Hi

1. ONAN (oil natural,air natural)

2. ONAF (oil natural,air forced)

3. OFAF (oil forced,air forced)

4. ODWF (oil direct,water forced)

5. OFAF (oil forced,air forced)

Hi

1. ONAN (oil natural,air natural)

2. ONAF (oil natural,air forced)

3. OFAF (oil forced,air forced)

4. ODWF (oil direct,water forced)

5. OFAF (oil forced,air forced)

Dear

The main source of heat generation in transformer is its copper loss or I2R loss. Although there are other factors contribute heat in transformer such as hysteresis & eddy current losses but contribution of I2R loss dominate them. If this heat is not dissipated properly, the temperature of the transformer will rise continually which may cause damages in paper insulation and liquid insulation medium of transformer. So it is essential to control the temperature with in permissible limit to ensure the long life of transformer by reducing thermal degradation of its insulation system. In electrical power transformer we use external transformer cooling system to accelerate the dissipation rate of heat of transformer.

different transformer cooling methods

1. ONAN (oil natural,air natural)

2. ONAF (oil natural,air forced)

3. OFAF (oil forced,air forced)

4. ODWF (oil direct,water forced)

5. OFAF (oil forced,air forced)

If the transformer is oil type then the cooling is achieved using Air and oil either natural or forced based on the transformer rating. However, if the transformer is dry type, then Air only is used for cooling.

All the above replies are relevant and appropriate and described the conventional and popular methods of cooling transformers. The essential requirement of cooling is designing a proper heat pipe so that heat can be taken out from the object under consideration so you may expand your imagination to cool a transformer particularly if it be a non standard transformer. As an example in a Transformer with very high current capacity and lower voltage rating the secondary winding can be in the form of a pipe and cold water flowing continuously through it for cooling the winding. Since Dry type transformers as rightly pointed out by Mr. El-Fergany can be cooled by natural or forced air only some time you may try with  typically designed heat sink with or without  forced air for more effective cooling particularly for low voltage system.

1. ONAN (oil natural,air natural)

2. ONAF (oil natural,air forced)

3. OFAF (oil forced,air forced)

4. ODWF (oil direct,water forced)

5. OFAF (oil forced,air forced)

Above already mentioned, there is also OFWF - oil forced water forced especially in case of generator step up transformer in HPP

Also in case of largest power transformers there is ODAF - oil forced with directed flow, air forced