05160799.pdf

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2Central Electricity Authority, Sewa Bhawan, R.K. Puram, New Delhi 110 066, India3Department of Electrical Engineering, Ecole de Technologie Superieure (ETS), 1100 Notre Dame Oust, Montreal, Quebec,

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www.ietdl.orgCanada, H3C1K3E-mail: [email protected]

Abstract: Fast acting static synchronous compensator (STATCOM), a representative of FACTS family, is a promisingtechnology being extensively used as the state-of-the-art dynamic shunt compensator for reactive power controlin transmission and distribution system. Over the last couple of decades, researchers and engineers have madepath-breaking research on this technology and by virtue of which, many STATCOM controllers based on the self-commutating solid-state voltage-source converter (VSC) have been developed and commercially put in operationto control system dynamics under stressed conditions. Because of its many attributes, STATCOM has emerged as aqualitatively superior controller relative to the line commutating static VAR compensator (SVC). This controller iscalled with different terminologies as STATic COMpensator advanced static VAR compensator, advanced static VARgenerator or static VAR generator, STATic CONdenser, synchronous solid-state VAR compensator, VSC-based SVC orself-commutated SVC or static synchronous compensator (SSC or S2C). The development of STATCOM controlleremploying various solid-state converter topologies, magnetics congurations, control algorithms, switchingtechniques and so on, has been well reported in literature with its versatile applications in power system. Areview on the state-of-the-art STATCOM technology and further research potential are presented classifyingmore than 300 research publications.

1 IntroductionLine commutating thyristor device-based solid-state reactivepower compensators were developed in the 1970s. These areused either as thyristor switched capacitors or thyristor-controlled reactor (TCRs) or a combination thereof withpassive lters eliminating dominant harmonics generatedfrom electronic switching phenomenon. These are basicallya VAR impedance-type controllers, commonly known asstatic VAR compensator (SVC), where susceptance of theTCR is controlled by varying the ring angle. Thetechnology is well matured, but its operational exibilityand versatile applications are limited.

With the advent of voltage-source converter (VSC)technology built upon self-commutating controllable solid-state switches viz. gate turn-off thyristor (GTO), insulatedgate bipolar transistor (IGBT), injection-enhanced gate

transistor (IEGT), integrated gate commutated thyristor(IGCT) or gate commutated thyristor (GCT) and so on, ithas ushered a new family of FACTS controllers such asstatic synchronous compensators (STATCOM) and uniedpower ow controller (UPFC) have been developed. Theself-commutating VSC, called as DC-to-AC converter, isthe backbone of these controllers being employed toregulate reactive current by generation and absorption ofcontrollable reactive power with various solid-stateswitching techniques. The major attributes of STATCOMare quick response time, less space requirement, optimumvoltage platform, higher operational exibility and excellentdynamic characteristics under various operating conditions.These controllers are also known as STATic COMpensator(STATCOM), advanced static VAR compensator (ASVC),advanced static VAR generator (ASVG), STATicCONdenser (STATCON), static var generator (SVG),synchronous solid-state VAR compensator (SSVC),Published in IET Power ElectronicsReceived on 28th January 2008Revised on 22nd April 2008doi: 10.1049/iet-pel.2008.0034

Static synchronous com(STATCOM): a reviewB. Singh1 R. Saha2 A. Chand1Department of Electrical Engineering, Indian Institute of TecPower Electron., 2009, Vol. 2, Iss. 4, pp. 297324i: 10.1049/iet-pel.2008.0034ISSN 1755-4535

pensators

a3 K. Al-Haddad3nology, New Delhi 110 016, India297

& The Institution of Engineering and Technology 2009

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www.ietdl.orgVSC-based SVC or self-commutated SVC or staticsynchronous compensator (SSC or S2C). EPRI in USA isa pioneer to conduct research in this area and has beeninstrumental to develop a number of existing STATCOMprojects in collaboration with power utilities/industries.Power industries such as GE, Siemens, ABB, Alsthom,Mitsubishi, Toshiba and so on, with their in-house R&Dfacilities have given birth to many versatile STATCOMprojects presently in operation in high-voltage transmissionsystem to control system dynamics under stressedconditions. The VSC-based STATCOM has emerged as aqualitatively superior technology relative to that of the line-commutating thyristor-based SVC being used as dynamicshunt compensator.

GTO-based VSCs (GTO-VSC), commercially availablewith high power capacity, are employed in high powerrating controllers with triggering once per cycle[fundamental frequency switching (FFS)]. Although IGBTand IGCT devices are available with reasonably goodpower ratings, these are being mainly used in low-to-medium rating compensators operated under pulse-widthmodulation (PWM) switching, that is, multiple switching(13 kHz) in a cycle of operation. Use of these switchingdevices in high power rating controllers is yet to be fullycommercialised and therefore its use is limited. In thestate-of-the-art STATCOM equipments, two majortopologies of VSC-bridges viz. multi-pulse and multi-levelare the most common for operation under FFS or PWMmode or selective harmonic elimination modulation. Forhigh power rating STATCOMs, GTO-VSC is still thechoice for operation under square-wave mode of switching,that is, once per cycle. A concept of multi-level voltage re-injection in DC circuit of VSC topology, as an alternativeto high-frequency device switching adopted under PWMcontrol or instead of adopting higher multi-level topologyunder FFS principle, has been reported to multiply thepulse-order several times without employing additionalVSCs. With commercialisation of this approach, therewould be a major saving of solid-state devices and magneticcomponents.

