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

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