Vacuum flashover characteristics of solid CL-polystyrene

Insulator vacuum surface flashover continues to be a limiting factor in high voltage, high-energy vacuum-to-oil interfaces. CL-polystyrene is being used in several applications due to its better flashover voltage hold-off. Insulator stacks, built following J.C. Martin´s criteria, require that the material constant be known as well as the proper time scaling. The authors have performed a series of vacuum flashover experiments with a sufficient number of CL-polystyrene samples to determine the material constant required for the JCM 45° empirical equation. It appears that most low voltage flashover events (due to possible conditioning) occur late in time (t_eff @ 63% or later), and most non-recoverable flashover events occur during the rise time (t_eff @ 89% or earlier). Testing was performed with a Marx generator producing up to 400 kV peak voltage with a rise-time of 80 to 90 ns and a typical RC decay of 3 μsec. Unrecoverable sample flashover was determined by three flashover events in a row. Scaling the JCM constant with t_eff @ 89%, and considering only the last three flashover events per sample, an average value of 235 is obtained for polystyrene. When all the flashover events are considered in the scaling, an average constant value of 210 is then obtained. They have also performed testing on a similar number of samples of Perspex and Acrylic. Experimentally determined Weibull distributions are presented for the three materials.