The electric field modelling of a high voltage bushing with contaminants on the lower porcelain surfaces

In industry, there have been several documented failures on the lower (oil side) region of high voltage power transformer bushings. In these cases, the failure mode was considered to be due to localised discharges, as a result of accumulated surface contaminants. Given failure is often catastrophic, it is therefore difficult to establish the exact point of failure. In this paper, a numerical model that simulates the electric field and equipotential distributions for an oil impregnated paper bushing with contaminants on the lower porcelain surface is presented. The associated tangential electric field stresses are established through simulation for four likely locations of contaminants. The contaminants dependence on dielectric properties, location and thickness is modelled and discussed. The results demonstrate bushings are highly sensitive to contaminants on the lower porcelain.