Partial discharge characteristics of oil impregnated paper insulation system with oil gap under repetive impulse voltage superimposed on AC voltage

Partial discharge (PD) characteristics of an oil-impregnated paper insulation system with an oil gap under a lightning impulse voltage superimposed on an alternating current (AC) voltage were investigated. Prior to the voltage superimposition, insulation oil in the oil gap was degraded due to continuous PDs. Next, an AC voltage giving an electric field lower than designed one of 275 kV OF cable systems was applied to the insulation system, and then a lightning impulse voltage equivalent to switching surge voltage was repeatedly superimposed on the AC voltage. As a result, PDs progressed to continuous discharges from intermittent ones after the repeated superimposition of the lightning impulse voltage and the AC voltage. As a result, it is suggested that repeated applications of switching surge voltage may be one of the main causes of continuous PD occurrence in OF cable systems especially when insulation oil in the oil gap is degraded.