A General Approach to the Endurance of Electrical Insulation under Temperature and Voltage

The paper deals with the models for estimating the life of electrical insulation under multiple stress conditions. In a first approach, simple models, the inverse-power law and the exponential, are considered for electrical aging; they are combined with the Arrhenius model to find the life equation of insulating materials under temperature and voltage. This equation is discussed and the corresponding surface studied and drawn. Since experimental results often disagree with these models in the low-stress range, an improved electrical model is proposed in which a threshold electrical gradient is introduced. This also involves the introduction of a threshold temperature, variable with electrical stress. Using some simple assumptions about the thresholds, it is possible to attain a new model for insulation life under combined stresses, perfectly symmetrical in thermal and electrical stress. The results of both voltage endurance tests on various insulating materials and multiple stress tests on a composite material agree with the proposed model and show its capability by accurately predicting the longtermerm behavior of insulation.