A comprehensive review on the STATCOM technologyand its development are carried out in this paper. Thepaper includes ten sections viz. (i) working principle ofSTATCOM, (ii) solid-state switching devices andtechnology, (iii) STATCOM topologies and congurations,(iv) control methodologies and approaches, (v) componentselection, (vi) specic applications, (vii) simulation tools,(viii) latest trends and perspective research potentials (ix)concluding remarks and (x) references. Based on theliterature survey, Refs. [1320] are classied into threecategories such as texts [117], patents [1840] andresearch papers [41320]. Based on the development ofSTATCOM technology, the articles [41320] have beenclassied into eight subgroups comprising of (i) state-of-the-art technology [4154], (ii) GTO-VSC basedSTATCOMs [5572], (iii) PWM-VSC basedThe Institution of Engineering and Technology 2009STATCOMs [7391], (iv) multi-level and multi-pulsetopologies [92132], (v) control methodologies [133227],(vi) specic applications of STATCOMs [228305], (vii)STATCOMs with energy source [306313] and (viii)STATCOM simulation techniques [314320].

2 Working principle of statcomVSC is the backbone of STATCOM and it is a combinationof self-commutating solid-state turn-off devices (viz. GTO,IGBT, IGCT and so on) with a reverse diode connected inparallel to them. The solid-state switches are operatedeither in square-wave mode with switching once per cycleor in PWM mode employing high switching frequencies ina cycle of operation or selective harmonic eliminationmodulation employing low switching frequencies. A DCvoltage source on the input side of VSC, which is generallyachieved by a DC capacitor and output, is a multi-steppedAC voltage waveform, almost a sinusoidal waveform. Theturn-off device makes the converter action, whereas diodehandles rectier action. STATCOM is essentiallyconsisting of six-pulse VSC units, DC side of which isconnected to a DC capacitor to be used as an energystorage device, interfacing magnetics (main couplingtransformer and/or inter-mediate/inter-phase transformers)that form the electrical coupling between converter ACoutput voltage (Vc) and system voltage (Vs) and a controller.The primary objective of STATCOM is to obtain analmost harmonic neutralised and controllable three-phaseAC output voltage waveforms at the point of commoncoupling (PCC) to regulate reactive current ow bygeneration and absorption of controllable reactive power bythe solid-state switching algorithm. As STATCOM hasinherent characteristics for real power exchange with asupport of proper energy storage system, operation of suchcontroller is possible in all four quadrants of QP plane [2]and it is governed by the following power ow relation

S 3VsVcXL

sina j3 VsVcXL

cosa V2s

XL

! P jQ (1)

where S is the apparent power ow, P the active power ow,Q the reactive power ow, Vs the main AC phase voltage toneutral (rms), Vc the STATCOM fundamental output ACphase voltage (rms), X ( vL, where, v 2pf ), theleakage reactance, L the leakage inductance, f the systemfrequency and a the phase angle between Vs and Vc.

Active power ow is inuenced by the variation of a andreactive power ow is greatly varied with the magnitude ofthe voltage variation between Vc and Vs. For lagging a,power (P) ows from Vc to Vs, for leading a, power (P)ows from Vs to Vc and for a 0, the P is zero and Q isderived from (1) as follows

Q VsXL

(Vc Vs) (2)IET Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324doi: 10.1049/iet-pel.2008.0034

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www.ietdl.orgVc Vs, no reactive power exchange takes place. In thehigh rating STATCOM operated under fundamentalfrequency switching, the principle of phase angle control(a) is generally adopted in control algorithm to compensateconverter losses by active power drawn from AC systemand also for power ows in or out of the VSC to indirectlycontrol the magnitude of DC voltage with charging ordischarging of DC bus capacitor enabling control of

Figure 1 Basic two-level six-pulse VSC bridge and its AC voltPower Electron., 2009, Vol. 2, Iss. 4, pp. 297324i: 10.1049/iet-pel.2008.0034second generation FACTS controller being used as adynamic reactive power compensator. This powersemiconductor device has no turn-off capability andrelatively high response time. The emerging technology issolid-state controllable turn-off switches. These switchesviz. GTO, IGBT, IGCT are being used extensively inconverter circuits for state-of-the-art FACTS controllers.Drive circuit requirements, switching frequency/speed,

output waveform in square-wave mode of operationThe AC voltage output (Vc) of STATCOM is governed byDC capacitor voltage (Vdc) and it can be controlledby varying phase difference (a) between Vc and Vs (and alsoby m, modulation index for PWM control). The basic two-level and three-level VSC congurations and respective ACoutput voltage (Vc) waveforms corresponding to a square-wave mode of operation are illustrated in Figs. 1 and 2,respectively.

Functionally, STATCOM injects an almost sinusoidalcurrent (I ) in quadrature (lagging or leading) with the linevoltage (Vs), and emulates as an inductive or a capacitivereactance at the point of connection with the electricalsystem for reactive power control, and it is ideally thesituation when amplitude of Vs is controlled from fullleading (capacitive) to full lagging (inductive) for a equalsto zero (i.e. both Vc and Vs are in the same phase). Themagnitude and phase angle of the injected current (I ) aredetermined by the magnitude and phase difference (a)between Vc and Vs across the leakage inductance (L), whichin turn controls reactive power ow and DC voltage, Vdcacross the capacitor. When Vc . Vs, the STATCOM isconsidered to be operating in a capacitive mode. WhenVc , Vs, it is operating in an inductive mode and for

reactive power ow into the system. Phasor diagramsthe operating principle are illustrated in (Figs. 3a3g). Taspect is well presented in [1, 2, 4, 6, 12, 15, 31, 32,58, 59, 63, 73, 92, 96, 109, 116, 136, 140, 144, 160, 1225, 235].

