Short Answer:

A transformer is cooled to remove the heat generated due to losses during its operation. These losses mainly come from winding resistance (copper loss) and core magnetization (iron loss). Without proper cooling, the temperature can rise and damage insulation, reducing the transformer’s life.

Cooling in transformers is done using various methods such as air, oil, or forced circulation systems. Small transformers use natural air cooling, while larger ones use oil-based or forced cooling methods like oil circulation with fans or water for heat dissipation.

Detailed Explanation:

Transformer cooling

Cooling is a very important part of transformer design because during operation, transformers generate heat due to energy losses. If this heat is not removed properly, it can overheat the windings and insulation, leading to reduced efficiency, faster aging, or even transformer failure. Therefore, a good cooling system helps maintain temperature within safe limits and improves the reliability and life of the transformer.

Why Cooling Is Needed:

1. Heat Generation:
   Transformers generate heat from copper losses (I²R losses in windings) and iron losses (hysteresis and eddy currents in the core).
2. Temperature Limits:
   Insulation used in transformers has a temperature limit. Exceeding this limit reduces its life and may cause electrical breakdown.
3. Continuous Operation:
   Transformers often operate 24/7. Without cooling, the internal temperature would rise beyond safe levels.

Common Cooling Methods:

1. Air Natural (AN):
   Used in small transformers. The heat is simply released to the surrounding air through the outer surface of the transformer.
2. Air Forced (AF):
   Fans are used to blow air over the transformer surface to speed up cooling. Suitable for medium-sized transformers.
3. Oil Natural Air Natural (ONAN):
   Transformer core and windings are immersed in oil. The oil absorbs heat and circulates naturally inside the tank. The hot oil transfers heat to the tank walls, which is then released into the surrounding air.
4. Oil Natural Air Forced (ONAF):
   Similar to ONAN, but uses external fans to blow air over the tank surface. It improves cooling for larger transformers.
5. Oil Forced Air Forced (OFAF):
   Oil is pumped using oil pumps, and air is blown using fans. This method is used in large transformers where natural circulation is not enough.
6. Oil Forced Water Forced (OFWF):
   In very large transformers, oil is passed through heat exchangers where water is used to carry away the heat. This method is highly effective and used in power stations.

Additional Cooling Features:

• Radiators or Fins:
  Attached to the tank to increase the cooling surface area.
• Cooling Tubes:
  Used to improve heat transfer from oil to the external surface.
• Temperature Sensors and Relays:
  Monitor internal temperature and control cooling devices like fans and pumps.

Selection of Cooling Method:

• Small Distribution Transformers: Use AN or ONAN.
• Medium Power Transformers: Use ONAF or OFAF.
• Large Power Transformers: Use OFAF or OFWF for efficient cooling.

The choice depends on the transformer’s size, location, load condition, and operating environment.

Conclusion:

Cooling of a transformer is essential to keep its temperature within safe limits, prevent damage, and ensure reliable operation. Various cooling methods like natural air, oil, and forced systems are used depending on the transformer’s size and application. Proper cooling extends the life of the transformer, maintains efficiency, and prevents failures caused by overheating.