It is a mineral oil obtained by refining crude petroleum. It serves the following purposes:
i. Provides additional insulation.
ii. Carries away the heat generated in the core and coils.
iii. Protects the paper from dirt and moisture.
The transformer oil should possess the following properties:
- High dielectric strength.
- Low viscosity to provide good heat transfer.
- Good resistance to emulsion.
- Free from inorganic acid, alkali and corrosive sulphur.
- Free from sludging under normal operating conditions.
- High flash/fire point.