Mechanism of HVDC wall bushing flashover in nonuniform rain

A model which assumes a dry zone of changing length is used to determine conditions which may lead to flashover of HVDC wall bushings in nonuniform rain. The key parameter in the proposed mechanism is the ratio of the resistance per unit length of the wet region to the resistance per unit length of the drier region. Measurements of the surface resistivity are presented for horizontal insulators, including wall bushings, with different surface conditions subjected to rain. Such measurements can be utilized to evaluate the potential capability of various materials to prevent wall bushing surface flashover in nonuniform rain