Role of Temperature in Partial Discharge Inception Voltage at Triple Junctions

In numerous industrial and electrical applications, within the zone near the junction between a metal conductor, a solid insulator, and the surrounding gas, here called a triple junction, the local electric field created by the electric potential of the metal conductor may be enhanced due to the difference in the relative permittivity of the solid insulator and surrounding gas. This local field enhancement may facilitate the formation of partial electrical discharges. Partial discharge (PD) activity can cause low-level damage as well as an accelerated aging, which weakens the insulation system and then leads in the long term to complete breakdown. Therefore, in certain applications, such as in the aeronautical or nuclear sector, differences in temperature and gas pressure from the ambient case should be considered because of their influence on the PD inception voltage (PDIV). The present work demonstrates a significant reduction in the PDIV in the case of a triple junction with alumina as the solid insulator when the system is heated to temperatures in the range of 300 °C–400 °C. It is proposed that this effect is due to the increased permittivity of the alumina ceramic insulator at these temperatures, which leads to an increased enhancement of the local electric field at the triple junction. The proposed mechanism for local field enhancement is discussed considering, among others, permittivity measurements at elevated temperature.