Transformer

The hermetically sealed transformer and the RMU are used in network distribution systems which often have multiple electrical sources feeding multiple loads in a network, mesh, or loop. The transformer is sealed for use in vault or manhole locations, where it could possibly even be submersed in water at times, hence "hermetically sealed" so no water can get in.

The RMU is the circuit interrupter and protection / control unit that interconnects the transformer to the network. It has sensing circuitry to determine proper power flows, and will separate from the network when abnormal conditions (due to a fault) are detected. In this way, a fault only takes a small part of the network out of commission, and the rest of the network continues to function. It is also designed for use in conditions similar to the hermetically sealed transformer, so that it is impervious to dust and water entry. In the US, it is called a "Network Protector."

Sridharganasen has it only partly correct: an ONAN (oil-natural, air-natural cooling) transformer may be hermetically sealed, but is not always. There are many ONAN transformers that are free-breathing, i.e., directly vented to atmosphere. However, a hermetically sealed transformer will be ONAN, because it will not (vast majority of the time) have external cooling fans or oil circulating pumps as found on larger transformers for substation use.

The hermetically sealed transformer is the transformer without the conservator tank. The pressure of the expansion/contraction of transformer oil (due to inside temperature increase and decrease) is absorbed by the conservator for a regular transformer. The hermetically sealed transformer is totally sealed and doesn't have conservator. This type of transformer required low maintenance and is used mostly in harsh operating conditions such as offshore petroleum facilities, chemical plant etc. Generally, there are two categories of hermetically sealed transformer based on the mechanism of absorbing the transformer inside pressure: (1) Corrugated transformer and (2) Nitrogen filled transformer

Corrugated transformer: The transformer is fully filled with transformer oil and its body is corrugated so that the body can expand and contract to absorb the change of inside oil pressure.

Gas (Nitrogen) filled transformer: A portion of transformer is filled with inert gas like nitrogen which can absorb the change of inside oil pressure with additional pressure/vacuum bleeders installed in the transformer.