General Equation of the Decline in the Electric Strength for Combined Thermal and Electrical Stresses

In this paper the equation for the decrease in the electric strength with time for combined thermal and electrical stresses is obtained, using the following assumptions: (a) the theory of total aging treated as a cumulative quantity, (b) the electric strength is a basic property for aging evaluation of an insulation, and (c) the life model, already proposed by the author for combined stresses is valid. The model is based on the inverse-power law for electrical aging, the Arrhenius relationship for thermal aging, and a linear dependence of the voltage endurance coefficient on thermal stress. The equation thus obtained represents a synthesis of all the main relationships used in electrical, thermal, and combined-stress endurance, and, in addition, includes a new model, that of the electric strength variation due to thermal stress. The comparison with the experimental curves shows a good agreement with the theory for both thermal and electrical stresses.