The free breathing transformers are equipped with on air cushion under the cover or a conservator to allow the oil to expand, at the temperature variations. As it is not hermetic, the oil is always in contact with the air. Moisture is kept at bay through the use of dehydrating salts contained in special filters (breather). The tank can be in finned or with radiators. This design applies to all powers, but especially from 4000kVA and up. For higher powers and to avoid contact between the oil and air, a rubber separator (rubber bag) or a nitrogen cushion is used.
This type of transformer is equipped with an expansion tank or conservator mounted above the main tank. The expansion of the insulating liquid is compensated inside the conservator by the raising of the oil level. In the conservator the top of the oil is in contact with the air which must remain dry to avoid any oxidation. This is achieved by admitting the outside air in the conservator through a desiccating device containing silica-gel crystals.