There is no expansion tank. Boiler completely filled with oil. Hermetic type transformers were closed to the atmosphere making hermetic pressure setting in factory environment.
The expansion of oil due to thermic effects is gotten under control with the elastic and wavy structure of boiler.
The mission of wavy walls are both transmission of heat to the outside and absorption of additional oil which expands with the movement of elongation and contraction. There is no oxygen in the environment because transformer oil does not interact with the air. Therefore, there is no aging in the oil depending on the oxidation. Because of no interaction with air, transformer oil is not moisture. Therefore, the hermetic type transformers are last longer than transformers with the expansion tank. Hermetic transformers do not need maintenance except the cleanliness of the bushing and control of protection devices. Their height are lower and there is an opportunity for installation in small areas. They are advantageous for compact transformer stations. The negative effects which occur cause of the cable of expansion tank and busbar connections in terms of safety distances are quite out of the question in Hermetic transformers.
