In these substations, the apparatus is installed within the substation building. Such substations are usually for a voltage up to 22 KV but can be erected for 35KV when the surrounding atmosphere is contaminated with impurities such as metal corroding gases and fumes, conductive dust etc. The switchgear on supply or primary side will consist of oil circuit breakers only.

The high voltage supply is given to the primary of the transformer through circuit breaker. From the bus-bar various feeders emerge out. The panel for each feeder consists of an isolator switch and circuit breaker. In addition to isolator and circuit breaker, the panel is provided with the measuring instruments. As regards protection of feeders usually reverse power relay is employed. For the protection of oil filled transformers with conservator tanks relay is most common.

Quang Anh CGTE build indoor substation or Substations  which the apparatus is equipped inside the substation building to fit specific applications from 320KVA through 4000KVA, with primary voltage ratings up to 22kv. In addition, we have three-phase outdoor substations from 320KVA to 4000KVA.