Breakdown behavior of oil-impregnated polypropylene as dielectric in film capacitors

The dielectric breakdown behavior of thin metalized PP films for use in HV capacitors was studied as function of temperature with and without rape-seed oil as impregnation medium. The impregnating oil penetrates into the amorphous regions of the dielectric. With oil impregnation, the dielectric breakdown strength of the PP film is increased by >25%. A correlation between the breakdown strength and the degree of impregnation was found. With increasing temperature the breakdown strength of PP films decreases. Moreover, the measurements showed abrupt changes of slope in the breakdown strength at defined temperatures. From these results a correlation was established between the dielectric breakdown strength of PP films and temperature induced structure changes. It was shown that the abrupt changes of slope in the breakdown strength occurring at defined temperatures are due to the additives contained in the PP capacitor films used. Finally, guidelines for further development of impregnated PP films as dielectrics for high power capacitor applications are presented.