3 State-of-the-art solid-stateswitching devices and switchingtechnologyIn power converter circuits [41, 44, 47, 48, 51], varcontrollable solid-state switches such as conventiothyristor, GTO, IGBT, IEGT, IGCT or GCT [1bipolar junction transistor (BJT) and MOS eld eftransistor are employed for various applications suchVSC, current-source converter and so on. Each devicedifferent operating characteristics in respect to switchfrequency/speed, device ratings, turn-off and turntimings, forward and reverses breakdown voltage, on-svoltage drop, switching losses and so on. The conventiothyristor, a line commutating switching device availcommercially at very high power ratings, is a matechnology and forms basic switching element for SVC299


ativelyd inCTStchingCT isy [47,igherents.OMalega

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www.ietdl.orgFigure 3 STATCOM operating principle and control characteristics

a Capacitive modeb Inductive modec Floating moded Capacitor charging modee Capacitor discharging modef VI characteristicsg VQ characteristicsThe Institution of Engineering and Technology 2009switching losses and cost of each device are the trade-off touse these devices effectively. Among the turn-off powerswitches, GTO thyristor is a mature technology andcommercially available at high power ratings. Its extensiveapplications in high power rating converter-cum-compensator circuits have ushered in a new era of FACTS[42, 43, 52, 54, 63, 70, 296] controllers, for example,STATCOM [46, 228, 239, 252, 269, 280282], UPFC[252, 281], convertible static compensator (CSC) [278],static synchronous series compensator (SSSC) [252, 281]

and so on. Solid-state IGBT switching device is a relnew technology in power electronics is employemedium-to-high power ratings PWM-based FAcontrollers [41, 44, 47, 271] due to its high swifrequency and speed. Among the turn-off switches, IGthe most promising and emerging solid-state technolog48] and has the merits of low switching loss, hswitching frequency/speed, no snubber circuit requiremIGCT-converter-based high power rating STATC[280] is under implementation stage at 138 kV T

Figure 2 Basic three-level six-pulse VSC bridge and its output AC voltage waveform in square-wave mode of operatioIET Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324doi: 10.1049/iet-pel.2008.0034

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www.ietdl.orgsub-station in California. Because of relatively high cost, itscommercial competitiveness is yet to be fully explored.

Switching topologies such as PWM or power frequencyswitching depend upon the type of solid-state devices usedin STATCOM. Primarily, fundamental frequency methodof switching (pulsed one per line frequency cycle) andPWM techniques (pulsed multi times per half cycle) arewidely accepted methods. In PWM control, solid-stateswitches are operated many times at frequent intervalswithin the same cycle of output voltage, and an improvedquality of output AC voltage waveforms [in terms of low-amplitude of low-order harmonics/low total harmonicdistortion (THD)] can be obtained. Based on thefrequency and amplitude of triangular shape carrier signaland modulating control signal, PWM converters aredesigned, in general, to eliminate triplen and other low-order harmonics (5th/7th), and by means of suitablelter design, predominantly higher-order harmonics arereduced in the AC voltage output. As the converterconduction and switching losses are a function ofswitching frequency, the PWM technique is notgenerally adopted in high rating STATCOMs onaccount of high switching losses, whereas low-to-medium rating STATCOMs used in power distributionsystem are built upon PWM control and suchSTATCOMs are generally termed D-STATCOM [55,61, 88, 90, 91, 117, 217, 243, 251, 260, 268, 274, 275,307, 310]. Switching frequency [16] of solid-statedevices is one of the key factors in designing PWM-VSCand it can be typically 3 kHz for IGBT and 500 Hz forIGCT or GCT. The various aspects of PWM-VSCbased STATCOM have been presented in [7391].However, soft-switching technique or rather zero-voltageswitching applications in multiple voltage source square-wave converters have been proposed in the literature [73,99] to considerably reduce switching losses and electro-magnetic interference.

As GTO is well-proven solid-state device andcommercially available with power-handling levels as that ofthe conventional thyristor, GTO-VSC is the backbone ofthe high power rating STATCOMs [5572] that are usedextensively in high-voltage transmission system. The PWMtechnique in such converter circuit has been found to beunpopular due to its higher gating energy requirements andswitching losses. Factoring this, STATCOMs built uponGTO-VSCs are designed primarily to operate it in asquare-wave mode of operation.

4 Statcom topologies andcongurationsMany VSC-based topologies and congurations are adopted inthe state-of-the-art STATCOM controllers and signicantly,multi-pulse and/or multi-level topologies [46, 92132] arewidely accepted in the design of compensators. For example,Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324i: 10.1049/iet-pel.2008.0034a two-level multi-pulse topology is a mature topology andcommercially adopted in +100 MVA STATCOM at 500/161 kV Sullivan S/S of Tennessee Valley Authority (TVA),US [231, 235, 239, 240] and in +80 MVA SVG at 154 kVInuyama switching station of Kansai Electric Power Co.(KEPC), Japan [228]. An elementary six-pulse VSC whichconsists of three legs (phases) with two valves per leg and anelectrostatic capacitor on the DC bus is illustrated in Fig. 1.Each valve consists of a self-commutating switch with areverse diode connected in parallel. In square-wave mode,eight possible switching states are possible with respect to thepolarity of DC voltage source (Vdc). A set of three quasi-square waveforms at its AC terminals, displaced successivelyby 1208, is obtained using fundamental frequency switchingmodulation. The phase to neutral (0, +Vdc/3, +2Vdc/3) andline-to-line voltage (0, +Vdc) of the converter shown inFig. 1 contain an unacceptable current harmonics causingsevere harmonic interference to electrical system. To reduceTHD, multi-pulse converter topology derived from thecombination of multiple number (N-numbers) of elementarysix-pulse converter units to be triggered at specicdisplacement angle(s), is widely adopted, and output ACvoltage waveforms from each unit is electro-magneticallyadded with an appropriate phase shift by inter-phasetransformer(s) to produce a multi-pulse (6 N pulses)waveform close to sinusoidal wave.

