FEM approach to design functionally graded transformer bushing

Porcelain bushings are universally employed for power transformers. However, non-condenser bushings are unsuitable for high voltage applications owing to their extreme non-linearity in potential and electric field distribution. A majority of the transformer failures are due to the flashovers taking place in these bushings. This paper introduces a concept of replacing one of the bushing´s insulation layer, bakelite by functionally graded material FGM, as a solution to the prevailing problem of non-linearity. A 33kV, 250A porcelain non-condenser bushing of 750mm height having Bakelite and transformer oil as the main insulating material and porcelain for the outer shed is modeled using 2-D FEM software Elecnet. The simulation studies indicate that, maximum stress is experienced radially at a height of 35.5cm to 38.5cm from the ground. It is seen that on replacing Bakelite by a 2 layer FGM (each layer of different permittivity), there is significant reduction in electric stress at the triple junction point of the grounded flange. Also, the potential and electric field distribution is found to be linear and uniform as compared to that of a single layer bushing.