Modelling of HVDC wall bushing flashover due to uneven wetting

An electric field based flashover model is proposed to predict the flashover voltage of an unevenly wetted HVDC wall bushing. Criteria for initiation and propagation of discharges have been incorporated in the model. The critical flashover voltage of a 600 kV wall bushing has been predicted and it is shown that the flashover stresses are very close to experimentally derived values, i.e. 60 - 70 kV/cm. Important aspects of wall bushing flashover such as critical dry zone length, the U-shaped characteristic of critical flashover voltage versus dry zone length, the effect of waterdrops, influence of ram conductivity and altitude are well reproduced and explained by the model.