AC BREAKDOWN STRENGTH OF AROMATIC POLYMERS UNDER PARTIAL DISCHARGE REDUCING CONDITIONS

Partial discharge occurring at HV in ac electric breakdown field measu rements on insulating polymeric films strongly reduces the breakdown f ield strength of the material and conceals the intrinsic breakdown pro cess. By performing ac electric breakdown field measurements on polyme r films in phthalic diesters, it was possible to suppress partial disc harge and obtain breakdown field data of more intrinsic character. Thi s is valuable for the evaluation of the influence of the structure and the properties of polymers on their insulating behavior. The ac elect ric breakdown field strength of five aromatic polymers at different fi lm thicknesses was measured in dibutyl phthalate. The results from the se measurements showed about a twofold increase in the breakdown field strength at sample thicknesses > 50 mum, compared to data obtained in transformer oil. The breakdown field strength for thick samples showe d a linear decrease with increasing sample thickness for all polymers except PEEK, which showed a constant breakdown field strength in the t hickness range investigated. The breakdown field strength of thin samp les was independent of the polymer structure. Measurements of the brea kdown field strength of the polymers under dc conditions show no obvio us correlation between chemical structure or polymer property and the breakdown field strength.