connected controllers
Series connected controllers
Combined shunt -series connected controllers

9. Shunt connected controllers
Static Synchronous Generator (SSG)
Static Synchronous Compensator (STATCOM)
Battery Energy Storage System (BESS)
Superconducting Magnetic Energy Storage (SMES)
Static Var Compensator (SVC)
Thyristor Controlled Reactor (TCR)
Thyrlstor Swltched Reactor (TSR)
Thyristor Switched Capacitor (TSC)
Static Var Generator or Absorber (SVG)
Thyristor Controlled Braking Resistor (TCBR)
Types of shunt connected controllersavailable in the market

10. Shunt connected controllers
StaticSynchronousCompensator(STATCOM):
AStaticsynchronousgeneratoroperatedasashunt-connectedstatic
varcompensatorwhosecapacitiveorinductiveoutputcurrentcanbe
controlledindependentoftheacsystemvoltage.
STATCOM based on
voltage-sourced converters
STATCOM based on
current-sourced converters
 Incostpointofview,thevoltage-sourcedconvertersseemtobe
preferred,andwillbethebasisforpresentationsofmostconverter-based
FACTSControllers.

11. Static Var Compensator (SVC):
 Ashunt-connectedstaticvargeneratororabsorberwhose
outputisadjustedtoexchangecapacitiveorinductivecurrentsoasto
maintainorcontrolspecificparametersoftheelectricalpowersystem
(typicallybusvoltage).
Thethyristor-controlledreactor(TCR)orthyristor-
switchedreactor(TSR)forabsorbingreactivepower
andthyristor-switchedcapacitor(TCS)forsupplying
thereactivepower.
SVCisconsideredbysomeasalowercost
alternativetoSTATCOM.

12. Series connected controllers
Static Synchronous Series Compensator (SSSC)
Interline Power Flow Controller (IPFC)
Thyristor Controlled Series Capacitor (TCSC)
Thyristor-Switched Series Capacitor (TSSC)
Thyristor-Controlled Series Reactor (TCSR)
Thyristor-Switched Series Reactor (TSSR)
Types of series connected controllersavailable in the market

13. Series connected controllers
StaticSynchronousSeriesCompensator(SSSC):
InaStaticSynchronousSeriesCompensator,outputvoltageisin
quadraturewith,andcontrollableindependentlyof,thelinecurrentforthe
purposeofincreasingordecreasingtheoverallreactivevoltagedrop
acrosstheline
ItislikeaSTATCOM,exceptthattheoutput
acvoltageisinserieswiththeline.

14. ThyristorControlledSeriesCapacitor(TCSC):
Acapacitivereactancecompensatorwhichconsistsofa
seriescapacitorbankshuntedbyathyristor-controlledreactorinorder
toprovideasmoothlyvariableseriescapacitivereactance.
ItisanalternativetoSSSC.
TheTCSCmaybeasingle,largeunit,ormayconsistofseveralequalor
different-sizedsmallercapacitorsinordertoachieveasuperior
performance.

15. Unified Power Flow Controller (UPFC)
Thyristor-Controlled Phase Shifting Transformer (TCPST)
Interphase Power Controller (IPC):
Combined shunt-series connected controllers
Types of shunt series connected controllersavailable in the market

16. Combined shunt and series connected controllers
UnifiedPowerFlowController(UPFC):
 Acombinationofstaticsynchronouscompensator(STATCOM)anda
staticseriescompensator(SSSC)whicharecoupledviaacommondclink,
toallowbidirectionalflowofrealpowerbetweentheseriesoutputterminals
oftheSSSCandtheshuntoutputterminalsoftheSTATCOM,andare
controlledtoprovideconcurrentrealandreactiveserieslinecompensation
withoutanexternalelectricenergysource.
ThisisacompleteControllerfor
controllingactiveandreactivepower
controlthroughtheline,aswellasline
voltagecontrol.

