Bushing (electrical)

A bushing is an electrical engineering component that insulates a high voltage conductor passing through a metal enclosure. Bushings appear on switchgear, transformers, circuit breakers and other high voltage equipment.

Description

A disassembled bushing. The copper conductor passes up the hollow centre of the bushing.

The bushing is a hollow insulator, allowing a conductor to pass along its centre and connect at both ends to other equipment. Bushings are often made of wet-process fired porcelain, and may be coated with a semi-conducting glaze to assist in equalizing the electrical stress along the length of the bushing.

The inside of the bushing may contain paper insulation and the bushing is often filled with oil to provide additional insulation. Bushings for medium-voltage and low-voltage apparatus may be made of resins reinforced with paper. The use of polymer bushings for high voltage applications is becoming more common. The largest high-voltage bushings made are usually associated with high-voltage direct-current converters.

Capacitor types

Some of the higher voltage types (layers of paper, film and aluminum foil used as insulating medium) are called capacitor bushings because they form a low value capacitor between the conductor and the wall. This is done to reduce the electrical field stress that would otherwise occur and cause breakdown.

Bushing failure

Bushings sometimes fail due to partial discharge degradation in the insulation. There is at present great interest in the electricity supply industry in monitoring the condition of high voltage bushings.

See also

In mechanical engineering, a bushing or plain bearing refers to a part which allows a rotating component to pass through a stationary one, the bushing carrying the load.

References

• Central Electricity Generating Board (1982). Modern Power Station Practice. Pergamon. ISBN 0-08-016436-6.