The role of trapped space charges in the electrical aging of insulating materials

Abstract

An investigation of the effect of trapped space charges on the aging of polymeric insulating materials subjected to thermo-electrical stress is reported in this paper. Possible scenarios of degradation mechanisms, thermally activated, but accelerated by the presence of space charges, are examined. The model which derives from this approach has some interesting features: it is characterized by electrical and thermal thresholds, its parameters have a physical background, it can be cast into a probabilistic framework. Acceleration of aging due to space charges is attributed to a reduction of the free-energy barrier to degradation, seen as a local partially-reversible reaction, which is caused by energy stored in space-charge centers. The validity of the model is limited to dc voltage, and to the time of formation of microcavity-crazes, rather than to breakdown times, since other mechanisms will occur under electrical field once large enough cavities are formed in the insulation. The model is applied to the results of thermo-electrical life tests performed on PET, showing very good fitting, as well as interesting relationships between parameter estimates and insulation morphology. It is shown that the model can also fit well to ac life data, where it takes on a phenomenological meaning.