Effects of Moisture on Partial Discharge and Breakdown Characteristics of Silicone Oils under AC Stress

This paper presents an experimental study on effects of moisture on partial discharge (PD) and breakdown characteristics of two silicone oils under AC stress. In this study, a needle-to-plane electrode configuration was employed to perform PD and breakdown tests in the silicone oils under three relative humidity (RH) levels, i.e. 5%, 30% and 90%. The experimental results revealed that as the RH is increased from 5% to 30%, the PDIV, PD magnitude and repetition rate and AC breakdown voltage do not change significantly. However, when the RH is further increased from 30% to 90%, it was found that both PD and breakdown characteristics are moisture dependent. The high RH results in a dramatic drop in the PD repetition rate. Compared to low RH oils, the 90% RH oil exhibits lower PD magnitudes and hence leads to a higher PD inception voltage; it, however, has higher PD magnitudes when applied voltage is increased, and as a consequence would have a lower breakdown voltage.