Oil immersed distribution transformers

Hermetic with full filling
The flexible walls of the case allow to provide sufficient cooling to the transformer by compensating the volume changes of the insulating liquid due to the temperature during its operation; thanks to the hermetic nature of the structure the fluid is not in contact with the external atmosphere, thus avoiding the absorption of humidity.
This type of transformer can be considered maintenance-free.

With conservator
In case where the volume of liquid inside the case is critical a conservator is used to compensate its expansion during operation at high ambient temperatures.
These transformers are equipped with an air dryer or as an option an oil/air separation membrane to protect it from humidity and oxidation. 

Hermetic with dry air / nitrogen cushion or with expandable radiators
A sealed cover protects the insulating liquid from contamination by external humidity; its volume expansion is compensated through the use of a dry air / nitrogen cushion or by radiators.