ASYMMETRICAL CONTROL OF UTILITY STATIC VAR COMPENSATOR FOR GRID VOLTAGE BALANCING

Transcription

1 ASYMMETRICAL CONTROL OF TILITY STATIC VAR COMPENSATOR FOR GRID VOLTAGE BALANCING A. S. Mäkinen, GE Grid Solions, Vehmaisenka 5, 3373 Tampere, Finland, J. Aho, GE Grid Solions, Vehmaisenka 5, 3373 Tampere, Finland, A. M. Pena, Companhia Energeica de Minas Gerais Brazil, ABSTRACT A saic var compensaor (SVC) is a shn conneced conrollable reacive power sorce which is sed o conrol power sysem volages and reacive power flow. Nowadays, iliy companies increagly reqire also he capabiliy o compensae he nework volage asymmery from he SVC. In order o flfil his ask, he SVC shold be conrolled asymmerically. This paper presens a flexible vecor conrol sysem of a SVC capable of conrolling nework volage posiive and negaive seqence componens individally. Pracical operaion of he conrol sysem is verified g sie measremens from a SVC (-3/ MVAr) locaed in Bom Despacho, Brazil. The aim of his paper is o show how he performance of a SVC significanly enhances when he inelligen asymmerical conrol is applied. The benefis of developed negaive seqence conrol feares are illsraed g simlaions carried o in a laboraory environmen consig of a Real-Time Digial Simlaor (RTDS) and he SVC conroller. The simlaions show ha he asymmerically conrolled SVC is able o serve he needs of a power sysem dring ransiens as well as nder seady sae nework condiions. KEYWORDS Saic Var Compensaor, Negaive Seqence Volage Conrol, DDSRF-PLL, RTDS 1. Inrodcion In order for a power sysem o operae efficienly and reliably he sysem volages and reacive power need o be conrolled. Main arges of he volage conrol can be caegorized as follows: [1] Node volages need o say wihin accepable limis. Long lag operaion nder condiions oside he limis may case poor performance or damage o eqipmen conneced o he nework. Excessive reacive power flow in he nework shold be minimized in order o minimize he power sysem losses. Frhermore, he ransmission sysem efficiency increases in addiion o available acive power ransfer in power sysem lines. Power sysem sabiliy enhancemen de o prevenion of volage insabiliy or volage collapse. [] nlike power sysem freqency, which depends on he acive power balance of overall power sysem, he volage is a local measre in nare. Ths, devices capable of conrolling he nework volage need o be disribed hrogho he power sysem. These devices are for example a SVC, a saic synchronos compensaor (STATCOM), a synchronos generaor and a synchronos condenser. This paper focses on he volage conrol capabiliy of an iliy SVC (-SVC). Tradiionally, he -SVC has conribed o he power sysem volage and he reacive power flow by conrolling he nework volage posiive seqence componen or hree-phase roo mean sqare (RMS) vale. From he power sysem poin of view, he -SVC has been caegorized as a shn conneced symmerically conrolled variable sscepance or reacive power sorce, which assiss he power sysem o flfil he reqiremens menioned above. However, he drawback of he symmerically conrolled SVC is he lack of nework volage asymmery miigaion capabiliy. The loads along he power sysem lines may be conneced asymmerically generaing a negaive seqence componen on he nework volages. nransposed ransmission and reacance of a long ransmission line increase he negaive seqence componen, i.e. volage asymmery. For example, i is repored ha occasionally he negaive seqence volage reaches wo percen in Sadi elecriciy nework. [3]

