Hermetic transformer is design which has no conservator.
The dielectric insulating fluid in the transformer tank is completely sealed and is in zero contact to the atmosphere.
This type of distribution transformer is used in applications where the transformer is to be installed in harsh climatic conditions(smoke,pollution,dusty environment,etc) or user prefers low maintenance for transformer.
The design avoids air in the transformer tankthereby avoiding oxidation of the dielectric fluid.
Hence it would be safe to say that these transformers need no maintenance of the dielectric insulating fluid (usually transformer oil).
Normally these type of oil transformers are accommodated by devices that measure heat related volume variations.
Gas cushion is used to compensate the volume variation due to heat.
Normally the gas is nitrogen. This gas is thermally decoupled from the dielectric liquid.