They are transformers that are subjected to purification from the air gaps in the coils in specially designed vacuum furnaces.
Since the contact of the oil, which is used for cooling and insulation in the transformer, with air is prevented, moisture and aging due to oxidation will be prevented. Since there is no contact medium with air, there is no need for oil change and extra maintenance.
Since there is no expansion tank in Hermetic Transformers, the height is low. It allows mounting in smaller spaces. It is particularly advantageous in compact transformer substations.
The wave walls on the transformer are designed in accordance with the expansion of the heated oil.