2 The negaive seqence volage componen has adverse impac on nework loads and eqipmens sch as roaing machines and power sysem lines. Indcion machines as well as synchronos machines sffer from exra heaing and power losses in addiion o mechanical sresses. Negaive seqence crren in ransmission lines does no conribe o sefl energy ransfer and i increases he power sysem losses, redcing efficiency. De o he adverse impacs menioned above, elecric iliy companies have sared o pay more aenion o he grid volage negaive seqence componen. Therefore, compensaion eqipmens like he -SVC are reqired o be able o miigae he negaive seqence componen in parallel wih he posiive seqence volage conrol. In pracice, he miigaion means ha he conrollable componens of he SVC, which are hyrisor conrolled reacors (TCR) and hyrisor swiched capaciors (TSC), shold be asymmerically conrollable. In addiion o he posiive seqence volage conrol and he negaive seqence volage miigaion dring seady sae he operaion performance of a SVC dring nework volage ransiens is of grea imporance. I is repored in [4] and [5] ha a gle phase-o-grond fal may lead o volage collapse of cerain pars of he nework de o significan amon of indcion moor load and limied reacive power sppor by he nework devices. However, he volage collapse is avoided wih help of a SVC designed o sppor he violaed nework volages by injecing capaciive reacive power dring and afer he fal. [4] [5] This example nderlines he imporance of he SVC reacive power sppor dring ransiens. Naral response of a volage conroller of a symmerically conrolled SVC is o move o a capaciive operaion poin as a resl of he nework gle phase-o-grond fal. As a conseqence, he SVC sars o sppor he violaed nework volage which is desired. However, he reacive power sppor increases also he healhy phase volages. If he SVC is conneced o a weak power sysem he incremen in healhy phases may be significan and he volages may exceed accepable limis. I shold be noed ha he healhy phase volages may increase frher de o operaion of a nearby locaed symmerically conrolled SVC or oher volage conroller. Ths, he symmerically conrolled SVC may be forced o redce is capaciive reacive power op in is all phases dring nework ndervolage in order o mainain he healhy phase volages wihin accepable limis. This is a drawback considering he volage sabiliy of he power sysem. The performance of he SVC can be significanly improved if he SVC is operaed asymmerically. The negaive seqence volage conrol prevens he healhy phase overvolage and he posiive seqence volage can be se o maximm possible. The asymmerical conrol of a SVC has received srprigly lile amon of aenion in he lierare alhogh he performance of he SVC can be improved remarkably. [6] This paper fills he gap by presening a conrol sysem of he SVC wih fncional feare of nework volage asymmery compensaion. The conrol sysem is based on vecor conrol in a synchronos reference frame and i ilizes advanced grid synchronizaion and seqence componen separaion mehod o generae closed loop conrol for posiive and negaive seqence volage componens. The pracical operaion has been esed g sie measremens from a SVC (-3/ MVAr) locaed in Bom Despacho, Brazil. I is shown ha he modern asymmerically operaing SVC can significanly enhance he power sysem operaion nder seady sae and dring grid fals.. Conrol of iliy Saic Var Compensaor This chaper presens main conrol sysem feares of a SVC. The conrol sysem blocks as well as he main componens of a generic SVC are shown in Figre 1. A ypical SVC consiss of a ransformer, harmonic filers, a TCR and a TSC. The reacive power op range can be exended wih mechanical swiched capaciors (MSC) or mechanical swiched reacors (MSR). The harmonic filers provide a low impedance pah in heir ning freqency for harmonic crrens generaed by he TCR operaion. The filers generae capaciive reacive power a a fndamenal grid freqency. The SVC main componen configraion may conain also a main reacor which is a reacor locaed beween he TSC and TCR bsses. [7] The benefis of he main reacor concep are redced impac of harmonics on he PCC volage as well as redced fooprin. The SVC conrol sysem conains he following operaor selecable modes of operaion: 1. Manal conrol mode or open loop operaion mode The op sscepance of he SVC B d + ref can be seleced by he operaor. This conrol fncion is ypically sed only dring commissioning. As a proecion measre, he manal mode is swiched aomaically o volage conrol mode if a poin of common copling (PCC) volage increases over or decreases nder operaor defined hreshold.

