ABB Review Special Report - ABB - ABB Group

V (pu)61.61.41.21.00.80.60.40.20V/I characteristic, showing the possible steady-state andtransient operating points of the SVC-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.2 2.5 3.0 3.5 4.0Capacitive I (pu) InductiveThe colored area represents continuous SVC operation. Above thisarea, the SVC can be operated up to 1.2 pu voltage for 3 s, up to 1.3 puvoltage for 400 ms, and up to 1.5 pu voltage for 300 ms.to be taken out of service, the 400-kVtransmission system could not be operatedwithout risking dangerous overvoltages.As a result, an availability figureof 99.7 % was specified, and this stronglyinfluenced the design, quality, functionalityand layout of its componentsand subsystems as well as of the SVCscheme as a whole.3 s400 ms300 msshould be thelast transformersin the Nam-Power systemto go into saturation.TCR reactorand valveEach TCRbranch consistsof two air-corereactors connectedon eachside of athyristor valve.The reactorshave specialexterior surfacesto protectthem from theeffect of sandstorms and sunin the harshdesert environment.A secondary voltage of 15 kV was chosenas an optimum value for both thethyristor valve and busbar design. Thethyristor valves consist of single-phasestacks of antiparallel-connected thyris-tors (16 thyristors, two of which areredundant, in each valve). Snubbercircuits (series-connected resistors andcapacitors) limit overvoltages at turnoff.The thyristors are fired electrically usingenergy taken directly from the snubbercircuit.An overvoltage protection device limitsthe voltage that can appear across thevalve, being triggered by control unitsthat sense the instantaneous voltageacross each thyristor level.Redundant TCR branchThree TCR units rated at 110 MVAr havebeen installed to cope with the Nam-Power network’s sensitivity to reactivepower and harmonic current injections.A fourth, identical TCR is kept on hotstandby. The SVC control system automaticallyrotates the current standbyTCR unit every 30 hours to ensure equaloperating time for all units.Redundant cooling systemAn unusual feature of the Auas SVC isthat each TCR valve has its own coolingsystem, making four in all. Thus, outagetime is minimized and availability is increased.A water/glycol cooling mediumis used to avoid freezing in case of aux-Operating rangeThe Auas SVC provides resonance controlover its entire operating range 6 , whichextends well beyond its continuous range.Controlled operation is possible all theway up to 1.5 pu primary voltage – a necessaryfeature for controlling the resonancecondition. Besides providing resonancecontrol, the SVC also controls thepositive-sequence voltage (symmetricalvoltage control) at the point of connection.Single-phase transformersFour single-phase transformers, includingone spare, are installed. Due to thehigh overvoltage demands made onthem during resonance these transformershave been designed with a lowerflux density than standard units; they22Special ReportABB Review