- Transformer Cooling System Definition: A transformer cooling system is defined as methods used to dissipate heat generated in transformers to prevent damage and ensure efficiency.
- ONAN Cooling: ONAN cooling uses natural oil and air circulation to cool the transformer, relying on convection for heat dissipation.
- ONAF Cooling: ONAF cooling employs fans to blow air over the transformer, enhancing heat dissipation by forced air circulation.
- ODAF Transformer: The ODAF (Oil Directed Air Forced) transformer uses directed oil flow and forced air to cool high-rating transformers efficiently.
- OFAF Cooling: OFAF cooling combines oil pumps and air fans to circulate oil and cool the transformer quickly and efficiently.
The main source of heat in a loaded transformer is copper loss, or I2R loss. Hysteresis and eddy current losses also heat the core, but I2R loss usually dominates on load. If that heat is not removed, temperature keeps rising and can damage paper insulation and the liquid insulation. Temperature must stay within the permitted limit so the insulation ages more slowly. On an electrical power transformer an external transformer cooling system is used to speed heat loss from the transformer. The next sections describe the usual transformer cooling methods.
Different Transformer Cooling Methods
Cooling is sped up with different transformer cooling methods chosen for size and rating. The methods follow.
ONAN Cooling of Transformer
This is the simplest transformer cooling system. ONAN means “Oil Natural Air Natural”. Natural convection of hot oil does the cooling. Hot oil rises to the top of the tank and cooler oil takes its place. Oil that has risen then loses heat to the air by conduction, convection and radiation and then cools. In that way the oil keeps circulating while the transformer is on load.
Heat lost to air depends on the tank surface, so extra surface is added as tubes or radiators on the tank. That extra surface is the radiator of transformer or radiator bank of transformer. The figure below shows a simple ONAN arrangement on an earthing transformer.
ONAF Cooling of Transformer
Heat dissipation can be improved by adding surface, and it is faster still with forced air. Fans blow air onto the cooling surface and remove heat faster than still air. This method is ONAF, “Oil Natural Air Forced”. Because ONAF cools faster than ONAN, an electrical power transformer can handle more load without exceeding temperature limits.
OFAF Cooling of Transformer
In an OFAF cooling system of transformer, fans still blow air over the dissipating surface, and oil is forced around the tank rather than left to natural convection alone.
Heat dissipation can be raised further by forcing the oil to circulate. In OFAF cooling, oil pumps move the oil inside the tank. OFAF means “Oil Forced Air Forced” cooling. The plant is compact: it can match earlier cooling duty in less space. With natural oil flow, heat leaves slowly by convection. With forced oil, heat is carried away as soon as it enters the oil, so cooling is faster.
OFWF Cooling of Transformer
Water usually enters cooler than the surrounding air and has a higher heat capacity, so it exchanges heat more readily. In OFWF cooling, hot oil is pumped to an oil-to-water heat exchanger. Cold water then cools the oil. OFWF means “Oil Forced Water Forced” cooling.
ODAF Cooling of Transformer
ODAF, oil directed air forced cooling, is a development of OFAF. Forced oil is steered along set paths in the winding. Cool oil entering the tank from the cooler or radiator is passed through winding ducts between insulated conductor so heat leaves the winding faster. ODAF is used on very high-rating transformers.
ODWF Cooling of Transformer
ODWF, oil directed water forced cooling, is like ODAF except the hot oil is cooled by forced water instead of air. Both of these transformer cooling methods are forced directed-oil cooling of transformer.
