ABB Review Special Report - ABB - ABB Group

iliary power outages during the colddesert nights.Filter branchesThe required capacitive MVAr are providedby two 40-MVAr filter banks. Eachfilter is double-tuned to the 3rd/5th harmonicsand connected in an ungroundedconfiguration. The double-tuneddesign was chosen to ensure sufficientfiltering even in the case of one filterbecoming defective.Black-start performanceSince the SVC is vital for operation ofthe NamPower system, everything hasto be done to avoid the SVC breakertripping, even during a network black-Voltage / 400 kV (pu)71.81.61.41.21.00.80.6Real-time digital simulation. 400-kV line energized from the north, with and withoutthe new resonance controller0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0t (s)a)out. In such a case the network couldbe energized from the Eskom side andthe SVC would have to be immediatelyready to control a possible resonancecondition. To handle this task, the SVChas three separate auxiliary supplies,one of which is fed directly from theSVC secondary bus. The SVC is capableof standby operation with its MACH 2controller active for several hours withoutauxiliary power, and automaticallygoes into resonance control mode assoon as the primary voltage returns.BrefAdd (pu)BrefDI (pu)0.50.0b)-0.5-1.0-1.5-2.0-2.5-3.00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0t (s)0.50.0c)-0.5-1.0-1.5-2.0-2.5-3.00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0t (s)a Voltage response, 400 kVRed Conventional PI controllerb SVC controller outputBlue Resonance controllerc Impact of resonance controllerWorst-case situation:energization from north to southThe worst–case scenario for the SVC andthe Nampower system is energization ofthe 400-kV line from the northern section(Auas substation). This system condition,which initiates the critical 50-Hz resonance,was therefore simulated in a realtimedigital simulator with and withoutthe new resonance controller. As shownin 7 the overvoltage that appears atAuas is 1.62 pu with a conventional PIcontroller. (The two resonance frequencies– 56 Hz and 81 Hz – that can beseen in the result correspond to the system’sfirst and second pole, respectively.)The new resonance controller has a considerableimpact on the system’s behaviorand the voltage controller’s additionalcontribution forces the SVC to becomeinductive. As a result, the peakvoltage appearing at Auas is reduced toa value of 1.32 pu.Special ReportABB Review23