Partial discharge inception characteristics of LN2/epoxy composite insulation system under thermal bubble condition

In this paper, we investigated electric field strength (PDIE) at partial discharge (PD) inception in liquid nitrogen (LN2)/epoxy composite insulation system with thermal bubbles at atmospheric and pressurized conditions. We measured PDIE and PD charge, and simultaneously observed the bubble diameter, number and their behavior using a high-speed video system of 200 frames/s. PDIE characteristics were analyzed using Weibull statistics. The pressure dependence of bubble diameter, PD charge and scale parameter of PDIE was systematized for different thermal bubble conditions. Experimental results revealed that (1) PD was initiated in thermal bubbles in LN₂, (2) the scale parameter of PDIE decreased with the increase of V_rate defined as the volume ratio of thermal bubbles to the gap space, (3) PDIE was found to mutually depend on not only V_rate but also gas density in the thermal bubbles