Transformer manufacturers design cooling systems to efficiently dissipate the heat generated due to copper (I²R) losses, core losses (hysteresis & eddy currents), and stray losses. The cooling method depends on the transformer’s size, power rating, and application.

Cooling Methods in Transformers

1. Dry-Type Transformer Cooling

Used for low to medium power transformers, typically up to 10 MVA.

AN (Air Natural): Heat dissipates through convection in open air.

AF (Air Forced): External fans improve heat dissipation.

2. Oil-Immersed Transformer Cooling

Used for high-power transformers (above 10 MVA) since oil has better heat dissipation than air.

ONAN (Oil Natural Air Natural): Oil circulates naturally; heat dissipates through radiators.

ONAF (Oil Natural Air Forced): Fans increase cooling efficiency by forcing air over the radiators.

OFWF (Oil Forced Water Forced): Pumps circulate oil, and heat is removed by water-cooled heat exchangers.

ODWF (Oil Directed Water Forced): Jet-directed oil flow for large power transformers (like in substations).

Key Cooling Design Features

Radiators & Fins: Increase surface area for heat dissipation.

Cooling Fans & Pumps: Enhance airflow and oil circulation.

Heat Exchangers: Used in large transformers to transfer heat from oil to water.

Oil Expansion Tanks (Conservator): Prevents excessive pressure build-up from oil expansion.

Thermal Sensors & Cooling Control Systems: Automatically regulate cooling mechanisms based on temperature.