3 . Volage conrol mode In volage conrol mode, he PCC volages are conrolled o an operaor desired vale g he posiive seqence volage conroller. The volage conrol mode can also be faciliaed wih negaive seqence conrollers which are sed o eliminae nework volage asymmery. 3. Reacive power or slow sscepance conrol mode The reacive power op of he SVC is conrolled o an operaor defined reference vale. The reacive power conroller op is he reference for posiive seqence volage conroller. The reacive power conroller is ned o be significanly slower han he volage conroller. Therefore, his conrol mode is also called as a slow sscepance conrol mode. The reacive power reference is ypically se o zero or in he middle of he SVC conrol range in order o have large conrol range available o conerac agains rapid nework volage drops or swells. In case of an asymmerically conrolled SVC, he negaive seqence volage conrollers can be in operaion also in his mode. Hence, he nework volage asymmery can be eliminaed and he average of insananeos reacive power can be simlaneosly conrolled. 4. Power oscillaion damping (POD) mode In POD mode, he conrol arge of he SVC is o damp iner-area oscillaions which may arise beween weakly inerconneced clsers of generaors as a resl of nework fals or sdden loss of generaion. The iner-area oscillaions appear ypically in he range of.15- Hz. The POD conrol inp can be an exernal signal or he conrol sysem freqency measremen. Independen of he conrol mode seleced by he operaor he ops of he conrol sysem are SVC phase-ophase sscepances B ab, B bc and B ca. These sscepances are fed o a TSC firing ni. If he phase-o-phase reference exceeds he swiching limi of he TSC, hyrisor firing plses are generaed. The TSC can be designed o operae eiher symmerically or asymmerically. The phase-o-phase sscepances are sen also o a TCR sscepance calclaion and firing ni, which calclaes he TCR sscepance based on he available componens on he SVC configraion. The TCR firing ni generaes he TCR firing plses based on he calclaed sscepances. Also he TCR can be designed o operae eiher symmerically or asymmerically. Saic VAr Compensaor Power sysem Transformer Filer 1 Filer TSC 1 TCR 1 V a,b,c I a,b,c V mv TSC 1 Firing plses TCR 1 Firing plses Measremen procesg and Conrol sysem synchronizaion ni f TSC Firing ni TCR sscepance Calclaion and Firing ni q svc d + d - q - Power oscillaion damping conrol Manal conrol B ab,bc,ca,ref Reacive Power conroller V Qcrl ref ON OFF Volage conroller B d + ref B d - ref B q - ref Transformaion ni B ab,ref B bc,ref B ca,ref Conrol mode selecor Figre 1: SVC main componens and conrol sysem block diagram.

4 Measremen procesg and conrol sysem synchronizaion The measremen procesg and conrol sysem synchronizaion ni embodies he hear of he SVC conrol sysem. I provides he reacive power measremen, PCC posiive and negaive seqence volage componens as well as a reliable and fas freqency measremen as op. In addiion, space vecor based volage conrol in a roaing reference frame is synchronized o he grid volage posiive seqence componen in he ni. The conrol sysem synchronizaion is based on he operaion of a decopled doble synchronos reference frame - phase locked loop (DDSRF-PLL). [8][9][1] The DDSRF-PLL consiss of a decopling nework and a phase locked loop operaing on a synchronos reference frame (SRF-PLL). The decopling nework provides posiive and negaive seqence componens from he inp volage vecor. The synchronizaion o he posiive seqence volage is done by conrolling he posiive seqence volage q-componen o zero g SRF-PLL. Decopling nework The nbalanced grid volage vecor αβ can be expressed in a saionary reference frame as follows: β α αβ (1) where corresponds o he magnide of he volage vecor, ω is he angle of he fndamenal freqency posiive seqence volage and φ is he volage vecor iniial angle. Sperscrips + and correspond o he posiive and negaive seqence references. The posiive and negaive seqence volage componens can be expressed in a synchronos dq-reference frame wih help of eqaion (1): T β α β α dq q d dq () T β α β α dq q d dq (3) I can be noiced ha eqaions () and (3) conain AC and DC componens. The AC componen in he posiive seqence reference frame originaes from he negaive seqence volage componen and he AC componen in he negaive seqence reference frame arises de o he posiive seqence volage componen. The idea of he decopling nework is o cancel he AC componens and o express he fndamenal freqency posiive and negaive seqence PCC volage componens as DC vales. The DC vales can be calclaed from eqaions () and (3) as follows: q d q d (4) q d q d (5) Eqaions (4) and (5) show ha he decopling nework is able o provide he desired seqence volage componens. However, he angle of he posiive seqence volage componen ω is needed in order o ilize he eqaions. The angle is obained g he SRF-PLL. SRF-PLL The balanced grid volage vecor in a saionary reference frame can be expressed as follows: β α αβ (6) Eqaion (6) can be ransformed ino a synchronos reference frame roaing wih an angle provided by he synchronizaion sysem as follows: sync sync β α sync sync sync sync q d (7) where θ sync is he angle provided by he synchronizaion sysem. The block diagram of he SRF-PLL is shown in Figre a. The arge of he SRF-PLL is o conrol he inp volage q-componen o zero g a PI-conroller. The feedforward erm ω ff is sed o se he anglar freqency ω sync of he PLL near he grid freqency in order

