1 ADVANCED TAPCHANGER CONTROL TO COUNTERACT POWER SYSTEM VOLTAGE INSTABILITY Zoran Gajiş zoran.gajic@se.abb.com Samir Aganoviş samir.aganovic@se.abb.com ABB AB, Subsaion Auomaion Producs, SE-721 59 Vaseras, SWEDEN SUMMARY The main purpose of he auomaic apchanger conrol (ATCC) for power ransformers wih load apchanger (LTC) is o keep he volage on low volage (LV) side of he ransformer wihin a prese deadband. Originally ATCC was designed in his way in order o compensae for he volage drop across power ransformer impedance caused by flow of he load curren. Therefore an ATCC shall reac and change ap posiion in accordance wih LV side load variaions. However, he ATCC will as well reac on abnormal volage variaions on he high volage (HV) side of he power ransformer. Someimes such ATCC behavior is no desirable because i jus furher increases he oal load on he HV sysem (i.e. ransmission sysem). Especially, such behavior shall be prevened during criical operaion saes of he ransmission sysem, such as a slow power sysem volage collapse. The major power sysem blackous hroughou he world in 2003 have clearly illusraed he need for differen modes of volage conrol, since he requiremens during normal operaion condiions and abnormal condiions, sliding owards volage insabiliy, are very differen. In his paper he focus will be on possibiliies o improve radiional load apchanger conrol in order o perform properly also during sressed siuaion in he power sysem. Mos of he curren commercially available auomaic apchanger conrollers, jus measure he LV side volage of he power ransformer in order o conrol ap posiion. Such a principle has a major drawback ha ypically speeds up a power sysem volage collapse. However, some modern inelligen elecronic devices (IEDs) used for such auomaic conrol do have he capabiliy o measure he power sysem volage on boh sides of he power ransformer. A scheme wih such buil-in feaure can offer excellen performance of ATCC scheme during large volage variaions on he ransformer HV side. In he same ime i can as well be used o improve ime coordinaion of he LTCs conneced in series and o minimize he overall number of LTC operaions in he whole power sysem.
2 INTRODUCTION When he load in a power nework is increased he volage will decrease and vice-versa. To mainain he nework volage a a consan level, power ransformers are usually equipped wih a load apchanger. The apchanger alers he power ransformer urns raio in a number of predefined seps and in ha way changes he secondary side volage. Each sep usually represens a change in LV side no-load volage of approximaely 0.5-1.7%. Sandard apchangers offer beween ± 7 o ± 17 seps (i.e. 15 o 35 posiions). The ATCC is designed o regulae a power ransformer wih a moor driven load apchanger. Typically such conrol scheme regulaes volage a he secondary side of he power ransformer. The conrol mehod is based on a sep-by-sep principle which means ha a conrol pulse, one a a ime, will be issued o he apchanger mechanism o move i up or down by one posiion. The pulse is generaed by he ATCC whenever he measured volage, for a given ime, deviaes from he se reference value by more han he prese deadband (i.e. degree of insensiiviy). Time delay is used o avoid unnecessary operaion during shor volage deviaions from he pre-se value. AUTOMATIC LTC CONTROL PRINCIPLES FOR SINGLE TRANSFORMER A ypical ATCC measures he busbar volage (U BB ) a he power ransformer LV side, and if no oher addiional feaures are enabled (i.e. line drop compensaion) his volage is used for volage regulaion. The volage conrol algorihm hen compares U BB wih he se arge volage (Use) and decides which acion should be aken. Because his conrol mehod is based