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

The use of polymeric bushings for high voltage equipment has been increasing mainly due to some advantages over ceramic bushings. Some of these advantages are: reduced weight, better performance under polluted conditions and seismic activity and reduced hazard due to explosion. In polymeric, as well as in ceramic bushings, the electric field on the surface of the insulating housing must be controlled in order to avoid the presence of surface discharges on the vicinity of the metal flanges. In instrument transformers, one of the most used techniques to control the electric stress on the surface of the housing is by using a capacitive grading system. In this technique, conductive foils are used between paper layers to produce the capacitive stress control. In this work is discussed how high permittivity composites prepared with functionalized barium titanate powder can be used to control the electric stress on the surface of the bushing without the need of conductive foils inside the main insulation. This option is analyzed in a special design of bushing used in combined instrument transformers with two central conductors, one at high voltage and the other at ground potential, each of them with its own capacitive grading foils. By using three dimensional finite element simulations, the performance of the high permittivity composite bushing is compared with the stress grading obtained in a bushing with capacitive grading. According to the results, by using a composite material of increased permittivity, the electric stress on the surface of the bushing can be reduced to acceptable values without need of conductive foils.