Experimental Study on Creeping Flashover Characteristics Along Dielectric Surfaces in Oil-Filled Transformer Using Electro-Optic Coupling Method

Abstract

Winding deformations and insulation dampness are the two main causes for the creeping flashover along the dielectric surfaces of oil-immersed power transformers, which could lead to main insulation failures and explosions. In this paper, these two types of insulation defects are modelled by evaluating creeping discharge tests due to moisture dripping and local electric field enhancement in a transformer protype. The completed creeping process along the dielectric interface, from partial discharge initiation to eventual flashover, was observed using an SLR camera and electro-optical coupled sensors. The results indicate that the evolution of the two types of discharges can be divided into four stages, that is, initial weak discharge at the tip of the high voltage side, appearance of carbonised traces at the tip, strong arc discharge in the gap between the high and low voltage sides and eventual flashover. Creeping discharges caused by insulation dampness grow faster than that caused by winding deformations, in terms of discharge intensity and carbonised pressboard areas. Therefore, the more serious surface discharge phenomenon under the condition of insulation and humidity in this paper is mainly caused by the more obvious space charge accumulation brought by low-density areas such as bubbles.