Lifetimes of polypropylene films under combined high electric fields and thermal stresses

The lifetimes of polypropylene films under high AC and DC field stresses were determined at 23, 70, and 90°C. A two-parameter Weibull distribution was used in the evaluation of the data, with the shape and scale parameters estimated using graphical and maximum-likelihood methods. For DC aging, the results indicate that a power-law type of relation may hold true, with the parameters changing with temperature. However, for AC aging more anomalous results were obtained. These anomalies were more noticeable in the intermediate voltage region where the 23°C lifecurve fell below the lifecurves at the other two temperatures. This is possibly due to the partial discharges which are mainly responsible for the breakdown in this range of voltage and the progressive impregnation of the dielectric films with transformer oil