Polarity effect of PD in oil-pressboard insulation under highly uneven electric field

Abstract

The polarity effect of the partial discharge (PD) in oil-pressboard (oil-PB) insulation under a highly non-uniform electric field is remarkable. For intensive analysis of the characteristics and mechanisms of this polarity effect, a non-uniform electric field was simulated by a needle-plate model with oil-PB composite insulation. The PD statistical features, PD pattern and PD pulse were studied via pulse current method, the charge distribution was simulated on the basis of hydrodynamic drift-diffusion model and bipolar charge transport model, and the cause of the polarity effect was analysed based on the PD mechanism which is quantificationally explained via a PD circuit model. It was found that the average apparent discharge amplitude and PD repetition rate of the neg-PD are higher; the average equivalent frequency and equivalent time of neg-PD are higher and lower respectively; the pos-PD pulse is unimodal and the neg-PD pulse is oscillating with some secondary discharges at AC voltage or a back discharge at DC voltage; and the neg-PD pulse has a narrower width owning to the fast rising edge and falling edge. The PD current pulse in the coupling loop is essentially caused by the charging process of the streamer-shorted defect. The cause of the polarity effect is that the positive streamer evolves slowly and can puncture the entire oil gap with an energy-concentrated channel, whereas the negative streamer with a diffused channel evolves faster but cannot puncture the entire oil gap.