On electric stresses at wedge-shaped oil gaps in power transformers with application to surface discharge and breakdown

The problem of a high-voltage applied to a wedge-shaped oil gap in power transformers is considered as far as induced distortions and tangential electric stresses are concerned. Near-tip distortions and electric stress concentration occur due to the essentially difference in the relative permittivity between the insulating oil and the pressboard. When the magnitude of tangential electric stress intensity reaches a critical threshold value, surface discharge and breakdown occurs at the oil/pressboard interface. In this work, a simplified computation model is developed for the quantitative analysis of the distribution of electric stress and the breakdown electric field strength in the wedge-shaped oil gap in power transformers. The detailed computation results on the effects of pressboard permittivity and thickness on the surface discharge inception voltage are presented. It shows that the surface discharge inception voltage is a function of pressboard permittivity and thickness; and the value of surface discharge inception voltage decreases with increasing dielectric permittivity.