Surface discharges along polymeric insulating materials

A 10-stage Marx generator has been used to apply voltages of up to 400 kV, with rise times in the region of 150 ns, to samples of polymeric insulating materials held between two electrodes immersed in transformer oil. The Marx is triggered by depressurisation of a spark column filled with dry air. Electrical fields of up to 500 kV/cm are applied across the electrodes, causing surface flashover of the sample between the electrodes. The materials investigated are polypropylene, low-density polyethylene, ultra-high molecular weight polyethylene, Rexolite (cross-linked polystyrene) and Torlon (polyamide-imide). Different sample topologies are tested in two different point-plane, and plane-plane, electrode configurations. Results showing how the breakdown (flashover) voltage varies with an increasing number of discharges are presented and discussed. The delay times to breakdown and their significance are also discussed.