ABB Review Special Report - ABB - ABB Group

the south. However, with a length of890 km it is also very long, in fact oneof the longest lines of its kind in theworld. This and the network’s tree-likeconfiguration, coupled with remote generationand the very long radial linesoperated at high voltage, results in thecharging capacitance being high. Theeffect of this is to shift the existing parallelresonance closer to 50 Hz, makingthe network more voltage-sensitiveduring system transients, for examplewhen the 400-kV line is energized orduring recovery after a line fault clearance.Each of these phenomena manifestsitself as an extremely high andsustained overvoltage.Resonance and overvoltagesThe NamPower network has a first naturalparallel resonance frequency wellbelow 100 Hz, namely in the 55–70 Hzrange (curves 1 and 2 in 3 ).The effect of adding the new 400-kVline section (Aries-Kokerboom-Auas)and its four 100-MVAr shunt terminalreactors has been to shift the system’sfirst resonance into the 60–75 Hz frequencyrange (curves 3 and 4). (Thereduction in system impedance at50 Hz is due to the new 400-kV line,and an indication of how the systemhas been strengthened.)Curves 5 and 6 in 3 show the networkimpedance as seen at the Auas 400-kVbus the instant the 400-kV line is energizedfrom the northern section (fromthe Auas side) and before the circuitbreakeron the Kokerboom side isclosed.The impact of the resonance problemin the NamPower system is best illustratedby simulating the condition atAuas substation, represented by curve6. The voltage situation is shown in 4 ,in which the line circuit-breaker atAuas is closed at time t = 1.0 s and itis assumed that the breaker at Kokerboomis synchronized at t = 1.2 s. Dueto the large charging capacitance of theline the voltage first dips, then overshoots.The extremely high overvoltages appearingat Auas, with a peak value inexcess of 1.7 pu and a sustained transientovervoltage (TOV) of more than1.5 pu, attest to the severity of the problem.It is clear that as soon as 50-Hzresonance is triggered very high dynamicovervoltages appear with large timeconstants under certain system load andgeneration conditions.5400 kV Auas substation15 kVSingle-line diagram of the AuasSVCX400 kV/15kVSVC transformerTCR1 TCR2 TCR3 TCR4 Filter1 Filter2(spare)Auxsupply80 MVAr capacitive) and is installedprimarily to control the system voltage,in particular the extreme (up to 1.7 pu)overvoltages expected as a result of thenear 50-Hz resonance. An uncommonfeature of the project is that the SVC isinstalled in a system with very longlines, little local generation and faultlevels lower than 300 MVA.V (kv)46004002000-200-400-600Energization of the Auas-Kokerboom400-kV line from thenorthern section, without the SVC0.95 1.00 1.05 1.10 1.15 1.20 1.25t (s)The blue, green and red curves represent thedifferent phases (instantaneous values).Preliminary studies indicated that overvoltageswould appear that would makethe Nam-Powersystem inoperableunlessvery fast,effectiveand reliablecountermeasures are taken. Several solutionswere considered as an answer tothe resonance problem, including fixedand switched reactors, before deciding toinstall a FACTS device in the Auas substation.Preference was given to conventional,proven SVC technology [1].SVC design featuresThe Auas SVC has a dynamic rangeof 330 MVAr (250 MVAr inductive toThe SVC that is installed is of a newtype, developed by ABB for power applications.Studies showed that overvoltagescould make the NamPower systeminoperable unless very fast, effectiveand reliable countermeasuresare taken.Its uniquecontrolprinciplehas sincebeenpatented.The inductivepower of 250 MVAr is provided by threethyristor-controlled reactors (TCRs), afourth, continuously energized TCR beingalways on standby 5 . Two identicaldouble-tuned filters, each rated at40 MVAr, take care of harmonics andsupply capacitive reactive power duringsteady-state operation.High availability is essential for the AuasSVC. If, for any reason, it should haveSpecial ReportABB Review21