An oil filled self cooled transformer most often uses natural oil circulation and natural air cooling, commonly described as ONAN. Heat generated in the windings transfers to the oil, hot oil rises, and cooler oil descends, forming convection flow through ducts and radiators. The radiator surfaces then transfer heat to surrounding air without fans or pumps. This design is simple and reliable for many distribution and medium power applications, but its cooling limit depends on ambient temperature, radiator size, oil viscosity, and how effectively internal oil channels guide flow.
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