Long front time switching impulse tests of long air gap in UHV projects at altitude of 2100 m

The past and recent investigations show that front time of switching overvoltage waveforms in Ultra High Voltage (UHV) system of long distance transmission are mostly in long wave front time exceeding 1000 μs. The long front time of switching impulse voltage has been deviated from the critical front time. This paper is involved with breakdown characteristics and external insulation design under long front switching impulse voltage in UHV system. All the Tests were performed on an outdoor UHV test site at altitude of 2100 m, China Southern Power Grid Co. Ltd. (CSG). Breakdown characteristics of rod-plane and actual size tower-conductor in ±800 kV UHV DC transmission line are obtained under wide front time switching impulse voltages from 100 to 2350 μs. From the comprehensive studies of such tests, it is proposed that U₅₀ of front time exceeding 1000 μs increases 20% compared with the results of standard switching impulse (250/2500 μs) tests and standard deviations of U₅₀ is 6% in UHV transmission system. Finally, an air gap insulation design procedure in conductor-tower of UHV transmission line at altitude of 0-2000 m is presented. Long front time switching impulse strength of conductor-tower gap is proposed into selection by reference in UHV transmission line insulation design.