Surface flashover of oil-immersed polymers with varying applied electrical field

The introduction of solids into liquid insulation located between two conductors is often necessary to provide mechanical support in sub-systems of high-voltage, pulsed-power machines. Flashover on or around the surface of the solid can result in permanent damage to the insulation system, resulting in reduced operational voltages and system downtime as the damaged component is located and replaced. Knowledge of the electrical fields that can be repeatedly applied to different liquid-immersed solids without resulting in surface flashover is therefore important for the appropriate design of insulating components for pulsed-power systems. Described in the present paper are the results of experiments at applied electrical field levels that did not result in breakdown events for every applied impulse. Cylindrical samples of polypropylene, low-density polyethylene, ultra-high molecular weight polyethylene, Rexolite, and Torlon were immersed in insulating oil and subjected to impulse voltages in uniform fields. Twenty impulses were applied to the insulation system at peak applied field levels 200 kV/cm, 400 kV/cm, 700 kV/cm, and 1 MV/cm. It is shown that average streamer propagation velocities are comparable to those in open oil gaps under point plane geometry, and that the velocities increase with increasing applied field, approaching 30 km/s at 1 MV/cm. Breakdown events are found to be initiated at lower applied fields for materials of higher relative permittivity.