Temperature-dependent dielectric breakdown strength of oil impregnated polypropylene foils

The influence of impregnating rapeseed oil on the dielectric breakdown behaviour of thin metallized polypropylene (PP) foils used for high power capacitors was investigated. With oil impregnation the dielectric breakdown strength of the PP foil increases strongly. The breakdown behaviour of rapeseed oil impregnated PP foils was compared to silicon oil and pentaerythrit-tetrapelargonate oil impregnated PP foils. Based on these experiments a guideline for the selection of impregnating oil for high power capacitors can be given. The breakdown strength of PP capacitor foils decreases with increasing temperature. In addition to this behaviour, breakdown strength measurements performed as a function of temperature showed discontinuities for dry and impregnated PP foils at defined temperatures. DSC (Differential Scanning Calorimetry) measurements reveal structure modifications in the same temperature regions as the breakdown discontinuities occur. Investigations have been performed to find out if the structural phase transitions can be attributed to pure mechanical relaxations observed in heat treatment experiments.