They are produced in power range of 25 – 2500 kVA, up to 36 kV high-voltage level, mono-phased or tri-phased, oily, natural cooler (ONAN), idle level convertor or automatic level convertor under load and in a way that they can be used in externally and internally. 

Hermetic transformers are produced in factory in a way that they shall not contact with air by setting pressure.

Oil pressure changing because of thermic effects in hermetic transformers are controlled by expansion-contraction of boiler wave walls and they are designed in way that they shall be able to resist these effects. 

Since there is no expansion vessel, oil filling process to the boilers must be made in special vacuum rooms in a way that there shall remain no void. Thus, since there is no moisture in the boiler, deterioration in oil because of oxidisation is decreased to zero.

Heights of hermetic transformers compared to the transformers with expansion vessel are lower. So, they can be used in smaller areas. 

Since oil is not in contact with air in hermetic transformers, oil change is not made in certain periods as it is in transformers with expansion vessel. No maintenance is needed except cleaning of bushings of YG and AG bushings.