Effects of Hydrothermal Aging on the Breakdown Voltage of Polyesterimide

Abstract

Polymers have always played a big role in the insulation of electrical machines and have been the focus of several studies when they are subjected to different stresses. The humidity and temperature are the main constraints that degrade the insulating performance of the materials when they act together causing hydrothermal aging of the polymers. This study deals with the influence of hydrothermal aging on the breakdown voltage of polyesterimide which is known as one of the most important factors which define the electrical insulation performance of the polymer. The breakdown test was executed under AC and DC voltage. A statistical analysis of breakdown data was carried out using the two-parameter Weibull distribution. According to the findings, the breakdown voltage changes with aging time. Its rise is due to a polymer crosslinking by thermal action, causing a reduction of the mean-free path of charge carriers decreasing their mobility. The decline is attributed to the material plasticization after water penetration into the polymer matrix at the beginning of aging, which is a physical effect. Eventually, the chemical effect reflects the hydrolysis reaction, which destroys the hydrogenous links of the polymer. The breakdown voltage under the DC ramp is higher than that obtained under the AC ramp. The DC breakdown voltage depends on the polarity of the applied electrical field. The polymer degradation is characterized by a change in color and the crumbling of samples.