17. Thyristor-Controlled Voltage Limiter (TCVL)
Thyristor-Controlled Voltage Regulator (TCVR) based
on tap changing;
Thyristor-Controlled Voltage Regulator (TCVR)
based on voltage injection.
Other controllers

18. Controlofpowerflowasordered.
Increasetheloadingcapabilityoflinestotheirthermalcapabilities,
includingshorttermandseasonal.
Increasethesystemsecuritythroughraisingthetransientstabilitylimit,
limitingshort-circuitcurrentsandoverloads,managingcascading
blackoutsanddampingelectromechanicaloscillationsofpowersystems
andmachines.
Providesecuretielineconnectionstoneighboringutilitiesandregions
therebydecreasingoverallgenerationreserverequirementsonbothsides.
Providegreaterflexibilityinsitingnewgeneration.
Reducereactivepowerflows,thusallowingthelinestocarrymore
activepower.
Increaseutilizationoflowestcostgeneration.
BENEFITS FROM FACTS TECHNOLOGY

19. Static VAR Compensator (SVC)

20. Static VAR Compensator (SVC)
-A TCRand TSCare the most important building blocks of thyristor
based SVCs
Thyristor control reactor (TCR)
ThecontrollablerangeoftheTCRfiringangle,
alpha,extendsfrom90degreeto180degree.
Afiringangleof90degreeresultsinfullthyristor
conductionwithacontinuoussinusoidalcurrent
flowintheTCR.
Thecurrentreducestozeroforafiringangleof
180degree.
Thyristorfiringatanglesbelow90degree
introducesdccomponentsinthecurrent,disturbing
thesymmetricaloperationofthetwoantiparallel
valvebranches.
B
L

21. Let the source voltage be expressed as
The TCR current is then given by the following differential equation:
Where,
V=Peakvalueoftheappliedvoltage
ω=Angularfrequencyofsupplyvoltage.

22. TCR V-I Characteristic TCR susceptance characteristic.
TCR operating Characteristic

23. Current and voltages for different αin a TCR.

24. Thyristor Switched Capacitors:
Thecapacitorvoltageisnotequaltothesupplyvoltagewhenthethyristorsarefired.
Immediatelyafterclosingtheswitch,acurrentofinfinitemagnitudeflowsandchargesthe
capacitortothesupplyvoltageinaninfinitelyshorttime.Theswitchrealizedbythyristors
cannotwithstandthisstressandwouldfail.
Thecapacitorvoltageisequaltothesupplyvoltagewhenthethyristorsarefired.The
analysisshowsthatthecurrentwilljumpimmediatelytothevalueofthesteady-state
current.Thesteadystateconditionisreachedinaninfinitelyshorttime.Althoughthe
magnitudeofthecurrentdoesnotexceedthesteady-statevalues,thethyristorshavean
upperlimitofdi/dtvaluesthattheycanwithstandduringthefiringprocess.Here,di/dtis
infinite,andthethyristorswitchwillagainfail.

25. ItcanthereforebeconcludedthatthissimplecircuitofaTSCbranch
isnotsuitable.
is a magnification
factor

26. Static VAR Compensator (shunt connected controller)
Note:
Thecontrolstrategyusuallyaimstomaintainthetransmissionlinevoltageatafixedlevel.

27. SVC V-I Characteristic

29. Thyristor-controlled series capacitor, TCSC

30. Thyristor-controlled series capacitor, TCSC
-Series-connected controller
Advantage of Series Compensation
Withseriescapacitors,thereactivepowerincreasesas
thesquareoflinecurrent,whereaswithshuntcapacitors,
thereactivepowerisgeneratedproportionaltothesquare
ofbusvoltage.
Inasamesystembenefitsasthoseofseries
capacitors,shuntcapacitorsthatarethreetosixtimes
morereactivepower–ratedthanseriescapacitorsneedto
beemployed.
Furthermore,shuntcapacitorstypicallymustbe
connectedatthelinemidpoint,whereasnosuch
requirementexistsforseriescapacitors.

31. Variable-Series Compensation
1.Enhancedbase-powerflowandloadabilityofthe
series-compensatedline.
2.Additionallossesinthecompensatedlinefromthe
enhancedpowerflow.
3.Increasedresponsivenessofpowerflowintheseries-
compensatedlinefromtheoutageofotherlinesinthe
system.