5 o accelerae he sar-p. The anglar posiion of he roaing reference frame is aained afer inegraion of ω sync. As can be seen from (7) he SRF-PLL conrol loop is nonlinear de o a soidal fncion. However, he sysem operaes almos linearly when he difference ω-θ sync is small i.e. (ω-θ sync ) ω-θ sync. Ths, he SRF- PLL can be linearized and he block diagram of he linearized SRF-PLL is shown in Figre b, where k LF and T i represens he loop filer, which is PI-conroller, gain and inegraion ime respecively. I can also be observed ha he phase deecor gain is direcly proporional o he magnide of he inp volage. a) b) Figre : a) Block diagram of SRF-PLL, b) linearized form of SRF-PLL. DDSRF-PLL The DDSRF-PLL is a combinaion of he decopling nework and he SRF-PLL. The block diagram of he DDSRF-PLL is shown in Figre 3. The q + calclaed by he decopling nework is sed as an inp for he SFR-PLL in order o synchronize he conrol sysem o he posiive seqence volage. De o he fac ha he PLL gain is proporional o he inp volage magnide he inp is acively normalized o he posiive seqence volage componen. Ths, he PLL gain remains consan independen of he nework volage magnide and loss of gain phenomenon dring ndervolage as well as oo high gain dring overvolage are prevened. [11] Frhermore, he freqency measremen remains accrae also dring nework volage dips. The PI conroller op is limied in order o preven nsable operaion of he DDSRF-PLL in specific a case repored in [1]. The block LPF represens a simple low pass filer wih a c-off freqency of ω f : LPF s f s (8) f Figre 3: Block diagram of DDSRF-PLL. As a conclsion o his secion, he benefis of he DDSRF-PLL synchronizaion and seqence componen calclaion mehod are lised in following: Fas and accrae calclaion of fndamenal freqency posiive seqence volage componens d + and q + dring seady sae and grid fals Posiive seqence volage magnide can be expressed g d + componen de o he reference frame selecion Fas and accrae calclaion of fndamenal freqency negaive seqence volage componens d - and q - dring seady sae and grid fals Adapive grid volage freqency measremen Synchronizaion immne o he grid volage asymmery Consan PLL gain independen of he nework volage magnide

6 Volage Conrol The seqence componens calclaed by he DDSRF-PLL are fed as inps o he volage conrol sysem. The block diagram of he volage conrol sysem is depiced in Figre 4. The reference of posiive seqence volage conroller is eiher an operaor defined volage reference d + * or a reference volage from he reacive power conroller Qcrl *. The slope volage slope is added o he volage conrol reference before generaing he error measre of he conroller. Two elecrically close volage conrollers may sar o operae agains each oher. In a wors case, one conroller may prodce maximm capaciive while he oher conroller generaes maximm indcive power. This phenomenon is called hning. These volage conrollers can be for example SVCs, STATCOMs or synchronos generaors wih heir aomaic volage reglaors. The prpose of he slope conrol is o preven hning phenomenon and hereby faciliae he sharing of reacive power among mliple parallel operaing volage conrollers. The prpose of slope comp is o redce he gain of he volage conrol loop when he X slope differs from zero. When X slope is increased he SVC conroller ends o become faser which decreases sabiliy margin. The impac of he slope on he conrol gain is compensaed by slope comp in order o obain similar response regardless of he sed X slope. The gain opimizer (GO) opimizes he conrol gain in relaion o nework shor-circi level (SCL) which is calclaed by he SVC wihin an inerval defined by he operaor or aomaically afer a nework volage disrbance. In case he conrol sysem deecs any ndamped oscillaions indicaing oo high conrol sysem gain he Gain Conroller (GC) redces he gain and hereby mainains he sable operaion of he SVC. Sch a siaion may occr, for example, as a resl of sdden redcion in he nework SCL while he PI conroller sill operaes wih high gain adjsed o a srong nework. The op of he posiive seqence volage conroller is he reference for he posiive seqence sscepance B d + *. The conroller maximm and minimm sscepance limis correspond o maximm indcive and maximm capaciive sscepances, respecively. The negaive seqence volage conrol consiss of direc and qadrare axis volage conrollers. Boh conroller references d - * and q - * are always se o zero becase he prpose is o cancel he nework volage asymmery. The ops of he conrollers are d- and q-axis negaive seqence sscepance references B d - * and B q - *. These sscepances are fed wih B d + * o a ransformaion ni which ransforms he sscepances from he synchronos reference frame o phase-o-phase sscepance vales B ab *, B bc * and B ca *. These sscepance reqess are limied o he maximm indcive and capaciive sscepance limis. If he prpose is o operae he SVC in symmerical mode he ops of he negaive seqence conrollers are limied o zero. The ransiion from symmerical mode o asymmerical mode can be made smooh by ramping he negaive seqence conrol limis from zero o a vale where he conrollers are no limied. In normal operaion, he negaive seqence conrollers are no limied in order for he SVC o be able o provide maximm amon of negaive seqence sscepance. The conrol arrangemen is implemened in a manner ha he posiive seqence sscepances are prioriized and a windp of he negaive seqence conrollers is prevened. Figre 4: Block diagram of volage conrol sysem.

