Investigation of aging rate in polymer films subjected to surface discharges under distorted voltage

This paper presents the first results of a research carried out with the purpose of investigating life performance and degradation mechanisms of electrical insulating materials subjected to distorted ac voltage. Accelerated life tests were performed until breakdown, measuring partial discharge evolution with time, by a test arrangement suitable to induce surface discharges on insulating material specimens. Harmonic voltages having different frequencies and phase, in the range of those usually recorded in electrical plants, were superimposed to the fundamental component. The tested material is polyethylene terephthalate. Either reduction or increase of life, with respect to sinusoidal conditions, were detected, dependently on the way as harmonic voltages are summed up. In particular, significant life shortening was obtained in the cases when voltage maximum is increased due to the contribution of harmonics. These result can be directly correlated with the partial-discharge pattern structure.