The design of dielectric barriers for HVDC bushings

High voltage bushings employ capacitive grading foils to control the electric field distribution under AC conditions. However, under DC conditions, the foils are unable to prevent a concentration of electric field within the bushing due to its high resistivity relative to that of the surrounding oil. A method of controlling the electric field is therefore required for DC conditions, and this is usually achieved through the use of dielectric barriers which surround the oil-immersed end of the bushing. This paper describes analytical techniques used to assess the performance of a particular barrier configuration in order to optimise its design.