on a sep-by-sep principle, a deadband LU (i.e. degree of insensiiviy) is inroduced in order o avoid unnecessary swiching around he arge volage. The deadband is ypically symmerical around Use as shown in Figure 1. Deadband should be se o a value close o he power ransformer s LTC volage sep. Typical seing is 75% of he LTC sep. Securiy Range Auo mode is Blocked Lower Cmd Raise Cmd L U L U Lower Cmd Fas Lower Cmd is Blocked LU in LU Raise Cmd is Blocked in U U Volage Magniude U block U min U 1 se U2 max Figure 1: Typical ATCC Volage Scale for Auomaic LTC Conrol
3 During normal operaing condiions he busbar volage U BB, says wihin he deadband. In ha case no acions will be aken by he ATCC. However, if U BB becomes smaller han U1 or greaer han U2 (see Figure 1), an appropriae lower or raise imer will sar. The imer will run as long as he measured volage says ouside he inner deadband. If his condiion persiss for longer han a prese ime, he appropriae lower or raise command will be issued. If necessary, he procedure will be repeaed unil he busbar volage is again wihin he inner deadband. The main purpose of he ime delay is o preven unnecessary LTC operaions due o emporary volage flucuaions. The ime delay may also be used for LTC co-ordinaion in radial disribuion neworks in order o decrease he number of unnecessary LTC operaions. This can be achieved by seing a longer ime delay for ATCCs locaed closer o he end consumer and shorer ime delays for ATCCs locaed a higher volage levels. CONTROL PRINCIPLES FOR PARALLEL TRANSFORMERS Auomaic load apchanger conrol of parallel ransformers can be made according o hree differen mehods: 1) Reverse reacance mehod 2) Maser follower mehod 3) Circulaing curren mehod Unlike he firs mehod, he las wo mehods require exchange of signals and measured values beween he ransformers, or beween he ransformers and a cenral conrol uni. However, he drawback wih he firs mehod is ha he volage conrol will be affeced by changes in he load power facor. The maser follower mehod is generally limied o applicaions wih similar ransformers, whils he circulaing curren mehod, which is ypically available in new numerical ATCCs, also handles, in an elegan way, he more generic case wih unequal ransformers in parallel operaion. Two main objecives of volage conrol of parallel ransformers wih he circulaing curren mehod are: 1) Regulae he LV side busbar volage o he prese arge value 2) Minimize he circulaing curren, in order o achieve opimal sharing of he reacive load beween parallel operaing ransformers The firs objecive is he same as for he volage conrol of a single ransformer while he second objecive ries o bring he circulaing curren, which appears due o unequal LV side no load volages in each ransformer, ino an accepable value. Figure 2 shows an example wih wo ransformers conneced in parallel. If ransformer T1 has higher no load volage (i.e. UT1) i will drive a circulaing curren which adds o he load curren in T1 and subracs from he load curren in T2. I can be shown ha he magniude of he circulaing curren in his case can be approximaely calculaed wih he following formula:
4 I = I = cc _ T1 cc _ T 2 U Z U + Z T1 T 2 T1 T 2 Because ransformer impedances are dominanly inducive i is possible o use only he ransformer reacance in he above formula. A he same ime his means ha ransformer T1 circulaing curren lags he busbar volage U BB for almos 90º, whils ransformer T2 circulaing curren leads he busbar volage by almos 90º. This also means ha he circulaing curren is mainly reacive in naure, and i only represens reacive power ha circulaes beween wo ransformers conneced in parallel. Therefore by minimizing he circulaing curren flow hrough he