High field electrical and thermal aging of polypropylene films

The lifetimes of polypropylene films under high AC and DC field stresses were determined at 23 degrees C, 70 degrees C and 90 degrees C. For DC aging, the results indicate that a power law type of relation might hold true, with the parameters changing with temperature. However, for AC aging, more anomalous results were obtained. At least three regions in the voltage-life curve were identified: a region where the stresses applied were very close to the breakdown field of the films, an intermediate region and a region where the films were stressed at comparatively low electric fields. It is speculated that these regions arise because of different mechanisms that are operative at different electric stresses and temperatures, as well as the progressive impregnation of the dielectric films with the transformer oil. The results are described and discussed.<>