In a multi-pulse converter conguration, the displacementangle between two consecutive six-pulse converter is 2p/(6N ) and three-phase voltage contains odd harmonicscomponent of the order of (6Nk+ 1), where k 1, 2,3, . . . . With the increase in pulse number, lower-orderharmonics are neutralised and a very close to sinusoidalAC output voltage waveform can be realised. Comparedwith basic six-pulse converter, the multi-pulse congurationof STATCOM increases the achievable VAR rating,improves the harmonic performance, decreases the DC sidecurrent harmonics and reduces signicantly the overalllter requirements. Basic two-level 12 (2 6-pulse),24 (4 6-pulse) and 6N (N 6-pulse)-pulse convertercongurations are depicted in Figs. 4a4b, 5 and 6,respectively. Basic congurations of magnetics in multi-pulse converters are discussed in [92, 228, 235]. It is notedthat increase in pulse order increases the number ofelectronics devices, magnetics and associated componentsand thus added to the cost. However, the high pulse-orderSTATCOM enables to improve harmonics and operationalperformances. Most industrial practices are to employ 48-pulse conguration [46, 228, 131, 235, 239, 240, 252, 269,278] where magnetics are designed generally in two stagesusing transformers. The inter-phase transformers (as manyas VSCs) are employed to sum-up the output AC voltagesof converters, which is further stepped-up through a maincoupling transformer to match with the main AC system.The typical two stages of magnetics architecture of theexisting +80 MVA SVG [228] at the Inuyama switchingstation are depicted in Figs. 7a and 7b. The feasibility ofother magnetics congurations in 48-pulse compensator,301


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www.ietdl.orgFigure 5 24-pulse (4 6-pulse) converter conguration Figure 6 6N-pulse (N6-pulse) converter conguratiowhich are proposed in the literature [46], are illustrated inFigs. 7c and 7d. Out of the few multi-pulse topologies, 12-pulse, 18-pulse, 24-pulse and 48-pulse congurations arevery common and based on which, many STATCOMpower circuits are proposed in the literature [20, 21, 26, 36,38, 60, 61, 71, 86, 93, 111, 114, 143, 148, 167, 228, 235,298, 299, 309]. The EMTP models of 12-pulse and 24-pulse VSC-based STATCOMs are presented in [111, 167].

Typically, 12-pulse two-level converter congurationsconsisting of two elementary six-pulse bridges [55, 58,167], DC side of each is connected in parallel and its ACside is either connected in series or in parallel are shown inFigs. 4a and 4b. Magnetics in a 12-pulse two-levelSTATCOM is congured such that, one bridge is fed toYY transformer and the other bridge to a DYtransformer maintaining thereby a phase shift of 308between two sets of fundamental AC output voltagewaveform. The converter side D-winding has

p3 times the

turns as the converter side Y-winding to keep the samevolts per turn in both the windings. The AC mains sidewindings (Y ) are connected in series and can have any turnratios to increase or decrease the output voltages. Thecombined output phase voltage leads to multi-steppedvoltage waveform and has 12-pulse waveform withharmonics of the order of (12k +1) that is, 11th, 13th,23rd . . . and with amplitudes of 1/11th, 1/13th, 1/23rd . . . of fundamental amplitude, respectively.

Figure 4 Multi-pulse parallel and series converter congurat

a 12-pulse parallel converter congurationb 12-pulse series converter congurationThe Institution of Engineering and Technology 2009Another variant of topology is a multi-level VSC structureto generate multi-stepped voltage waveform close tosinusoidal nature. Owing to the complex series-parallelconnection of transformers windings/circuits in multi-pulseconverters, multi-level congurations have been receivingincreasing attention for high voltage and high power ratingapplications. In multi-level topology, a synthesised stair-case voltage waveform is derived from several levels of DCvoltage sources obtained normally by using capacitorvoltage sources, and in this category, three-level convertertopologies with square-wave mode of operation is mostcommon [252, 280, 281]. An N-level topology is achievedby splitting of DC capacitors into (N2 1) sectionsproduces N-level output phase voltage and a (2N2 1) leveloutput line voltage waveform. When number of levels ishigh enough, harmonic content in AC output voltage isreduced to low enough to avoid the need of lters. Themain features of multi-level converter are the low harmoniccontent of the output voltage compared with a square-wavepulse converter, decreased device voltage stress (a fractionof the total DC bus voltage) and potentially higherconverter voltage and thus power rating. It is proposed in[95] that the multi-level topology employing capacitorvoltage synthesis technique is to be preferred to the multi-pulse topology employing magnetic coupling technique.Three basic types of multi-level VSCs are reported in [95,114, 123] viz. (i) multi-point clamped converter in whichthree-level neutral point clamped (NPC) converter topologyis a matured technology [61, 114, 116, 153] and on this

sIET Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324doi: 10.1049/iet-pel.2008.0034

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www.ietdl.orgFigure 7 Two stages of magnetics architecture and feasibility of other magnetics conguration in two-level 48-pulse (86-pulse) STATCOM circuit

a Magnetics of 48-pulse, two-level +80 MVA STATCOM at Inuyama sw. station, KEPCb 48-pulse STATCOM terminal AC voltage waveform at PCCc Typical magnetics congurations of true 48-pulse STATCOMd Magnetic conguration of Quasi 48-pulse STATCOMPower Electron., 2009, Vol. 2, Iss. 4, pp. 297324 303i: 10.1049/iet-pel.2008.0034 & The Institution of Engineering and Technology 2009

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www.ietdl.orgconcept many STATCOM controllers have beencommercially developed. Contrary to two-level converters,this three-level converter allows controlling of themagnitude of AC voltage by a variation of dead-angle (b)maintaining xed DC capacitor voltage. The second type ischain converters based on standard H-bridge arrangementsand the third is nested-cell converter or ying capacitormulti-level converter. Implementations of these convertersrequire the same number of switches for the same numberof levels, but there is a wide variation in terms of passivecomponent requirements and operational and controlstrategies. Such topologies are complex and thereforeapplications of these converters are limited. Typically,three-to-nine level converter topologies have been reportedin the literature [95, 119, 132]. For relatively slowswitching devices like GTO, application of three-levelconverter topology with fundamental frequency switchinghas got wide acceptability in designing STATCOM forhigh power rating applications. A simplied scheme ofthree-level NPC converter comprising four-switches ineach converter leg and four-level single-phase NPCconverter conguration is given in Figs. 8 and 9, respectively.