ransformers, he oal reacive power flow hrough he parallelconneced ransformer group is opimized as well. A he same ime, a his opimum sae he apparen power flow is disribued among he ransformers in he group in direc proporion o heir raed power. U T1 I cc_t2 U T2 I cc_t2 T1 T2 <=> + Z T1 I T1 I cc_t1 + Z T2 I T2 I cc_t1 UB U B UBB I T1 I T2 U BB U B I L I L U L Load U L Load Figure 2: Equivalen scheme for wo parallel ransformers in accordance wih minimizing circulaing curren mehod Therefore an ATCC, regardless of wheher i is used for single or parallel ransformer conrol, always reacs and changes ap posiion in accordance wih LV side load variaions. However, he ATCC will as well reac on abnormal volage variaions on he high volage (HV) side of he power ransformer. Someimes such ATCC behavior is no desirable because i jus furher increases he oal
5 load on he HV sysem (i.e. ransmission sysem). Especially, such behavior shall be prevened during criical operaion saes of he ransmission sysem such as a slow power sysem volage collapse [1], [2] & [3]. REALIZATION POSSIBILITIES FOR TRANSFORMER PARALLELING The ransformer paralleling can be realized in wo differen ways: 1) ATCC funcionaliy for each ransformer is inegraed in he ransformer proecion IED. Inegraion of several proecion and conrol funcions in a single IED is common praxis for some elecric uiliies across he world since 1998. Thus, all power ransformer proecion funcions like 87T, 87N, 50/51, 50N/51N, 49 as well as LTC conrol funcion 90 are inegraed ino a single device. All required informaion for eiher maser-follower of circulaing curren operaing modes are communicaed beween he paricipaing IEDs using he IEC 61850-8-1 proocol. Thus, exchange of all analog and binary daa for ATCC operaion is made by using IEC 61850 GOOSE messages [9]. Up o eigh ransformers can be conrolled in parallel for such insallaion. Noe ha for such se-up paralleled ransformers can even be locaed in differen subsaions if corporae LAN is available. Example of such insallaion is shown in Figure 3. 2) ATCC funcionaliy for all ransformers in he saion is inegraed in a single IED which is only used for parallel LTC conrol. Such soluion eliminaes any need for exernal communicaion bu i requires CT and VT wiring from all LTC ransformers o be brough o a common locaion. Noe ha i is possible o achieve ho sandby funcionaliy by using backup IED wih idenical conrol faciliies as shown in Figure 4 which is aken from one exising insallaion. Which of he wo soluions is used depends on he end user preference. T1 T2 Figure 3: Inegraion of ATCC funcionaliy in he ransformer proecion IEDs
6 Figure 4: ATCC funcionaliy for four ransformers inegraed ino single conrol IED wih a ho sandby funcionaliy KNOWN WEAKNESSES OF TRADITIONAL ATCCS The lis of well-known weaknesses of radiional ATCC is given here: 1) Radial acive power flow from HV o LV side is assumed for correc operaion. Special measures shall be aken in case of acive power reversal. 2) Time coordinaion of cascading ATCCs can be quie difficul ask in order o minimize number of overall LTC operaion in a power sysem and sill keep accepable ime delay for ATCC insalled closes o he loads [4]&[5] 3) Quie inefficien way o conrol volage for power ransformers which inerconnec wo quie srong neworks (i.e. beween wo ransmission neworks like 400/220kV auoransformers) 4) Increase of volage on LV power ransformer side worsens he siuaion on he oher side (reacive power flow increases from HV o LV side of power ransformer) 5) LV side load recovery by ATCC acion during slow volage collapse in power sysem [1] However in his paper only he problems menioned in poins 2) and 5) above will be addressed.
