Life prediction of a full-scale transformer winding insulation through statistical analysis of AC voltage endurance test data

It is now acknowledged that for many solid dielectrics below a threshold electrical stress, electrical aging can either be neglected or considered to progress very slowly. Hence, it is recommended to exclude from life prediction data obtained at high electrical fields. In general, it is common to be faced with an insufficient number of life data obtained at low fields. Therefore, we first introduce the Inverse Power - Weibull model, which can estimate statistically the confidence bounds on the lifetime at a given reliability at operating voltage. Then, as the main contribution of this paper, the existing models pertaining to the size enlargement effect are extended to a non-uniform stress situation, and two methods are developed to calculate the confidence bounds of the lifetime of a full-scale high voltage component. Finally, the Inverse Power - Weibull model and the calculation method we derived are applied to the life data collected from transformer winding insulation specimens. The voltage versus lifetime characteristics of the epoxy resin insulation of the windings comply with earlier research. After completion of the statistical analysis, a warranty life is suggested for the winding of a full-size transformer, with a high reliability and confidence level.