The basic power substation theory for students

Oil-filled (immersed) transformers

Oil filled transformer makes use of oil for the cooling of major parts of a transformer. Oil filled transformers are transformers filled with a highly refined mineral oil that is used to insulate internal live parts of the transformer. The oil prevents corona and manages temperature control inside the transformer for the prevention of equipment and machinery overheating during the operation of large job applications.

Because of oil inside the transformer being of non-combustible properties, these transformers are very safe and can operate machinery for long period of time.

Oil or liquid preservation system

A preservation system is essential for a liquid-immersed transformer to allow- expansion and contraction of the liquid due to the changes in the temperature with-out exposing the insulating liquid to external contamination. The expansion space is known as “the oil preservation system”.

There are four different designs :

1. Sealed tank
2. Gas-oil seal
3. Conservator
4. Conservator diaphragm

The conservator must therefore breathe to the surrounding air, and moisture in the air can be absorbed into the oil and insulation system.

To prevent the moisture from entering the conservator, a silica gel breather is provided that will dry the air as the breathing takes place. As the silica gel absorbs the moisture, it will change from blue to pink in color and must be changed to retain its drying capability. So maintenance is required.

The accessories for mineral-oil transformer generally consists of :

1. A conservator tank.
2. Buchholz relay ( gas-detector relay ) for conservator-type transformer. Buchholz relay is located in the pipe between the highest part of the transformer tank and conservator. The relay  consists of two sets of contacts, one contact for trip upon surge and other alarm upon gas accumulation.
3. Breather of dehydrating type for conservator-type transformers containing a colour indicator( usually silica gel), piped to the conservator and mounted in such a way that it is accessible from ground level.
4. Winding temperature-indicator.
5. Oil temperature indicator.
6. Cooling equipment like radiators, fans for forced air cooling, and pumps for forced oil – and water- cooled transformer.