Experiences in testing of AC and DC bushings

Bushings are very important parts of the electrical power supply. They are used as transformer bushings to allow the electrical connection from the transformer winding to external equipment like busbar or overhead lines or as wall bushings for bringing “high voltage” through a wall. The design of bushings usually consists of an insulating body, for instance oil impregnated paper with concentrically inserted layers for electrical field grading or a gas chamber, a housing, usually porcelain or polymeric, and the flange. For testing of bushings it is necessary to simulate service conditions. This means oil vessels for transformer bushings and a special structure for wall bushings to simulate installation conditions are necessary. The performed electrical tests should verify that the bushing withstand the electrical stress in service. Challenges for testing bushings in the UHV range are the big dimensions of the test objects, avoiding any kind of external influences to the measuring results and the lack of relevant standards for the UHV range. This paper summarizes the experiences in testing AC and DC bushings up to 800 kV system voltages. Problems during the preparation process or during the tests which can occur and there possible solution will be shown.