Transformer Bushings: Types and Selection

Published 27 Aug 2026Updated 27 Aug 20268 min readIntermediateComponents
Quick answer — A bushing carries the high-voltage conductor through the grounded transformer tank/cover while insulating it from the earth and grading the electric field. The main families are solid (with a solid insulation core), oil-impregnated-paper (OIP), resin-impregnated-paper (RIP) and resin-impregnated-synthetic (RIS, e.g. epoxy/cycloaliphatic). Selection depends on voltage class, current, environmental duty, partial-discharge/creepage requirements and whether the bushing is oil-to-air, oil-to-oil or oil-to-SF6.
Contents
What types of transformer bushings are there and how are they selected?

By construction

Solid bushings are simple, cheap, for low/medium voltage. OIP bushings use a paper core vacuum-dried and oil-impregnated; they are robust and traditional but require oil. RIP bushings use resin-impregnated paper with no free oil and are a common choice for GIS and sealed units. RIS bushings use a moulded epoxy/cycloaliphatic core, very compact and pollution-resistant, widely used for MV and some HV applications.

Selection factors

Rated voltage, lightning/impulse withstand, rated current, creepage and pollution class, installation orientation, contamination, altitude derating, and PD limits. The bushing must also be compatible with the fluid or gas above and below it.

Testing and condition

Bushings are tested for power-factor/dissipation factor, capacitance, PD and withstand. Monitoring tap measurements let operators track the bushing condition (capacitance and tan-delta drift are early signs of moisture or contamination).

Worked example

Select the bushing by the higher of the system voltage and the BIL, then confirm creepage distance for the site contamination class (e.g. e = 20 mm/kV for heavy pollution) against the relevant standard.

References & standards

Related

Book seriesAcademy

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

Related components

Transformer bushings & terminations Oil, fluids & preservation
Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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