7 As a conclsion of his secion he main benefis of he presened conrol arrangemen are lised below: Fas and accrae closed loop posiive seqence volage conrol wih slope conrol inclding a gain opimizer and a gain conroller Fas and accrae closed loop negaive seqence volage conrol Priorizaion beween posiive seqence and negaive seqence volage conrol Sscepance limiaion is implemened o phase-o-phase componens allowing he ilizaion of he whole SVC power range for negaive seqence compensaion Possibiliy o ne he posiive and negaive seqence conrollers independenly Smooh ransiion from symmerical volage conrol mode o asymmerical volage conrol mode and vice versa 3. Pracical Experimen of Negaive Seqence Volage Conrol The pracical operaion of he presened negaive seqence conrol sraegy is esed g sie measremens from a SVC, Figre 5, locaed in Bom Despacho, Brazil. The nominal capaciive and indcive power of he SVC is 3MVAr and MVAr, respecively. The SVC configraion consiss of wo harmonic filers, which are ned o he 3 rd and 5 h harmonic, wo TCRs and wo TSCs. The nominal PCC volage is 5kV. However, a he ime of he sie measremens he posiive seqence volage alered arond 54kV. 3 rd filer 5 h filer TCR Transformer TSC TCR TSC Figre 5: Bom Despacho SVC. The measred PCC volage negaive seqence componen prior he acivaion of he negaive seqence conrol was.6%. The nework where he SVC is conneced is relaively srong and he measred nework SCL was approximaely 15MVAr. The pracical measremens are capred g Reason Digial Fal Recorder. The SVC sscepances B ab, B bc and B ca dring acivaion of he negaive seqence conrol are presened in Figre 6. The ransiion o asymmerical conrol is made smooh by ramping he negaive seqence conroller limis. Afer he end of he ramp he conrol sysem generaes phase-o-phase sscepances which compensae he nework volage asymmery.

8 Figre 6: SVC sscepances dring acivaion of negaive seqence conrol. 4. Inelligen Addiional Negaive Seqence Conrol Feares As menioned above, a modern asymmerically conrolled SVC can significanly enhance he power sysem operaion nder seady sae and dring grid fals. In his chaper, i is shown how he ransien performance can be improved g inelligen conrol. De o he flexibiliy of he presened conrol design i is possible o inclde more complex negaive seqence conrol feares sch as maximized negaive seqence volage conrol or parial negaive seqence volage conrol. In addiion, discssion abo he operaion characerisics of he SVC operaing in reacive power mode wih negaive seqence conrol enabled is carried o. Teg of he following conrol fncions is carried o g laboraory es sep consig of a RTDS and he SVC conrol sofware. The RTDS simlaes a Thevenin eqivalen based nework model and a SVC model. The SVC model receives he hyrisor firing plses from he conrol cbicle and provides essenial measremens o he conrol sysem. The power range of he SVC sed in he conrol fncion ess is 3MVAr capaciive and 15MVAr indcive. The hardware arrangemen of he laboraory es sep is shown in Figre 7. Conrol sysem cbicles RTDS HMI Figre 7: Hardware arrangemen of laboraory es sep. Inelligen Transien Conrol As menioned above, he SVC shold always be able o provide maximm reacive power op wiho violaing nework volage limis dring he nework gle phase-o-grond fal. This arge canno be accomplished g radiional symmerical conrol. This secion presens an inelligen conrol sraegy ilizing a combinaion of posiive and negaive seqence conrol. The gle phase-o-grond fal is simlaed wiho and wih inelligen ransien conrol (TC) in Figre 8. The sorce volage prior he fal is se o 1.63p and he nework SCL is se o 18MVA. The fal drops he PCC a-phase volage o.34p and endres 5ms. As can be seen from Figre 8a, he operaion poin of he SVC changes o maximm capaciive wiho TC. Ths, all he phase sscepances move o capaciive limi as shown in Figre 8c. This increases he PCC healhy V ab volage o 1.3p as depiced in Figre 8e. I shold be

