Estimation for a life model of transformer insulation under combined electrical and thermal stress

A life model for power transformer insulation is proposed. It is the Arrhenius model with a weighting factor for the enhanced reaction under combined thermal/electrical stress. Laboratory tests are used to evaluate the model´s parameters. The thermal and thermal/electrical stress aging data on Fish paper are presented and analyzed. The estimates from the life model are close to the experimental results. It is hoped that the life model is extendable to other engineering materials. The model´s parameters can be evaluated from two laboratory tests, thermal aging and combined thermal/electrical aging. For the Fish paper, when thermal stress is increased by 30°C, the life is reduced by a factor of 2; at 70°C life was 238 hours and at 100°C life was 120 hours. The material life was further reduced by a factor of nearly 2 when subjected to thermal/electrical stress: at 80°C life was 200 hours, whereas at 80°C and 600 Vac life was 118 hours. Electrical stress is important in accelerating the material degeneration