Lightning Surge Overvoltages at Substations Due to Backflashover With Assumed Lightning Current Waveforms Based on Observations

Recent lightning current observations have suggested that a front duration of 1.7 s is more appropriate than the conventional 1.0 s for the lightning protection design of ultra-high voltage (UHV) systems in Japan. This study examined overvoltages at gas-insulated subsystems (GIS) and transformers in a UHV (the rated voltage of 1100 kV) substation during backflashovers, with the front duration of lightning stroke current waveforms taken as a parameter. The dependence of overvoltages on the front duration of the lightning stroke current is significant at GIS and relatively small at the transformer. Generally speaking, the shorter the front duration is, the higher the voltage will be. The transformer capacitance, ranging from 0 to 1000 pF, equivalently increases the distance to the surge arrester, and a voltage increase was observed at the transformer terminal. However, the steepness of the intruding surge waveform is reduced and the voltage increase suppressed at 1000 pF and over. The effect of overvoltages on insulation design was also evaluated and the results thereof indicated the possibility of reducing the lighting impulse withstand voltage.