An oil-immersed transformer changes voltage by electromagnetic induction: alternating current in the primary winding sets up a changing magnetic flux in the core, and that flux induces a voltage in the secondary winding. The ratio of turns sets the ratio of voltages. Mineral oil fills the tank and does two jobs at once: it insulates the live parts and carries heat away from the core and windings.
Main parts
- Core. Stacked sheets of cold-rolled grain-oriented silicon steel form the magnetic circuit. Thin, coated laminations keep eddy-current losses down; larger units have oil ducts inside the core.
- Windings. Copper or aluminium conductors wound in cylindrical layers, normally with the low-voltage winding nearest the core and the high-voltage winding outside it.
- Tank. Welded steel that holds the oil and the active part. Small distribution units use a fully sealed corrugated tank; larger power transformers use a conventional or bell-type tank with radiators.
- Conservator. On non-sealed designs, a small tank above the main tank that takes up the oil as it expands and contracts, keeping the main tank full and limiting contact with air.
- Buchholz (gas) relay. Fitted in the pipe between tank and conservator. A minor internal fault gives an alarm; a serious one trips the breaker.
- Radiators. Hot oil rises, passes through the radiators, cools and returns to the bottom of the tank.
- Bushings. Insulated outlets that bring the high- and low-voltage leads through the tank cover.
- Tap changer. Adjusts the turns ratio to hold the secondary voltage steady. Off-circuit changers are set with the transformer de-energised; on-load changers work in service.
- Pressure relief device. Releases pressure if a severe internal fault builds up gas faster than the protection can act.
Cooling methods
- ONAN (natural oil, natural air): simple and low-maintenance; the usual choice for distribution transformers.
- ONAF (natural oil, forced air): fans on the radiators raise the rating of medium and large units.
- Forced oil circulation: pumps move the oil through coolers; used on large, heavily loaded power transformers.
Where they fit
Oil-immersed units are the standard choice outdoors and in substations: they handle overloads well, cost less than dry-type units of the same rating, and last for decades with routine oil checks. Where fire risk inside a building is the main concern, a dry-type transformer is usually specified instead.
See our oil-immersed ranges: S11, S13, S18, S20, S22 and the on-load tap changing SZ series.