AC breakdown strength of aromatic polymers under partial discharge reducing conditions

Partial discharge occurring at HV in AC electric breakdown field measurements on insulating polymeric films strongly reduces the breakdown field strength of the material and conceals the intrinsic breakdown process. By performing AC electric breakdown field measurements on polymer films in phthalic diesters, it was possible to suppress partial discharge and obtain breakdown field data of more intrinsic character. This is valuable for the evaluation of the influence of the structure and the properties of polymers on their insulating behavior. The AC electric breakdown field strength of five aromatic polymers at different film thicknesses was measured in dibutyl phthalate. The results from these measurements showed about a twofold increase in the breakdown field strength at sample thicknesses >50 μm, compared to data obtained in transformer oil. The breakdown field strength for thick samples showed a linear decrease with increasing sample thickness for all polymers except PEEK, which showed a constant breakdown field strength in the thickness range investigated. The breakdown field strength of thin samples was independent of the polymer structure. Measurements of the breakdown field strength of the polymers under dc conditions show no obvious correlation between chemical structure or polymer property and the breakdown field strength