Characteristics of the Partial Discharge-induced Current along Epoxy resin film under Superimposed AC- DC Voltage

Insulation of power electronic devices and converter station equipment is often under superimposed AC-DC voltage. The understanding of PD characteristics under AC-DC combined excitation plays an important role in designing the insulation of power electronic device. Previous researches focused on the stochastic features and mechanism under AC or DC voltages, while little attention was paid to the PD-induced charge movements under superimposed AC-DC field. The charge movements include the transient behavior of PD and the slow movement, which can be characterized by the transient PD current and the excess current respectively. In this paper, a composite AC-DC voltage is applied to a pair of rod-plane electrodes sandwiching epoxy film. And a sensitive high-frequency Rogowski coil and a capacitor-resistor shunt impedance are used to simultaneously measure the transient PD current and the excess current during PD. The excess current can be measured by cancelling the capacitive current from the total current. In the measurement, first set the AC voltage at 1.5 times the AC PDIV, then gradually raise the DC voltage up. The results show that, as the DC bias increases, the phase distributions of PD events are expanded, but the average PD currents and excess currents are lower than that under the pure AC electric field. Moreover, only few PD events are observed when the DC voltage rises close to the peak of AC voltage, and the surface flashover starts once the DC voltage rises above the peak of AC voltage. It is proved that the additional DC voltage suppress the corona discharge excited by the AC voltage to a certain extent, but it may also result in surface flashover if the DC bias exceeds the AC amplitude.