Abstract :
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.
