The hermetically sealed oil transformers are normally manufactured with a sealed tank equipped with fins that allow the expansion at the temperature variations. The tightness of the tank is up to 0.5 Bar. This type of transformer is the most widely used in the world. In the hermetically sealed transformer, the oil does not come into contact with the air and its electrical properties are therefore not compromised, ensuring a long life span of the transformer. For powers exceeding 3150kVA, or in case of a transformer with radiators, the transformer can still be hermetically sealed by means of a nitrogen cushion.
For this type of transformers the expansion of the insulating liquid is compensated by the elastic deformation of the oil-cooling radiators attached to the tank. The protection against internal faults is ensured by means of a DGPT device: Detection of Gas, Internal Over Pressure and Oil Over Temperature.