9 Volage [p] Volage [p] Sscepance [p] Sscepance [p] Sscepance [p] Sscepance [p] noed ha if here is a symmerically operaing SVC or anoher conrolled reacive power sorce locaed in he viciniy of he SVC he volage wold increase even frher. The overvolage dring he fal can be prevened g TC. In case any of he volages increase above an operaor defined limi he posiive seqence sscepance shown in Figre 8b is decreased generaing room for negaive seqence conrol o operae. The phase sscepance assig he sppressed volage is mainained in he capaciive maximm as can be seen in Figre 8d b he highes volage remains in a vale which can be seleced by he operaor. In his case, his vale is seleced o be 1.15p as can be seen from Figre 8e. This is a raher conservaive vale and i is seleced o adeqaely illsrae he operaion of he TC. If a higher volage limi is seleced by he operaor he SVC wold provide more capaciive power dring he fal. I can be conclded based on he simlaion resls shown in Figre 8 ha he asymmerically conrolled SVC wih inelligen TC can provide maximm posiive seqence volage sppor wiho violaing nework volage limis. The imporance of TC increases if here is anoher symmerically conrolled reacive power sorce locaed nearby Sscepances in dq-reference frame Bdpos Bdneg Bqneg.6.4. Sscepances in dq-reference frame Bdpos Bdneg Bqneg a) b) Phase sscepances Bab Bbc Bca.6.4. Phase sscepances Bab Bbc Bca c) d) Line-o-line rms volages Vab Vbc Vca Line-o-line volages Vab Vbc Vca e) f) Figre 8: SVC conrol dring ransiens: a) dq-sscepances wiho TC, b) dq-sscepances wih TC, c) phase sscepances wiho TC, d) phase sscepances wih TC, e) phase-o-phase RMS volages wiho TC, f) phase-o-phase RMS volages wih TC. Maximized Negaive Seqence Volage Compensaion Conrol The main conrol arge of he SVC is he conrol of he nework volage posiive seqence componen. This is he reason why he posiive seqence volage conrol is prioriized in he SVC conrol sysem. The priorizaion of he posiive seqence conrol means ha he desired posiive seqence sscepance reference is always generaed and he available power range of he SVC deermines he negaive seqence sscepance. For example, if he posiive seqence sscepance reference is near he SVC capaciive or indcive limi he room

