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 insula-tion make the partial discharge characteristics more complicated. In this paper

the time-variation trend of partialdischarge parameters and the stochastic characteristics of the pre-breakdown processes under AC-DC combined volt-age are investigated through multiple experiments for oil-paper insulation with needle-plate defect based on the pulsecurrent method. The mea

n partial discharge number and the mean partial discharge amplitude totally decrease withthe voltage time increasing. However

the variation trends of each experiment are different

and can be divided intofluctuation type and monotone type. The differences of partial discharge performances between the two types aremainly associated with the gathering of gas bubbles at paper surfaces. In the partial discharge equivalent time-fre-quency spectrograms

most points move towards arch top with the developing of pre-breakdown processes

wherepartial discharge equivalent time is 1 300

1

600 ns and partial discharge equivalent frequency is 7.5

1

0 MHz. Par-tial discharge equivalent time-frequency spectrogram may be half-arched type or whole-arched type at the end of pre-breakdown processes.