Hermetically sealed transformer is a transformer design which has no conservator. Here the dielectric insulating fluid in the transformer tank is completely sealed and is in zero contact to the atmosphere. The design avoids air in the transformer tank thereby avoiding sludging and oxidation of the dielectric fluid.

Hermetically sealed transformers are totally maintenance free and are particularly suited for use in exposed outdoor environments such as moisture, salt or dust laden atmospheres. They are used extensively in chemical plants, oil and gas terminals where poor accessibility makes regular maintenance impracticable. Transformers immersed in synthetic coolants are suitable for use indoors with adequate ventilation or near to the load center where oil would not be considered environmentally acceptable.

Normally these type of transformers are accommodated by devices that measure heat related volume variations. Gas cushion is used to compensate the volume variation due to heat. Normally the gas is nitrogen. This gas is thermally uncoupled from the dielectric liquid.