10 Sscepance [p] Volage [p] for negaive seqence sscepance becomes small. However, some iliy companies reqire priorizaion of nework volage negaive seqence conrol in case he posiive seqence volage is inside he operaor specified limis. This means ha he SVC is allowed o aomaically adjs he posiive seqence volage reference in order o make room for he negaive seqence conrol as long as he posiive seqence volage limis are no exceeded. As a resl, he SVC is able o ilize maximm available power o compensae he nework volage asymmery. The main feares of he maximized negaive seqence conrol are: The conrol fncion can be acivaed only if he ser defined posiive seqence reference and measred posiive seqence volage are inside he operaor specified safe limis wih selecable pickp ime The conrol fncion can be immediaely deacivaed if posiive seqence volage exceeds he limis de o he nework disrbance The conrol fncion can be deacivaed g ramp if posiive seqence volage reference is changed oside he limis by he operaor. The ramp fncion garanees smooh ransiion The conrol fncion shold no psh he posiive seqence volage oside he limis. This can be ensred g adapive conrol fncion limiaion In Figre 9, a.5% negaive seqence componen is added o he sorce volage a.6s when maximized negaive seqence volage compensaion conrol or negaive seqence prioriy conrol (NSPC) is deacivaed. The nework volage SCL is se o 44MVAr and sorce volage is se o 1.9p. The SVC negaive seqence conrol responds o his by deviaing he op phase sscepances from each oher. However, he negaive seqence volage componens shown in Figre 9b are no oally compensaed de o he priorizaion of posiive seqence volage conrol..5 Phase sscepances Bab Bbc Bca Negaive seqence volage in dq-reference frame Vdneg Vqneg a) b) Figre 9: Sep change of.5% negaive seqence componen o he nework volage: a) phase sscepances, b) negaive seqence volages in dq-reference frame. The acivaion of he NSPC is depiced in Figre 1. The operaor deermined volage safe limis are assmed o be 1.p and 1.5p. Ths, he posiive seqence volage is allowed o drif inside he limis if necessary. As a resl of he acivaion of he NSPC, he posiive seqence sscepance shown in Figre 1a moves o indcive wih a ime consan deermined by he NSPC conroller. This gives more room for negaive seqence conrol cag he phase sscepances depiced in Figre 1b o deviae more from each oher. Becase he posiive seqence sscepance moves owards an indcive operaion poin he posiive seqence volage decreases as shown in Figre 1c. However, i sill remains inside he allowable range. As a resl, he SVC is able o compensae he.5% nework volage asymmery as visible from Figre 1d. From Figre 1b i can be seen ha he B bc remains in he capaciive limi even hogh he NSPC is acivaed and only B ab and B ca move o indcive. The reason is he design crieria of minimm possible change of he posiive seqence sscepance o compensae he nework volage asymmery. In oher words, he operaion poin of he SVC is conrolled so ha he SVC operaes as close o he ser defined volage reference as possible while simlaneosly compensaing he nework volage asymmery. This holds re also in he reverse direcion. The NSPC aomaically ramps he B d + back o a vale deermined by he posiive seqence volage conroller if here is no need o modify he sscepance vale in order o compensae he asymmery.

11 Sscepance [p] Volage [p] Volage [V] Volage [p] Sscepance [p] Sscepance [p] Sscepances in dq-reference frame Phase sscepances.5.5 Bab Bbc Bca Bdpos Bdneg Bqneg a) b) Posiive seqence volage Vdpos Negaive seqence volage in dq-reference frame Vdneg Vqneg c) d) Figre 1: Acivaion of maximized negaive seqence volage compensaion conrol: a) dq-sscepances, b) phase sscepances, c) posiive seqence volage, d) negaive seqence volages in dq-reference frame. Parial Negaive Seqence Volage Compensaion Conrol Some iliy companies reqire parial compensaion of he nework volage asymmery. In his case, he SVC does no reac o small conen of volage asymmery. However, if he asymmery exceeds predefined limi seleced by he operaor he SVC shold enable he negaive seqence conrol and reglae he negaive seqence conen o he limi vale. If he nework condiions change so ha he negaive seqence conen decreases nder he limi he negaive seqence conrol is disabled. Hence, he SVC never generaes he asymmery o he nework volage becase i is ndesirable. The operaion of he parial negaive seqence volage conrol is illsraed in Figre 11. The nework operaing condiions are idenical o he previos case b he limi vale is se o.6%. The operaion of he phase sscepances nder a.5% sep change of he nework volage negaive seqence componen is illsraed in Figre 11a. I can be noiced ha he amon of negaive seqence sscepance is smaller compared o Figre 9a becase he negaive seqence volage is conrolled o a reference vale of.6% as can be seen from Figre 11b. I shold be noed ha he response ime in his case is no deermined by he ning of he negaive seqence conroller. I is deermined by he ning of he parial negaive seqence conroller which is prposely ned o be slower. I shold also be noed ha he parial conrol can be designed o operae simlaneosly wih he NSPC. Phase sscepance Negaive seqence volage in dq-reference frame.5 Bab Bbc Bca..1 Vdneg Vqneg a) b) Figre 11: Sep change of.5% negaive seqence componen o he nework volage wih parial negaive seqence conrol: a) phase sscepances, b) negaive seqence volages in dq-reference frame.

