Oil expands and contracts as load and ambient temperature change. A distribution transformer must accommodate that change in volume — the two common approaches differ in how.
## Conventional (free-breathing)
A conservator tank above the main tank absorbs oil expansion. Air enters and leaves through a silica-gel breather that removes moisture. The design is simple and easy to inspect, but the breather needs periodic maintenance and oil is exposed to some air, so it ages faster.
## Hermetically sealed
The tank is sealed with a bolted or welded cover and has no conservator. Expansion is absorbed either by a gas cushion above the oil or, in fully filled designs, by the flexing of corrugated fin walls. Oil never contacts outside air, which slows oxidation and moisture ingress and reduces routine maintenance.
## Practical differences
- Sealed units are usually more compact and lighter, and suit unattended distribution sites.
- Conventional units allow easier oil sampling and level checks, and are common at higher ratings.
- Protection devices differ: sealed units often use a pressure-relief device and a combined gas/pressure/temperature relay.
General engineering information.