The Effect of Relative Humidity on the Flashover Strength of Solid Insulation

Abstract

This paper informs on the flashover strength of 3 materials: Delrin (Polyoxymethylene), HDPE (High Density Polyethylene) and Ultem (Polyetherimide) with ‘smooth’ and ‘knurled’ surface finishes, in zero-grade air at −0.5, 0 and 0.5 bar gauge, and at 90% relative humidity (RH). All tests were completed using a 10-stage Marx generator, producing HV impulses with a nominal 100/700 ns waveshape. Each test conformed with the ASTM D3426-97 standard of ‘step up’ testing, to find the average flashover voltage for each set of conditions. The electrode system with each dielectric material demonstrated a decrease in breakdown voltage as the RH was increased. In high humidity environments, the knurled surface finish was seen to have a positive effect on the flashover strength of Delrin and Ultem samples, compared to a smooth, machined surface. Increasing pressure yielded an increase in the flashover strength also. For samples with a smooth surface finish, a decrease in flashover strength was found as the permittivity of the material increased, irrespective of humidity and pressure. However, a knurled surface finish had a much more erratic effect on flashover strength, with no discernible trend with increasing material permittivity.