Effect of surface deviation of solid insulation on impulsive flashover voltages under varying environmental conditions

In pulsed power engineering, solid spacers are used to insulate high voltage parts from extraneous metal parts. The applied voltage at which a discharge process initiates is important in the design process. In this paper, a method to potentially increase the failure voltage under multiple environmental conditions, without increasing the length of the solid spacer, was investigated. Three insulating materials: Delrin (Polyoxymethylene), Ultem (Polyetherimide) and HDPE (High Density Polyethylene), were tested under a 100/700 ns impulse voltage. Cylindrical spacers made of these materials were located in the center of a parallel-plane electrode setup in air, which provided a quasi-uniform field distribution. Breakdown tests published in this paper were performed in a sealed container at air pressures of −0.5, 0 and 0.5 bar gauge, with a constant relative humidity level of <10% RH. 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 slight 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. For negative polarity impulses, the flashover voltage for smooth machined surfaces was generally higher than for knurled finishes. Under positive polarity impulses, the opposite effect was found to occur, where the flashover voltage of the samples with knurled surfaces was higher for all tests. The results will give designers, particularly within the pulsed power industry, information on flashover voltages of materials under an array of environmental conditions.