New Voltage-Endurance Curves for Combined Thermal and Electrical Aging of Coil Insulation

The paper describes a method for estimating the life of high-voltage coil insulation under multiple stress conditions. Thermal and electrical stresses are considered. The final aim is to predict the life of insulation under normal voltage at normal temperature. The lifetime of the insulation is defined as the time until the residual ac short-time breakdown voltage decreases to the withstand test voltage, 2E+3 kV, where E is the rated voltage. We propose a new V-t curve, in which the lives under various multiple stress conditions can be interpolated by a graphical method. For example, from a series of accelerated aging tests on class B, 11 kV mica-epoxy coil insulation, new V-t curves for various temperatures have been derived. The life of the insulation under normal voltage (6.4 kV) at normal temperature (130 ° C) was estimated to be about 49 years. However, for real machine windings we must consider the effect of additional stresses, mechanical and environmental, and therefore, the revised lifetime of the insulation was assumed to be about 19 years. It was concluded that detailed examinations or insulation diagnosis should be made on machine insulation in service for over 15 years.