12 Volage [kv] Crren [ka] Sscepance [p] Insananeos reacive power [MVAr] Reacive Power Conrol As menioned above, he negaive seqence conrol can be enabled when he SVC operaes in reacive power conrol mode. The aim of he reacive power conrol is o generae a reference for he posiive seqence volage conroller. Ths, he posiive seqence volage conrol is prioriized in he inner conrol loop and he reacive power conroller deermines he operaion poin of he SVC. The phase sscepances in a case where he SVC is operaing in reacive power mode wih zero reference and simlaneosly compensaing a 1.5% nbalance from he nework is shown in Figre 1a. The average vale of he insananeos reacive power depiced in Figre 1b is zero. However, he de o he ineracion of symmerical componens he insananeos reacive power conains oscillaing par wih he freqency of wice he fndamenal. The PCC phase-o-grond volages as well as he PCC crrens are presened in Figre 1c and Figre 1d, respecively. I is inereg o noice ha he seqence of crrens is i a -i c -i b. Ths, from he power sysem poin of view he SVC operaes as a negaive seqence crren generaor in his specific operaion poin. However, he nework volage asymmery is compensaed..5 Phase sscepances 5 PCC insananeos reacive power Qsvc -.5 a) Bab Bbc Bca b) PCC phase-o-grond volages Va Vb Vc PCC crren Ia Ib Ic c) d) Figre 1: Compensaion of 1.5% nbalance from he nework volage dring SVC zero power operaion: a) phase sscepances, b) PCC reacive power c) PCC phase-o-grond volages, d) PCC crrens. As an example of he operaion of he SVC in reacive power mode he conrol reference is ramped from 1MVAr indcive o 7MVAr capaciive. The nework SCL is se o 14.7p. I can be seen from Figre 13a ha he reacive power reference can be execed despie of he 1.5% sorce volage negaive seqence componen. De o he reacive power op change owards capaciive, he posiive seqence volage shown in Figre 13b increases. The impac of he posiive seqence conrol priorizaion is illsraed in Figre 13c and Figre 13d where negaive seqence volage dq-componens and phase sscepances are presened respecively. As long as any of he phase sscepances are no limied de o he finie power range of he SVC he nework volage asymmery is compleely eliminaed. In case he posiive seqence conroller operaion poin moves owards he capaciive or indcive limi here is no sscepance available o compensae he nework volage asymmery.

13 Volage [p] Sscepance [p] Reacive power [MVAr] Volage [p] PCC reacive power Qsvc 1.6 Vdpos Posiive seqence volage a) b) Negaive seqence volage in dq-reference frame Vdneg Vqneg.5 Phase sscepances Bab Bbc Bca c) d) Figre 13: Reacive power reference change from 1MVAr indcive o 7MVAr capaciive nder nework volage 1.5% nbalance: a) PCC reacive power, b) posiive seqence volage, c) negaive seqence volages in dq-reference frame, d) phase sscepances. 5. Conclsions Saic var compensaors are ypically sed o conrol he power sysem posiive seqence volage and reacive power flow. Recenly, iliy companies have exended he reqiremens for SVCs o inclde he compensaion of nework volage asymmery in addiion o he posiive seqence volage conrol. In order o flfil his ask, he SVC conrol sysem shold be designed o operae asymmerically. Hence, he legs of he TCR and TSC shold be individally conrollable. This paper presens a modern vecor conrol based mehod o conrol he SVC asymmerically. The conrol sysem operaes in a synchronos reference frame oriened o he nework volage posiive seqeence componen and he posiive and negaive seqence volage componens are conrolled individally. The correc conrol sysem operaion has been esed and verified in pracice wih a SVC locaed in Bom Despacho, Brazil. I is shown in his paper ha he operaional capabiliy of he SVC is significanly improved g he presened flexible asymmerical conrol sraegy. The performance enhances dring ransiens as well as nder seady sae operaion condiions. I is described how he SVC can provide maximm possible posiive seqence volage sppor wiho sffering from a healhy phase overvolage problem dring a gle phase-o-grond fal. In addiion, he SVC can be conrolled o generae a desired reacive power while simlaneosly compensaing he nework volage asymmery. Frhermore, he SVC is able o ilize is whole power range o compensae he nework volage asymmery compleely or parially. I can be conclded ha wih he conrol sysem presened in his paper, he SVC power range can be ilized in is enirey o serve prposes desired by power sysem operaor. I is jsified o sae ha he performance of an asymmerically conrolled SVC is sperior compared o radiional symmerical design. In addiion, he inelligenly conrolled SVC is compeiive compared o a STATCOM from he conrol capabiliy poin of view.

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