Study of Oil/Pressboard Creeping Discharges Under Divergent AC Voltage-Part III: Effects of Temperature and Stress Type

The present article studies the effects of temperature and stress type on the oil/pressboard creeping discharge under divergent ac voltages. A ramp-stress test determines the short-time electric strengths of the oil/pressboard interface. Subsequently, long-term tests are carried out at different temperatures using constant and step stresses, respectively. Afterward, a novel methodology is proposed to detect the developing fault using synchronous partial discharge (PD) and online dielectric measurements. The interface short-time electric strengths correlate positively with the temperature. In long-term tests, the creeping discharges can generate three failure modes to oil/pressboard insulation: interface flashover (FO), surface tracking (ST), and internal treeing (IT). Different failure modes vary significantly in the discharge characteristics and the severities of damage to the pressboard. Temperature plays an important role in determining the failure modes of creeping discharge, and its influence mechanism is expounded in terms of the change of material properties, the interface of electric and dielectric phenomena, and their sophisticated interplays. The influence of stress type is related to the cumulative effect of stressing and the voltage rising steepness. The proposed methodology proves to be a potent tool in detecting the developing IT.