An insulating device called a transformer bushing makes it easier for an energised conductor to pass through the grounded tank of the transformer. A bushing may be made with a provision for a separate conductor to be drawn through its centre, sometimes known as a draw-lead or draw-rod bushing, or it may be built with the conductor already built in, in which case it is known as a bottom-connected bushing.
The two main types of bushing construction are capacitance-graded and solid or bulk type (sometimes called condenser type). The bushings used for a transformer's low voltage winding(s) are frequently of the solid kind with an insulator made of porcelain or epoxy. The high voltage winding of a transformer uses bushings that have been capacitance-graded for higher voltage ratings.
In contrast to a solid type construction, a capacitance-graded transformer bushing has conducting layers put into the insulation separating the centre conductor from the bushing's insulator (housing) at predefined radial intervals. These numerous conductive inserts produce capacitive components that connect the bushing's centre conductor to ground. To ensure a more even distribution of voltage across the surrounding insulation system in the bushing, they are designed to manage the voltage field around the centre conductor.
Between the conductor and the insulator in solid type bushings, electrical grade mineral oil is frequently utilised. This oil may be contained within the bushing or shared with the transformer. Typically, oil-impregnated paper (OIP), resin-impregnated paper (RIP), and resin-bonded paper are used as insulation in a capacitance-graded bushing (RBP). Mineral oil, which is often integrated into the bushing, is also used in capacitance-graded bushings.
Transformer asset owners are very interested in the health of the transformer bushings because bushing failures are frequently cited as one of the leading causes of transformer failures. The most common bushing failure modes are moisture infiltration, electrical flashover, lightning strike, short-circuited capacitance-graded layer(s), incorrect application of the bushing, corrosive sulphur, broken connection between ground sleeve and flange, and broken tap connection. The integrity of the bushings is determined by the electrical field tests that are performed after that.
