Oily type transformers are manufactured at power between 50-20000 kV and up to 36 kV maximum system voltage. Standard types of 3 phase, oily, natural- cooled (onan ) transformers are used as internal and external. There are two types of oily type transformers; hermetic and expansion tank. In both types, the transformer windings are completely in oil. However, Hermetic type oily transformers are closed to the atmosphere and does not need maintenance. There is no expansion tank in Hermetic type oily transformers and their boiler filled with oil. Hermetic type transformers were closed to the atmosphere making hermetic pressure setting in factory environment. Expansion of oil is controlled with boiler design which is elastic and wavy. 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. So oil is not exposed to oxidation. Because of no interaction with air, transformer oil is not moisture. Due to these reasons, hermetic type transformers are last longer than transformers with the expansion tank. In view of there is no expansion tank in the hermetic transformers, their heights are lower than transformers with expansion tank. It provides the opportunity of installation in small areas. Periodic maintenance of oily transformers with expansion tank needs to be done. Due to oil continuously interacts with the atmosphere, a situation is discussed such as the oil degradation and losing the feature of electrical resistivity. Therefore, their life is shorter comparing hermetic transformers. The processes of drying the active part and periodic oil changing in transformers with expansion tank is necessary.
