Accelerated aging of high voltage encapsulated transformers for electronics applications

This paper presents the result of accelerated aging tests under AC, DC and DC-AC superimposed voltages on high voltage encapsulated transformers of multi-layer insulation system. The aim of this work is to study the aging behavior of insulation system and find a mathematical model to estimate the lifetime of the insulation exposed to AC and DC voltages. The statistical distribution of times-to-breakdown were presented by Weibull probability distribution, and related to the test aging voltage by the inverse power law model. The arbitrary constants of the combined Weibull-inverse power law model were evaluated using maximum likelihood estimation. It was found in this study that aging of the insulation system at AC voltage is mainly caused by dielectric losses and partial discharges in the voids in the bulk of the insulation, whereas at DC voltage insulation failure is attributed to thermal breakdown. It was also found that the presence of small AC voltage component superimposed on DC high voltage accelerates the aging process very significantly and causes insulation failure in very short time