It is experienced that fundamental switching based 48-pulse converter topology is extensively used in high powerrating STATCOMs due to its excellent operational andharmonics performance, whereas low pulse-ordercompensators such as 12-pulse, or 18-pulse or 24-pulsecongurations under square-wave mode of operation arenot adopted due to high impact of voltage harmonics,causing unacceptable harmonic distortion. Such low-pulseorder and multi-pulse VSC topology-based STATCOMsare proposed in [71, 298, 299] for voltage regulation, powerfactor improvement in transmission system and these caneffectively improve harmonic performance by adopting atypical magnetics structure and simple control algorithm,the magnetics architectures of which are illustrated inFigs. 10a10c, 11a11c and 12a12b. Among the two-level, 48-pulse GTO-VSC topology-based STATCOMswith GTO triggering under FFS principle, two mostpioneering and practical compensators exist at the 154 kVInuyama switching station of KEPC and at 161/500 kVSullivan substation of TVA. In multi-level topology,

Figure 8 Single phase of a three-level NPC converter4The Institution of Engineering and Technology 2009three-level architecture is extensively adopted in high powerrating STATCOMs being used in high-voltagetransmission system. Interestingly, the rst UPFC [252] of+160 MVA capacity, which has a STATCOMcomponent, has been built using three-level NPC GTO-based converter conguration and it has been in service at138 kV Inez S/S of American Electric Power since 1997.A three-level IGBT-based NPC converter congurationwith a rating of +36 MVA being operated as a back-to-back inter-tie between Texas and Mexico with afunctionality of STATCOM has been working since 2001[271]. Three-level VSC topology is adopted in thedevelopment of a versatile +200 MVA CSC at 345 kVMarcy S/S, NY [278] and a +40 MVA STATCOM[281] under 80 MVA UPFC project of Korea ElectricPower Corporation. In Gleenbrook 115 kV sub-station,Northeast Utilities, +150 MVA STATCOM [282] is builtupon GTO-based chain-link VSC conguration. Multi-level topology and various STATCOM circuitcongurations and related control strategies are presentedin the literature [61, 78, 82, 92, 95, 96, 98, 106, 108, 110,114, 119, 123]. A nine-level high power rating convertertopology with a combination of IGCT and IGBT-basedconverter congurations, called hybrid approach, isproposed in [119].

The concept of multi-level voltage re-injection in DCcircuit of VSC topology is envisioned in [49, 66, 68, 69,72] to increase pulse-order (like conventional high-pulseSTATCOM) by minimising converters requirements andmagnetics. Simple congurations of voltage reinjection fortwo-level and three-level structurs are shown in Figs. 13and 14, respectively. Based on this principle, a model of36-pulse STATCOM is proposed in [68] using only twoelementary six-pulse VSCs operated under FFS principle.A model of a 60-pulse STATCOM is proposed in [72]using multi-level voltage re-injection in DC circuit of2 6-pulse STATCOM operated under square-wavemode. With the advent of this innovative approach, basicpulse-order is increased multi-fold improving harmonic

Figure 9 Single phase of a four-level NPC converterIET Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324doi: 10.1049/iet-pel.2008.0034

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www.ietdl.orgperformance signicantly. Instead of adopting VSC in thedesign of STATCOM, the current-source convertertopology with multi-level current re-injection technique isfocused in [50, 70], where a ve-level current reinjection isderived to meet harmonic standards.

5 Control strategies andapproachesThe control system is the heart of state-of-the-artSTATCOM controller for dynamic control of reactivepower in electrical system. Based on the operationalrequirements, type of applications, system conguration andloss optimisation, essential control parameters arecontrolled to obtain desired performance and many controlmethodologies in STATCOM power circuits have beenpresented in [133227]. In a square-wave mode ofoperation, phase angle control (a) across the leakagereactance (L) is the main controlling parameter. This

control is employed in a two-level converter structure,where DC voltage (Vdc) is dynamically adjusted to above orequal to or below the system voltage for reactive powercontrol. In a three-level conguration, the dead-angle orzero-swell period (b) is controlled to vary the converter ACoutput voltage by maintaining Vdc constant. The controlsystem for STATCOM operated with PWM modeemploys control of a and m (modulation index) to changethe converter AC voltages keeping Vdc constant. The basiccontrol architecture is shown in Fig. 15. For voltageregulation, two control-loop circuits namely inner currentcontrol loop and external/outer voltage control loop areemployed in STATCOM power circuit. The currentcontrol loop produces the desired phase angle difference ofthe converter voltage relative to the system voltage and inturn, generates the gating pulses, whereas the voltagecontrol loop generates the reference reactive current for thecurrent controller of the inner control loop. This controlphilosophy is implemented with proportional and integralcontrol (PI control) algorithm or with a combination of

Figure 10 Interfacing magnetics of 12-pulse (26-pulse) two-level +100 MVA GTO-VSC based STATCOM and STATCOM ACvoltage waveform at PCC

a Interfacing magnetics conguration-1 of 2 6 pulse convertersb Interfacing magnetics conguration-2 of 2 6 pulse convertersc 12-pulse STATCOM terminal AC voltage waveform at PCCPower Electron., 2009, Vol. 2, Iss. 4, pp. 297324: 10.1049/iet-pel.2008.0034305


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www.ietdl.orgFigure 11 Interfacing magnetics of 18-pulse (36-pulse) two-level +100 MVA GTO-VSC based STATCOM and STATCOM ACvoltage waveform at PCC

a Stage-I and stage-II Transformer magneticsb ()2082 082 (2)208 under stage-II of magneticsc 18-pulse STATCOM terminal AC voltage at PCC

