Free Breathing Transformer

The free-breathing transformers are equipped with a cushion under the cover or a conservator to allow the oil to expand at 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 filled with radiators. 

This design applies to all powers, but especially those of 4000 kVA 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.

What is a free‑breathing transformer?

A free‑breathing transformer is an oil‑immersed transformer fitted with a conservator or cushion beneath the cover so the oil can expand and contract with temperature changes; because the tank is not hermetically sealed, the oil is in contact with the atmosphere, and a breather containing dehydrating salts is used to limit moisture ingress.

How does a free‑breathing transformer work?

As the transformer heats during operation the oil expands into the conservator or cushion and returns as it cools, while the breather removes moisture from incoming air so water vapour does not accumulate in the oil or solid insulation; for very large ratings or where oil–air contact must be avoided, a rubber bag (separator) or a nitrogen cushion can be fitted to keep the oil isolated from the atmosphere.

What are the benefits of a free‑breathing transformer?

The design is simple, cost‑effective and well-suited to high‑power units, especially from around 4,000 kVA upwards, because conservator systems handle large oil volume changes efficiently and breathers are an established way to control moisture; the option to add radiators, rubber separators or nitrogen cushions gives flexibility to balance cost, maintenance and service life.

Where are free‑breathing transformers used?

They are commonly used in utility and industrial installations where large power ratings and straightforward oil management are required, such as power stations, large substations and heavy industry; when environmental or safety constraints demand reduced oil–air contact, the same design can be adapted with rubber bags or nitrogen cushions to meet site requirements.