The Simulation And Research On Over-voltage Suppression Measures For UHVAC Transmission Lines

With UHV (Ultra High Voltage) AC transmission lines having the characteristicof low distributed resistance and inductance, high distributed capacitance and longdistances, the over-voltage becomes more severely. And the insulation level isbasically determined by the over-voltage rate. So there are many benefits from theover-voltage limitation, such as reducing the insulation level and the constructioninvestment, mitigating the damage to the power equipments and so on.The paper presents the relationship between all kinds of factors (distances of theline, fault time, closing time etc) and the power frequency overvoltage and switchingovervoltage according to a great deal of simulation results by the PSCAD/EMTDC.And the effectiveness of many conventional and non-conventional over-voltagesuppression measures is examined, such as shunt reactors (uncontrollable andcontrollable), MOA (metal surge oxide arresters), circuit breakers with pre-insertionresistors, series compensation and controlled switching technology.The paper puts emphasis upon the application of the controlled switchingtechnology in the energization and re-energization of compensated UHV transmissionlines. The optimum instant for closing by phase is that of each phase voltage-zero onthe source-side. In the case of compensated line the optimum reclosing time is thevoltage minimum across the circuit breaker pole, preferably during a period ofminimum voltage beat. Both the theory and simulation results prove that the circuitbreaker with controlled switching can effectively limit the UHV transients, and thefield experience with controlled switching system applied in transmission line showsit operates reliably and accurately.This paper proposes that it is can take effect of controlled switching circuitbreaker to control the closing & re-closing time of the digital protection, which cansave the equipment investment and make the control mode more flexible.The simulation results indicate that controllable shunt reactors installed at bothends can effectively restrain the power frequency over-voltage, increase the transfercapability of the transmission lines and improve the reactive power compensation,which has tremendous economic interest, and it is necessary to set a switching stationat the centre of the transmission line for the long distance transmission lines. Theovervoltage level can be restrained to acceptable level, if the shunt reactors and metalsurge oxide arresters are set at both terminals for the short distance transmission lines,the MOA installed in the middle of the line (if possible), and the controlled switchingtechnology is applied in the energization transmission line and the adaptivesingle-phase reclosure.