Figure 12 Interfacing magnetics of 24-pulse (46-pulse) two-level +100 MVA GTO-VSC based STATCOM and STATCOM ACvoltage waveform at PCC

a Interfacing magnetics layout

b 24-pulse STATCOM terminal AC voltage waveform at PCC

IET Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324The Institution of Engineering and Technology 2009 doi: 10.1049/iet-pel.2008.0034

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www.ietdl.orgFigure 14 Typical voltage reinjection circuit layout of three-level 12-pulse (26-pulse) converter conguration fortransforming into 60-pulse AC voltage waveform at PCC17 illustrate the PI methodology for two-level and three-level GTO-VSC based STATCOM power circuits. Thegeneral mathematical approach, modelling and design ofcontrol systems for compensator circuits are proposed in[136, 153, 167, 180, 181, 186188, 194, 202, 220].Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324: 10.1049/iet-pel.2008.0034the essential AC and DC voltages and current signals(instantaneous values/vectors) are sensed using sensors. In thenext step, these signals are synthesised by techniques such asdq synchronous rotating axis transformation, alphabetastationery reference frame of transformation and so on. Phaseproportional (P), integral (I ) and derivative (D) controlalgorithm in dq synchronous rotating frame. Figs. 16 and

In the process of designing and implementation of controlsystem, acquisition of many signals is involved. Initially,

Figure 13 Typical voltage re-injection circuit layout of two-level 12-pulse (26-pulse) converter conguration fortransforming into 36-pulse voltage waveform at PCC307


power system,the injectedM mode of

there are twocontrol (VC)que. The CCensation anditching states,/or nonlinearethodologies,rison currenttate feedbackntrol are the, 169]. The8, 157, 161,4, 165, 244],t control andr improvingSTATCOMseural network,echniques andintroduced in

tr

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www.ietdl.orgvoltage regulation [137, 167, 178, 235] inthe basic control is realised by controllingreactive current by the STATCOM. In PWcontrol [73, 74, 78, 80, 138, 160, 178],control strategies adopted viz. voltagetechnique and current control (CC) technitechniques [73, 84], where error compvoltage modulation determine the various swhave been widely adopted with linear andcontrol strategies. In the linear control mstationery PI controller or ramp compacontrol, synchronous vector PI control, scontrol, predictive control and deadbeat covarious approaches followed [73, 84, 151nonlinear group of controllers [73, 84, 14175, 205] includes hysterisis control [75, 8delta modulation (DM) or pulse DM currenonline optimised controller [84]. Focontrollability and operational performance ofunder various system conditions, fuzzy logic, nneuro-fuzzy articial intelligence/rule-based trelated supplementary pre-compensators are

Figure 16 PI-control algorithm of two-level GTO-VSC basedSTATCOM power circuitlocked loop circuit is normally employed to calculate phase andfrequency information of the fundamental positive sequencecomponent of system voltage which synchronises ACconverter output voltage. Third step involves generation ofcompensating command signals based on three kinds of state-of-the-art control methodologies, linear, nonlinear and specialcontrol techniques. Fourth step is to generate required gatingsignals for the solid-state devices.

Signal actuation: Instantaneous current and voltage signals suchas system voltage are the basic input parameters to the controller

Figure 15 Basic GTO-VSC based STATCOM operation and conhe Institution of Engineering and Technology 2009and are sensed using CTs and PTs or using other sensingdevices. DC voltage across the capacitor and current on DCside are sensed using Hall effect and other sensing devices.

Compensating signals derivation: The compensating signalsare generally derived either in time domain or infrequency domain. Time-domain signals of instantaneousvoltage and/or current vectors are sensed and decomposedusing widely popular method such as the dq synchronousrotating axis transformation [80, 136, 137, 139, 160]. Thetransformed values are processed by various controltechniques like PI or PID controllers to derive thecompensating command signals [31, 32, 167, 235]. For

ol architectureIET Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324doi: 10.1049/iet-pel.2008.0034

[31, 106, 136, 137140, 160]. Mathematical models of of high power rating STATCOMs.

age has to beoutput voltagesystem voltagesformer [32] attion from thiscapacitor ratingnt is explainedic compensatorthe DC voltage23, 134, 135].optimised byel at the PCCe possibility ofe [170]. Theristics of then the DC sidend signicantly

taken intohe capacitor. Ife system for aor needs to berequirement ofoperation is

r a comparable

TAT

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www.ietdl.orgSTATCOM controller to control system parameters duringasymmetric conditions have been proposed based on thesequence analysis [80, 148, 190] and an analysis oncontrolling unbalanced voltage conditions is presented in[159, 174].

6 Component selectionand ratingsBased on the specic applications, operating requirements,system congurations and control strategies, ratings of variouscomponents of STATCOM such as DC capacitor, leakageinductance of coupling transformers, converter VA ratingsand so on, are selected. Solid-state self-commutating switches(GTOs, IGBTs, IGCTs or the like) and a diode connectedin parallel with reverse polarity constitute a valve in converter.Based on the current and voltage ratings of controllableswitches or devices, a group of valves is connected in series toobtain the desired voltage rating (sum of rated voltages) of theconverter. The rms current ratings translate in restrictions onthe converter current at AC side and peak current ratingsrelate to the device turn-off capabilities. One or moreredundant valve is also provided for reliability reasons [145,235]. Typical maximum voltage and current ratings of various

