Bushing (electrical)

A bushing must be designed to withstand the electrical field strength produced in the insulation, when any earthed material is present.

A typical bushing design has a conductor, usually of copper or aluminium, occasionally of other conductive material, surrounded by insulation, except for the terminal ends.

For a bushing to work successfully over many years, the insulation must remain effective both in composition and design shape and will be key factors in its survival.

Porcelain was originally used due to its properties of being impervious to moisture once sealed by fired glaze, and low manufacturing cost.

The main disadvantage with porcelain is that its small value of linear expansion has to be accommodated by using flexible seals and substantial metal fittings, both of which present manufacturing and operational problems.

Where partial discharge is required to conform to IEC 60137, paper and resin insulated conductors are used in conjunction with porcelain, for unheated indoor and outdoor applications.

The use of resin (polymer, polymeric, composite) insulated bushings for high voltage applications is common, although most high-voltage bushings are usually made of resin impregnated paper insulation around the conductor with porcelain or polymer weather sheds, for the outdoor end and occasionally for the indoor end.

Cast resin technology has dominated insulated products since the 1960s, due to its flexibility of shape and its higher dielectrical strength.

However, above 12 kV, there is a need to control the external electrical field and to even out the internal energy storage which marginalises the dielectric strength of paper insulation.

However, care must be observed in resin insulated bushings designs which use internally cast screens such that the benefit of electrical stress field control is not off set by increasing partial discharge caused by the difficulties of eliminating micro voids in the resin around the screens during the casting process.

This is sometimes due to the slow and progressive degradation of the insulation over many years of energized service; however, it may also be a rapid degeneration which destroys a good bushing in a matter of hours.