32. A basic TCSC module
TCSC CONTROLLER
A typical TCSC system.

33. Variation of the TCSC reactance with firing angle alpha

34. Advantages of the TCSC
Rapid,continuouscontrolofthetransmission-lineseries-
compensationlevel.
Dynamiccontrolofpowerflowinselectedtransmission
lineswithinthenetworktoenableoptimalpower-flow
conditionsandpreventtheloopflowofpower.
Suppressionofsubsynchronousoscillations.
Decreasingdc-offsetvoltages.
Enhancedlevelofprotectionforseriescapacitors.
Voltagesupport.
Reductionoftheshort-circuitcurrent.Duringeventsof
highshort-circuitcurrent,theTCSCcanswitchfromthe
controllable-capacitancetothecontrollable-inductance
mode,therebyrestrictingtheshort-circuitcurrents.

35. Unified Power Flow Controller (UPFC)

36. UnifiedPowerFlowController(UPFC):
Acombinationofstaticsynchronouscompensator(STATCOM)anda
staticseriescompensator(SSSC)whicharecoupledviaacommondc
link,toallowbidirectionalflowofrealpowerbetweentheseriesoutput
terminalsoftheSSSCandtheshuntoutputterminalsoftheSTATCOM.
 STATCOM
 SSSC

37. Static synchronous compensator (STATCOM)
Power circuit
Equivalent circuit

38. Iftheamplitudeoftheoutputvoltageisincreasedabovethatof
theutilitybusvoltage,Et,thenacurrentflowsthroughthereactance
fromtheconvertertotheacsystemandtheconvertergenerates
capacitive-reactivepowerfortheacsystem.
Iftheamplitudeoftheoutputvoltageisdecreasedbelowthe
utilitybusvoltage,thenthecurrentflowsfromtheacsystemtothe
converterandtheconverterabsorbsinductive-reactivepowerfrom
theacsystem.
Iftheoutputvoltageequalstheacsystemvoltage,thereactive-
powerexchangebecomeszero,inwhichcasetheSTATCOMissaid
tobeinafloatingstate.
Power exchange
STATCOM Operation

39. Inpractice,thesemiconductorswitchesoftheconverterarenot
lossless,sotheenergystoredinthedccapacitoriseventuallyused
tomeettheinternallossesoftheconverter,andthedccapacitor
voltagediminishes.
WhentheSTATCOMisusedforreactive-powergeneration,the
converteritselfcankeepthecapacitorchargedtotherequired
voltagelevel.Thistaskisaccomplishedbymakingtheoutput
voltagesoftheconverterlagbehindtheac-systemvoltagesbya
smallangle(usuallyinthe0.18–0.28range).
Inthisway,theconverterabsorbsasmallamountofrealpowerfrom
theacsystemtomeetitsinternallossesandkeepthecapacitor
voltageatthedesiredlevel.

40. Static series compensator (SSSC)
Power circuit
Equivalent circuit

41. SSSC Operation
Forinstance,iftheinjectedvoltageisinphasewiththelinecurrent,
thenthevoltagewouldexchangerealpower.
Ontheotherhand,ifavoltageisinjectedinquadraturewiththeline
current,thenreactivepower—eitherabsorbedorgenerated—wouldbe
exchanged.

42. Unified Power Flow Controller (UPFC):
Inverter 2
SSSC
Inverter 1
STATCOM

43. Inverter2providesthemainfunctionoftheUPFCbyinjectingan
acvoltageVpqwithcontrollablemagnitudeVpq(0<Vpq<Vpq
max)andphaseangleα(0<α<360),atthepowerfrequency,in
serieswithlineviaaninsertiontransformer.
Thetransmissionlinecurrentflowsthroughthisvoltagesource
resultinginrealandreactivepowerexchangebetweenitandtheac
system.Therealpowerexchangedattheacterminal(i.e.atthe
terminaloftheinjectiontransformer)isconvertedbytheinverterinto
dcpower,whichappearsatthedclinkaspositiveornegativereal
powerdemand.
ThebasicfunctionofInverter1istosupplyorabsorbthereal
powerdemandedbyInverter2atthecommondclink.
Inverter1canalsogenerateorabsorbcontrollablereactivepower,
ifitisdesired,andtherebyitcanprovideindependentshuntreactive
compensationfortheline.
UPFC Operation

44. Interline Power Flow Controller (IPFC):
Power circuit
ThebasicdifferencewithaUPFCisthatthesupportsystemintheIPFC
istheseriesconverterinsteadofashuntconverter.