Those finned metal panels surrounding a transformer tank are neither armor nor decoration; they are the radiators, the transformer's lungs, which expel loss-generated heat into the surrounding air and preserve the life of the insulation.
Why does a transformer need radiators?
Core and winding losses are converted into heat inside the oil. The surface area of the tank alone is not sufficient to dissipate this heat in medium and large transformers, so radiators are added to multiply the contact area between the hot oil and the air many times over.
How does the cooling cycle work?
- The hot oil around the windings becomes less dense and rises to the top of the tank.
- It enters the radiators through their upper openings and flows downward through their thin tubes, releasing its heat to the air through the walls.
- The cooler, heavier oil returns from the bottom of the radiators to the bottom of the tank, completing a continuous natural convection cycle without any pump.
- When fans are added to blow air over the radiators, the cooling becomes forced and the transformer's thermal capacity increases — see ONAN and ONAF cooling systems.
Field inspection points
- Oil leaks from radiator joints and valves are among the most common leak locations.
- Dust and dirt buildup on the surfaces reduces cooling efficiency — keeping radiators clean is genuine preventive maintenance.
- An unusual temperature difference between the top and bottom of a radiator may indicate a blocked valve or oil passage.
Sample answer: Heat transfers from the windings and core to the oil in contact with them, heating the oil and reducing its density so it rises to the top of the tank. It then passes through the radiators, where it cools by releasing its heat to the air through the large-area walls, and returns cooled to the bottom in a natural convection cycle. This can be enhanced with fans (forced air) to increase thermal capacity.
Neglecting the cleanliness of the radiators and their valves. A thick layer of dust or a radiator valve accidentally left closed after maintenance reduces actual cooling, causing the transformer's temperature to rise despite everything else being sound.
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