LESSONS LEARNED FROM SWEDISH BLACKOUT IN SEPTEMBER 2003 7 The major disurbances hroughou he world in 2003 have clearly illusraed he need for differen modes of volage conrol, since he requiremens during normal operaion condiions and abnormal condiions, sliding owards insabiliy, are very differen. In he following, focus will be on possibiliies o improve apchanger conrol in order o perform properly also for disurbed condiions. Figure 5 shows HV side volage and ap posiion for a power ransformer conneced beween 400kV ransmission sysem and 130kV sub ransmission sysem, in he affeced area, a he end of he Swedish blackou in 2003 [6]. The used ATCC is designed only o keep he volage a he low volage side of he power ransformer wihin cerain limis, around he se poin. When he ransmission side volage decreases, he ap posiion is increased by ATCC in order o fulfill is ask. As a consequence, he ap posiion increases nine seps wihin las 80 seconds of he blackou, keeping up he sub ransmission volage and hereby he load drawing more acive and reacive power from he already weakened ransmission sysem. Similar ATCC behaviors have as well been repored during oher blackous, which happened in las 20-30 years all around he world. Volage Simpevarp Busbar D TC-posiion Simpevarp 500 30 Volage [kv] 400 300 200 100 25 20 15 10 5 0 0 60 120 180 240 300 360 420 Time [s] afer 12:30 0 0 60 120 180 240 300 360 420 Time [s] afer 12:30 Figure 5: Recorded ransformer HV side volage and ap posiion a he end of he Swedish blackou in Sepember 2003 ADVANCED ATCC OPERATING PRINCIPLES The main purpose of he auomaic apchanger conrol for power ransformers wih load apchanger is o keep he volage on low volage side of power ransformer wihin a prese deadband. Originally ATCC was designed o compensae for he volage drop across power ransformer impedance caused by flow of he load curren. Therefore an ATCC shall reac and change ap posiion in accordance wih LV side load variaions. However, he ATCC will as well reac on abnormal volage variaions on he high volage side of he power ransformer. Of
8 en such reacion is no desirable because i jus furher increases oal load on he HV sysem (i.e. ransmission sysem). Especially, such behavior should be prevened during criical operaion saes of he ransmission sysem, such as a slow power sysem volage decrease, as shown in Figure 5. Typically modern commercially available apchanger conrollers jus measure he LV side volage of he power ransformer in order o make decisions abou suiable ap posiion. Such a principle has a major drawback ha ypically speeds up a power sysem volage collapse [1]. However, some modern inelligen elecronic devices (IEDs) [7] used for such auomaic conrol do have he capabiliy o measure power sysem volage on boh sides of he power ransformer, as shown in Figure 6. Addiionally he oal reacive and/or acive power flow hrough he power ransformer can be measured as well. Noe ha volage ransformers are ypically available on he HV side of he power ransformer due o oher reasons e.g. HV disance proecion. By using a number of over- and under-volage sages i is hen possible o monior HV side volage magniude and consequenly influence he operaion of he ATCC or oher equipmen in he subsaion. For he bes scheme securiy i is desirable o measure all hree phase-o-earh volages from he HV side, in order o ake necessary acion only when all hree volages are above or below he pre-se level. A he same ime prolonged presence of negaive or zero sequence volage will indicae possible problems wih HV VT. Therefore, operaion of he ATCC can be easily influenced, in he secure way, by he level of measured volage on he HV side of he power ransformer. The following are some ypical acions, which hen can be auomaically aken by such ATCC scheme: 1) Temporary ATCC block (e.g. for 20 s). 2) HV shun capacior (reacor) swiching. 3) ATCC volage se poin change (ypically reducion). 4) Complee ATCC block. 5) Undervolage load shedding. Temporary block of local ATCC for smaller volage deviaion on power ransformer HV side can also be used o drasically improve he cascading ATCC ime coordinaion in a power sysem. Small volage variaions on he HV power ransformer side can be only correced by he appropriae acion of he upsream ATCC. Therefore he local (i.e. downsream) ATCC can be emporary block in order o give addiional ime o he upsream ATCC in order o reac and correc HV volage magniude. By doing so he operaing ime of all cascading ATCCs can be se o he exac he same value. Such scheme will as well guaranee faser volage conrol a disribuion loads han wha is achieved oday wih radiional ime coordinaion approach. A he same ime he emporary blocking will guaranee operaion of downsream ATCC in case of failure of he upsream ATCC. Wih such approach overall number of LTC operaions in complee power sysem can be minimized. This will represen cos benefi for he power uiliy regarding required LTC mainenance.
