Electric stress grading on bushings of combined instrument transformers using high permittivity polymeric composites

Bushings are used in high voltage equipment to bring the high voltage conductors through a grounded structure. Due to the high electric field around the area next to the grounded structure it is required to keep the intensity of the field as low as possible in order to avoid surface discharges. In this paper, it is discussed how high permittivity composites prepared with doped and functionalized barium titanate powder can be used to control the electric stress on the surface of a polymeric bushing which is used in a combined instrument transformer with two central conductors, one at high voltage and the other at ground potential. First, it is described how the properties of the filler were improved and then it is shown how the prepared polymer/ceramic composite can modify the voltage profile along the surface of a prepared sample. By using three dimensional finite element simulations the performance of the composite bushing is compared with the stress grading obtained in a ceramic bushing with capacitive grading. According to the results, by using composite materials of increased permittivity, the electric stress on the surface of the bushing can be reduced to acceptable values without need of conductive foils.