Influence of Preexisting DC Voltage on Switching Surge Flashover Characteristics

Flashover characteristics are presented for air gaps and insulators when a switching surge and a dc voltage are simultaneously applied to test specimens. In the first case, both a switching surge and a dc voltage were applied to the same electrode. This test is equivalent to the stresses generated by the reclosing of a circuit breaker. It was found that positive switching surge flashover voltages of rod-plane gaps are reduced by four to seven percent due to the existence of negative dc voltage. In the second case, a switching surge and a dc voltage were applied to each electrode. For rod- rod gaps, positive switching surge flashover voltages to ground decreased by 70 to 80 percent of the dc voltage value due to the existence of the negative dc voltage at the opposite electrode. These data are useful to EHV insulation design.