Flashover of Smooth and Knurled Dielectric Surfaces in Dry Air

In pulsed power engineering, solid spacers are used to insulate high-voltage parts from extraneous metal parts, providing electrical insulation as well as mechanical support. The breakdown/flashover voltage, at which a discharge process initiates across the solid/air interface, is important in the design process, as it informs designers of specific threshold ‘failure’ voltages of the insulation system. In this article, a method to potentially increase the failure voltage, tested under multiple environmental conditions, without increasing the length of the solid spacer, was investigated. Three dielectric materials: high-density polyethylene (HDPE), Ultem (polyetherimide), and Delrin (polyoxymethylene), were tested under a 100/700 ns impulse voltage. Cylindrical spacers made of these materials were located in the center of a plane-parallel electrode arrangement in the air, which provided a quasi-uniform electric field distribution. Breakdown tests were performed in a sealed container at air pressures of −0.5, 0, and 0.5 bar gauge, with a relative humidity (RH) level of < 10%. The materials were tested under both, negative and positive polarity impulses. The surfaces of a set of solid spacers were subjected to a “knurled” finish, where
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0.5 mm indentations are added to the surface of the materials, prior to testing, to allow comparison with the breakdown voltages for samples with “smooth” (machined) surface finishes. The results show that the flashover voltage can be increased by the addition of a spacer with a knurled surface, by up to 60 kV under certain conditions, in comparison to a “smooth” (machined) surface finish.