交直流复合电压下油纸绝缘针板缺陷的预击穿阶段局部放电特性

The partial discharge characteristics of oil-paper insulation with needle-plate defect under AC-DC combined voltage are very unstable and highly stochastic. The gases generated in pre-breakdown processes of the insulation make the partial discharge characteristics more complicated. In this paper, the time-variation trend of partial discharge parameters and the stochastic characteristics of the pre-breakdown processes under AC-DC combined voltage are investigated through multiple experiments for oil-paper insulation with needle-plate defect based on the pulse current method. The mean partial discharge number and the mean partial discharge amplitude totally decrease with the voltage time increasing. However, the variation trends of each experiment are different, and can be divided into fluctuation type and monotone type. The differences of partial discharge performances between the two types are mainly associated with the gathering of gas bubbles at paper surfaces. In the partial discharge equivalent time-frequency spectrograms, most points move towards arch top with the developing of pre-breakdown processes, where partial discharge equivalent time is 1 300~1 600 ns and partial discharge equivalent frequency is 7.5~10 MHz. Partial discharge equivalent time-frequency spectrogram may be half-arched type or whole-arched type at the end of pre-breakdown processes.