What Is a Transformer Bushing? Types, Structure and Function

A transformer bushing is one of the most important — and most easily overlooked — components of high-voltage equipment. It safely carries current from the internal winding to the external network while withstanding system voltage and mechanical stress. Understanding its structure and types is the first step to correct selection and maintenance.

What a bushing does

A bushing performs three core functions:

• Electrical insulation: provides adequate insulation between the live conductor and the grounded enclosure (e.g. the transformer tank wall).
• Current conduction: carries rated current from the winding without excessive temperature rise.
• Mechanical support: supports leads, connectors and external connections against wind, seismic and short-circuit forces.

Main structure

A typical high-voltage bushing consists of:

• Central conductor: copper or aluminium, carries the current.
• Main insulation core: the primary insulation, usually oil-impregnated paper (OIP) or resin-impregnated paper (RIP).
• External sheds: porcelain or composite silicone sheds that increase creepage distance and prevent pollution flashover.
• Flange (mounting base): connects to the enclosure and is grounded.
• Test tap / measuring tap: used for tan delta, capacitance and partial discharge testing.

Classification by insulation medium

• Oil-impregnated paper (OIP): traditional, excellent insulation but oil-filled and needs maintenance.
• Resin-impregnated paper (RIP): dry, oil-free, low partial discharge, maintenance-free — the current mainstream trend.
• Resin-impregnated synthetic (RIS): synthetic fibre paper with higher mechanical strength.
• Resin-bonded paper (RBP): mainly for medium and low voltage.

Classification by application

• Transformer bushings: oil-to-air, one end in oil and the other in air.
• Reactor bushings: withstand higher electrodynamic forces and vibration.
• GIS bushings: SF₆-to-air, for gas-insulated switchgear.
• Wall bushings: pass through a wall or barrier, air on both sides.

Selection must consider rated voltage, rated current, basic impulse level (BIL), creepage distance, mounting arrangement and environment. Ever Power supplies high-voltage porcelain bushings compliant with DIN / IEEE / ANSI standards — contact us for selection support.