Discharges at low pressures in dielectric voids

When voids are fully enclosed in insulation, the inception of partial discharges is affected by the walls of voids. We have measured the discharge inception at different pressures below atmospheric pressure and shown the effect of pressure for different d/t′ (void diameter/void depth) which is used as a parameter of void dimensions. When d/t′ increases, there is a limiting case beyond which the discharge inception voltage Vi is similar to open voids. The rate of increase of Vi depends on the void diameter. At relatively low pressures the discharge inception is not much affected by void dimensions and this can be attributed to the fact that in this range of pt′ (pressure × void depth), the breakdown voltage of gas changes slowly. The observations are useful in understanding discharges in the insulation system of cables, capacitors, bushings or epoxy cast equipment which are dried and impregnated at low pressures containing gas pockets at sub-atmospheric pressures. The studies are also important for high altitude insulation design and spacecraft insulation.