In general, the nominal DC-link voltrelatively a large value to generate converterwith amplitude similar to that of the ACon the secondary side of the coupling tranzero VAR generation and moderate variavalue for rated VAR output. Deriving DCbased on the peak-stored energy requiremein [58]. The DC capacitor design for statis greatly inuenced by the ripple factor ofand these aspects are depicted in [58, 1Nevertheless, the capacitor size isconsidering the ripple on the harmonic levand also by taking into consideration thresonance for a given coupling reactancsteady-state and transient-state charactecontroller and the quantum of AC ripple owhich is less during balanced conditions ahigh during unbalanced situation areconsideration in determining the size of tthe controller exchanges real power with thshort time, the higher size of the capacitprovided. It is proposed in [4] that theDC-link component in PWM-modesignicantly smaller than those needed fothe control of STATCOMs [73, 84, 154, 156, 161, 162, 166,183, 201, 215, 218, 222]. For qualitative improvement ofelectrical system, DSP-based indirect current controltechniques [180182, 193, 247, 273] have assumed asignicant role. Analytical analysis of various controlparameters by space vector analysis has been presented in

Figure 17 Control algorithm of three-level GTO-VSC based SPower Electron., 2009, Vol. 2, Iss. 4, pp. 297324i: 10.1049/iet-pel.2008.0034state-of-the-art turn-off switches are presented in [16, 41, 45,47] such as GTOs: 6 kV, 6 kA; IGBTs: 6.5 kV, 2000 A [16]or 6.5 kV, 600 A or 4.5 kV, 900 A; IGCTs: 6 kV, 6 kA andconventional thyristors 8 kV, 3.5 kA. From Table 1, it isseen that the self-commutating solid-state GTO device is themain power switching element used in most converter circuits

COM power circuit309


pulses. However, in multi-level converter topologies, the

d in

ATC

hat

(s

(+tib

7

uno

100

31

&

www.ietdl.orgcapacitor rating is almost independent of the number of levelsand they are larger than the VA rating of the compensators. Itis proposed in [123] that DC capacitor rating is 418 timeslarger than the VA rating of the compensator in the multi-level topologies.

The selection of parameter for coupling reactance ofthe transformer mostly determines the full VAR output ofthe converter, and it is typically not more than 15% of thenominal system voltage [32]. The selection of the couplingreactance is heavily constrained by the harmonicrequirements of the network and, in general, a high value ispreferred to minimise the harmonic distortion at the PCC[170]. However, for low leakage reactance, converter ratingneeds to be increased.

The converter loss is one of the signicant aspect, whichaffects the overall efciency of the controller [74]. Theconverter loss increases almost proportionally with switchingfrequency and quadratically with the DC voltage. With theincrease of modulation index (m), losses decrease and thesystem runs at higher DC voltage for a given reactive current.For optimisation of converter operating losses, switchingfrequency should be low but m should be maximum.Mathematical modelling and designing of passive componentsof many prototype and/or commercial STATCOM

7 Specic application areasof statcomSTATCOM technology has multi-dimensional applications[228305] to control power system parameters in steady-state and dynamic system conditions. As a representative ofFACTS controller, STATCOM is a matured technology forpower quality improvements [55, 61, 117, 260, 268, 274,275], reactive power control, voltage regulation [78, 235],power swings/oscillations damping [78, 237, 259, 266, 284,286], damping torsional oscillations/SSR damping [202,250, 262], transmission line capacity enhancement, dynamicstability improvement including steady state, transient andvoltage stability [175, 228, 255, 275, 277, 288, 291, 292,295, 297, 300, 303], and for application under power systemfaults [86, 159, 184, 304]. It is also used as hybridcontrollers in combination with passive elements [235, 273].STATCOM has many interesting features such as highspeed of response (sub-cycle), versatile controlling andoperational characteristics, ability to implement controllersof low/medium/high MVA ratings, low-space requirement,higher stability margins and so on. It is rapidly replacing theconventional forced-commutating reactive power controllers,SVC and other slow-acting controllers in power system. Inthe eld of distribution system, the acronym of thiscontroller is D-STATCOM [55, 61, 88, 90, 91, 117, 217,square-wave mode operation. Generally, the capacitor rating inthemulti-pulse circuits decreases with the increasing number of

Table 1 Self-commutating power semiconductor devices use

Sl. Station-utility-Year of operation ST

1 Inuyama- Kansai Electric Power Corp.,Japan-1991

+80 MVA (at t

2 Sullivan-Tennessee Valley Authority(TVA), US-1996

3 Inez-American Electric Power (AEP),US-1998

+160 MVA

4 Henan-Henan Power Administration,China-1999

5 Marcy-New York Power Authority(NYPA), US-2001

+200 MVAconver

6 East Clayodon-National Grid Company(NGC), UK-2001

+

7 Essex-Vermont Electric PowerCompany (VELCO), US-2001

8 Kangjin-Korea Electric Power Corp.(KEPCO)-2002

+40 MVA (sh

9 Talega-San Diego Gas & Electric(SDG&E), California-2002

+0The Institution of Engineering and Technology 2009controllers and solid-state-device rating techniques have beenpresented in [20, 48, 58, 83, 135, 145, 309].

converters of high power rating statcoms

OM effective capacity Solid-state turn-offdevice ratings

time called as static var generator-SVG)

GTO: 4.5 kV, 3 kA

+100 MVA GTO: 4.5 kV, 4 kA

hunt part of unied power owcontroller-UPFC)

GTO: 4.5 kV, 4 kA

+20 MVA GTO: 4.5 kV, 3 kA

2 100 MVA converter units ofle static compensator-CSC)

GTO: 4.5 kV, 4 kA

5 MVA (re-locatable) GTO: 4.5 kV, 3 kA

133/241 MVA GTO: 6 kV, 6 kA

t part of +80 MVA unied powerw controller-UPFC)

GTO: 4.5 kV, 4 kA

MVA (+2 50 MVA) GCT: 6 kV, 6 kAIET Power Electron., 2009, Vol. 2, Iss. 4, pp. 297324doi: 10.1049/iet-pel.2008.0034

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www.ietdl.org243, 260, 268, 274, 275, 307] being widely used for power-quality improvement, custom power, voltage regulation,compensation and balancing of nonlinear loads and/orunbalanced loads, load power factor improvement, harmonicelimination and so on. Versatile applications ofD-STATCOM for system improvements in distribution levelhave been well documented in many references [251, 257,243, 256, 260, 263, 268, 274, 310]. Considerableimprovement in electrical machine controls like self-excitedinduction generators (SEIGs) by hysteresis current controltechnique and other nonlinear approaches have beenpresented in [181, 182, 244, 253, 272]. For harnessing non-conventional energy sources such as wind power, applicationsof STATCOMs and its controlling features to control SEIGsin wind farm are discussed in [217, 248]. In combinationwith an energy storage system (battery or magnetic storagedevice), STATCOM are being widely utilised [57, 306313]for power-quality improvements and also for uninterruptiblepower supply and real power exchange during emergency.

