Impulse-driven surface flashover of polymeric dielectric materials immersed in insulating oil

Surface flashover of dielectric components chosen to insulate high-voltage, pulsed-power machines is a limiting factor in the attainment of stable operation of such machines, and selection procedures for materials to be used under conditions with impulse rise-times in the nanosecond regime and associated high rates-of-change of electrical field are not well defined. The focus of the present work was therefore an experimental comparison of the flashover performance of four different polymeric dielectric materials, immersed in insulating oil, when subjected to fast-rising (~100 ns) impulse voltages. The time to breakdown/breakdown voltage of samples of: polypropylene; low-density polyethylene; ultra-high molecular weight polyethylene; and Rexolite; was analysed in both non-uniform and uniform fields. Low-density polyethylene showed the most consistent point of breakdown in non-uniform fields, for example with 94% of the data points located in the range between 138-204 ns and 282-332 kV for one set of test conditions. Ultra-high molecular weight polyethylene exhibited longer delay times to breakdown than the other materials. Measurements in uniform fields were restricted by the occurrence of bulk breakdown events. The results will provide data for high-voltage system designers for the appropriate choice of dielectric materials to insulate machines.