Transformer Bushing

A Bushing is an insulated device which is most commonly used in transformer and in many electical device that allows an electrical conductor to pass safely through an earthed conducting barrier such as the wall of a transformer.

Purpose of Bushing

• Bushing has to carry the full load current.
• Bushing has to provide electrical insulation to the conductor for working voltage and for various ove-voltages that occur during service.
• Bushing has to provide support against various mechanical forces like radial forces, axial forces etc.

Bushing Behaviour

The behaviour of an arc under AC and DC conditions in different. Thus, the bushing for DC terminal has to be different. The DC wothstand voltage of contaminated insulator is 20% to 30% to that of AC. For satisfactory performance, bushings with higher creepage is required to avoid frequent flashovers. Creepage distance as high as 40mm/kV is general specified.

Classification of Bushing

Alternator Bushing

• AC generators or Alternators require busings up to 33kV.
• Generally, 22kV is more usual in practice.
• In modern alternators, current ratings up to 20000 A are required.

Switchgear Bushing

• Bushing plays an important role in Switchgear.
• Bushing are to carry the conductors through the tank wall, and support the switch contacts.

Transformer Bushing

• Transformer require terminal bushings for both promary and secondary windings.
• Sometimes, a high voltage cable is directly connected to the transformer via bushing.

Wall Bushing

• In general, walls of the electrical devices such as circuit breaker are made of insulating device called bushing.

Loco Bushing

• Loco bushings are mainly used in traction application.