Characteristics of Creeping Discharge Caused by a Needle Electrode in Oil-Pressboard Insulation under +DC Voltage

Abstract

This paper presents an analysis of the creeping discharge characteristics caused by a needle electrode in oil-pressboard insulation under a positive DC voltage. The discharge behavior under a strongly (model I) or weakly (model II) vertical component of the electric field is studied using partial discharge (PD) detections, camera observations, and charge distribution simulations. It is found that the development process of the creeping discharge can be divided into three stages according to the stepwise growth trends of apparent charge amplitude and PD repetition rate. The time-resolved PD pattern and equivalent time-frequency pattern are different in the two models. Most discharge pulses are unimodal with underdamped oscillations, but bimodal pulses can occur in model II. The streamer in model I has a tail caused by the stranded charge. A secondary streamer can occur in model II following the initial streamer, causing a bimodal PD pulse. The study findings can provide suggestions for the optimization of insulation design and the promotion of converter transformer maintenance.