Investigation of Thickness Effect on Electric Breakdown Strength of Polymers Under Nanosecond Pulses

The thickness effect on electric breakdown strength (E_BD) of four kinds of polymers under nanosecond pulses is investigated. The polymers are polyethylene, PTFE, PMMA, and nylon. The test samples are 0.5-3.5 mm in thickness (d) and are immersed in transformer oil. The nanosecond pulse is based on a Tesla-type generator, TPG200, which is with values of pulsewidth of 8.5 ns and rise time of 1.5 ns. The experimental results show that E_BD is 1-2 MV/cm and decreases as d increases. The dependence of E_BD on d is analyzed with the Weibull statistical distribution. It is concluded that logE_BD versus log d is linear. By replotting the experimental data and by comparing with Martin´s results, it is found that the slope for the linear dependence is about -1/8. With this conclusion, the breakdown probability is researched. It is shown that, to get a breakdown probability as low as 0.5%, the applied field should be decreased to about half of E_BD.