Switching overvoltage analysis and air clearance design on the KEPCO 765 kV double circuit transmission system

This paper presents the results of the air clearance design on the KEPCO 765 kV double circuit transmission system. At first the authors performed the switching overvoltage study. The switching overvoltage was analyzed by EMTP (Electro-Magnetic Transient Program) and TNA (Transient Network Analyzer). The full scale test for the air insulation strength of 765 kV double circuit transmission towers was performed by a 4 MV outdoor impulse generator. The simultaneous fault clearing case of a single line-to-ground fault at each circuit was considered because the probability of simultaneous single line-to-ground fault at each circuit is very high. However, the reclosing overvoltages with preinsertion resistor are lower than the simultaneous fault clearing overvoltages when the transmission line is short (about 170-180 km). The maximum phase to ground switching overvoltage in the simultaneous fault clearing case is 1.9 p.u. and the maximum switching overvoltage between phase to phase is 3.3 p.u. The required air clearance in a tower at the altitude of below 1000 m is 4950 mm to endure the maximum switching overvoltage of 1.9 p.u. The standard deviation of 5% in switching surge flashover voltage was verified from the full scale test.