9 UHV > UHV > U HV Deec increased reacive power (i.e. Q) flow hrough power ransformer U_raed Temporary block ATCC for 20s Normal Volage Range UHV Temporary block ATCC for 20s HV capacior bank swich-in ATCC se poin reducion Programmable LOGIC by using AND gaes, OR gaes, TIMERS, ec. Block ATCC operaion Undervolage load shedding REDUCE Use BLOCK AUTO ULV IED RAISE LOWER ATCC Figure 6: Principles of an improved auomaic apchanger conrol scheme. When HV volage drops o even lower value his migh indicae he possible problems in he HV ransmission sysem e.g. slow volage collapse phenomenon. Therefore he proposed scheme can ake cerain precauions locally as for example: 1) HV shun capacior swiching and shun reacor disconnecion, in order o ry o increase volage on he HV side of he power ransformer. 2) ATCC volage se poin reducion, in order o keep low volage profile on sub ransmission sysem and herefore cause reducion of oal acive and reacive power demand from HV ransmission sysem. 3) Complee ATCC block in order o preven any apchanger auomaic operaion. This will inhibi undesirable ATCC operaions during sressed condiion on he HV side of he power ransformer. 4) Finally undervolage load shedding of pre-seleced ougoing feeders on power ransformer LV side can be performed in order o ry o preven complee power sysem blackou. Which exac acions shall be aken depends on he paricular power sysem characerisics, locaion of power ransformer wihin he power sysem and ype of load conneced on power ransformer LV side. Therefore a complee power sysem sudy mus be performed in order o deermine he opimum scheme seup. However, wih help of graphical configuraion ools, modern numerical IEDs [7] can be ailor made o fulfill sric requiremens of any power sysem operaor and characerisics of he individual power sysem.
10 Noe ha in Figure 6 scheme for a single ransformer is shown. However exacly he same scheme can be engineered for parallel operaing ransformers. CONCLUSIONS This paper focuses on new possibiliy for advanced auomaic LTC conrol sraegy for power ransformers. The main improvemen from he radiionally used schemes is ha he newly proposed scheme akes in consideraion he volage magniude on he HV side of he power ransformer. By doing ha he overall coordinaion of series conneced power ransformers wih LTC can be much improved and in he same ime performance of such ATCC scheme will be much beer during criical siuaions in HV power sysem e.g. slow volage collapse phenomenon. ACKNOWLEDGEMENT The paper is based on work performed wihin he CRISP: disribued inelligence in CRiical Infrasrucures for Susainable Power, financially suppored by he European Commission, conrac nr. ENK5-CT-2002-00673: CRISP, which is graefully acknowledged [8]. REFERENCES: [ 1 ] Taylor, C.W.: "Power Sysem Volage Sabiliy", McGraw-Hill, 1993. [ 2 ] Taylor, C.W.: "Conceps of Undervolage Load Shedding for Volage Sabiliy", IEEE Transacions on Power Delivery, vol. 7, no. 2, pp. 480-488, April 1992. [ 3 ] Ingelsson, Karlsson, Lindsröm, Runvik and Sjödin: Special Proecion Scheme agains Volage Collapse in he Souh Par of he Swedish Grid, 1996 CIGRE Conference in Paris [ 4 ] Mas Larsson: Coordinaed Volage Conrol in Elecrical Power Sysems, Disseraion, Lund Universiy, Sweden, December 2000. [ 5 ] Daniel Karlsson: Volage Sabiliy Simulaions Using Deailed Models Based on Field Measuremens, PhD Disseraion, Chalmers Universiy, Sweden, 1992 [ 6 ] "Elavbroe 23 sepember, 2003 (Blackou 23 Sepember, 2003)", Repor Nr 1:2003, Svenska Krafnä, 2003-11-03, available on www.svk.se. [ 7 ] ABB Documen 1MRK 504 086-UEN, "Technical reference manual, Transformer Proecion IED RET670", Produc version: 1.1, ABB AB, SA Producs, Vaseras, Sweden, (2007). [ 8 ] CRISP Final Summary Repor, available a: hp://www.ecn.nl/crisp/deliverables/d5.3.pdf [ 9 ] Gajiş, Aganoviş, Benoviş, Leci and Gazzari: Using IEC 61850 Analogue GOOSE Messages for OLTC Conrol of Parallel Transformers, Sep. 2009, CIGRE Conference on Acual Trends in Developmen of Power Sysem Proecion and Auomaion in Moscow