In high-voltage transmission and high-power ratingapplications, many practical STATCOM controllers arein real-time applications and their multi-dimensionaladvantages are well realised [228, 235, 239, 252, 258, 269,278, 280, 281, 282, 301, 306]. STATCOM back-to-backinter-tie [271] is a relatively new area of application toexchange power between two inter-ties and to improvevoltage stability. It is analogous to HVDC back-to-backsystem named as HVDC light with inherent MVARsupporting feature.

8 Latest trends and futuredevelopmentsIGCT and IGBT devices [47, 48] are the promising self-commutating solid-state controllable switches that areincreasingly being used in STATCOMs under PWM modeof operation due to its low switching losses and fast responsetime relative to GTO switches. Out of these two powerelectronic devices, IGCT is the most promising technologyfor high power rating STATCOMs. Owing to its qualitativeimprovement and rapid commercialisation, these devices arenow available with reasonably higher power ratings. Designand development of high power ratings STATCOMs usingIGCT-based VSCs with PWM mode of operationemploying multi-pulse and/or multi-level topologies are thepromising area of research. Out of the various multi-levelconverter topologies, three-level conguration has been provento be most practical. It is proposed that in multi-leveltopologies, beyond three levels, the controller design forbalancing voltages across the various segments of DCcapacitors to be used as energy storage devices is difcult andtherefore higher-level converter congurations are rarely used.Evolving proper controller to meet such specic controlobjective for multi-level STATCOMs is a potential area ofresearch. There is a further scope of improving the controllerfunctions in STATCOMs which would enable to controlPower Electron., 2009, Vol. 2, Iss. 4, pp. 297324i: 10.1049/iet-pel.2008.0034system dynamics during symmetrical and asymmetrical faultsin high-voltage transmission system. In this context,improving control algorithms employing fuzzy-logic or neuralnetwork or neuro-fuzzy logic needs to be investigated forachieving better controllability. In a system using multiplenumbers of the state-of-the-art compensators at variouspotential locations, coordinated control mechanism seems tobe an interesting area of research in respect to capacityoptimisation of the compensators ensuring effective utilisationof the transmission assets and thus, saving in cost. Theconcept of voltage re-injection principle in DC-link circuit ofSTATCOMs operated at fundamental frequency switching isa good technique to be greatly utilised to improve harmonicsperformance using less number of sold-state devices andassociated components in STATCOM power circuits.

9 Simulation toolsMany experimental and prototype models of STATCOMcontrollers have been reported in research publications.Simulation of various congurations/topologies, controlstrategies, magnetics, lter requirements, component leveldesigning and so on, have been presented in [314320] withthe help of many standard software simulation tools.MATLAB/SIMULINK/PSB, EMTP, PSCAD/EMTDC,SPICE, EUROSTAG and so on, are some of the softwaretools being extensively used by researchers and engineers tosimulate various power electronics devices in power systemcircuits, electrical machines and so on. Detailed modelling ofSTATCOM controllers and performance analysis andsensitivities of various passive components under varyingsystem-operating conditions ensure the researchers andengineers to rm up the design parameters in pre-fabricationstage. Employing EMTP simulation program, prototypemodelling of typical STATCOM controllers and analysishave been presented in [111, 167, 315, 316, 320]. A specicmodelling of D-STATCOM with IGBT converters arepresented by using power system block set tool box underMATLAB environment [317, 318, 319].

10 ConclusionsSTATCOMis the state-of-the-art dynamic shunt compensatorin FACTS family, which is widely used to control systemdynamics under stressed condition. The self-commutatingVSC built upon controllable solid-state devices (viz. GTO,IGBT, IGCT and so on) with operation under FFS or PWMswitching principle is the backbone of this compensator.Many commendable features of STATCOM viz. four-quadrant operation in PQ plane (in support of proper energysource), high speed of response (sub-cycle), versatilecontrolling and operational characteristics, optimum voltageplatform and so on, have been reported in researchpublications. STATCOM being a versatile reactive powercompensator has taken the place of the line commutatingSVC, a relatively slow-acting dynamic shunt controller. TheEPRI in USA, who is a pioneer to conduct research andevolve high power rating STATCOMs employing311


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www.ietdl.orgGTO-VSCs as its backbone, has developed a number of existingSTATCOM projects in collaboration with many utilities/organisations. In many research papers, this controller hasbeen called as ASVC or ASVG or SVG or STATCON orSSVC or VSC-based SVC or self-commutated SVC or staticsynchronous compensator (SSC or S2C). Acronym ofSTATCOM in electrical distribution system is D-STATCOM operating under PWM control. Power industriesviz. GE, Siemens, ABB, Alsthom, Mitsubishi, Toshiba andso on, with their in-house R&D facilities have given birth tomany STATCOM projects that are commercially in operationin high-voltage transmission system. In addition to its stand-alone usage in electrical network, this controller has been anintegral component of other state-of-the-art FACTScontrollers viz. UPFC and CSC. In the process ofSTATCOM technology development, numerous convertertopologies, magnetics congurations, control algorithms,switching principles and so on, have been reported in literaturefor various applications in transmission and distributionsystems. A comprehensive review on the state-of-the-artSTATCOM controllers has been carried out focusing the newhorizon of research potentials in this eld.

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