The influence of impregnating rapeseed oil on the dielectric breakdown
behaviour of thin metallized polypropylene (PP) foils used for high p
ower capacitors was investigated. With oil impregnation the dielectric
breakdown strength of the PP foil increases strongly. The breakdown b
ehaviour 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 oi
l 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 functi
on of temperature showed discontinuities for dry and impregnated PP fo
ils at defined temperatures. DSC (Differential Scanning Calorimetry) m
easurements reveal structure modifications in the same temperature reg
ions as the breakdown discontinuities occur. Investigations have been
performed to find out if the structural phase transitions can be attri
buted to pure mechanical relaxations observed in heat treatment experi
ments. (C) 1998 Elsevier Science S.A. All rights reserved.