Partial Discharge Characteristics in Pre-Breakdown Processes of Oil-Pressboard Insulation with Needle-Plate Defect Under AC-DC Combined Voltage

The PD characteristics of oil-pressboard insulation with needle-plate defect at AC-DC combined voltage are unstable. In this paper, the time-variation trends of PD characteristics in pre-breakdown process at AC-DC combined voltage are studied via the tests in oil-pressboard insulation with needle-plate defect. The average PD repetition rate and average PD amplitude decrease with the PD developing, and the time-variation trends of them can be divided into fluctuation type and monotone type. The average PD amplitude has no obvious polarity effect, the average PD repetition rate has a polarity effect. In equivalent time-frequency spectrograms, most points move towards arch top with the developing of pre-breakdown processes, where PD equivalent time is 1300–1600 ns and PD equivalent frequency is 7.5-10 MHz. PD equivalent time-frequency spectrograms can be divided into half-arched type and whole-arched type at the end of pre-breakdown processes. The defect in pressboard caused by PD is a possible reason for the appearance of whole-arched time-frequency spectrogram.