Prediction of residual breakdown electrical field strength of epoxy-mica paper insulation systems for the stator winding of large generators

A method of predicting the residual breakdown electrical field strength of an epoxy-mica paper insulation system is described. This method was developed by using data on service-aged stator bars from generators as well as accelerated aging model bars. Thermal aging, mechanical aging, and combined thermal and mechanical aging were done individually as accelerated aging. The partial discharge characteristics, dissipation factor, and breakdown voltage were measured on the model bars and service-aged bars from generators. From these results, the residual breakdown electrical field strength was predicted based on the partial discharge characteristics and dissipation factor. First, the dominant type of deterioration (thermal or mechanical aging) on a bar´s insulation was determined by the pattern of voltage-partial discharge magnitude characteristics. Then the residual breakdown electrical field strength was predicted by using a predicting equation for both thermal and mechanical aging. Each predicting equation uses dissipation factor tip-up (i.e. difference in dissipation factor values between 2 kV and rated voltage) and partial discharge inception electrical field strength. The accuracy of this method was confirmed using data measured on 112 service-aged stator bars from generators and 10 accelerated aged model bars. The 95% confidence interval level included 94% of the measured residual breakdown electrical field strength values. This result shows that the method enables precise predictions of the residual breakdown electrical field strength of an epoxy-mica paper insulation system.