A transformer bushing is an insulating structure that facilitates the passage of an energized, current-carrying conductor through the grounded tank of the transformer. The conductor may be built in to the bushing, i.e., a bottom-connected bushing, or the bushing may be built with the provision for a separate conductor to be drawn through its centre, a.k.a., a draw-lead or draw-rod bushing.
The two principal types of bushing construction are solid or bulk type and capacitance-graded (sometimes called condenser type). The bushings used for the low voltage winding(s) of a transformer are often solid type with a porcelain or epoxy insulator. Capacitance-graded bushings, designed for higher voltage ratings, are used for a transformer’s high voltage winding.
Unlike a solid type construction, in a capacitance-graded transformer bushing, conducting layers are inserted at predetermined radial intervals within the insulation that separates the centre conductor from the insulator (housing) of the bushing. These multiple conductive inserts create capacitive elements linking the centre conductor of the bushing to ground. Their purpose is to control the voltage field around the center conductor so that the voltage distributes more uniformly across the surrounding insulation system in the bushing.
In solid type bushings, electrical grade mineral oil is often used between the conductor and the insulator, which may be contained within the bushing or shared with the transformer. Typical insulation used in a capacitance-graded bushing is oil-impregnated paper (OIP), resin-impregnated paper (RIP), and resin bonded paper (RBP). Capacitance-graded bushings also use mineral oil, usually contained within the bushing.
Transformer bushing failures are often credited as one of the top causes of transformer failures so the condition of the bushings is of high interest to transformer asset owners. Typical bushing failure modes include moisture ingress, electrical flashover, lightning strike, short-circuited capacitance-graded layer(s), bushing misapplication, corrosive sulphur, broken connection between ground sleeve and flange, and a broken tap connection. The following electrical field tests provide information about the integrity of the bushings.