Transformer rating and cooling methods

like all other electrical machines, transformers rating is limited by temperature rise, which depends on losses. in transformer there are mainly two type of losses

1. copper losses - depends on current through windings and resistance of winding (I²R). keeping resistance constant this part is restricted by current.

2. iron losses - depends on core (size, material, lamination thickness), flux density and power frequency. here physical properties of core and power frequency is constant, so iron losses will depend on flux density. which will depend on voltage applied.

in this way transformer heat generated will depend on voltage applied on transformer and current flowing through windings, it has nothing to do with power factor. so we may find transformer ratings such as 300VA, 315KVA, 100 MVA ....

Further transformer rating can be improved if we maintain the temperature in limits. For cooling of transformers various methods are used:

• Oil Natural Air Natural (ONAN): Oil circulation and air circulation is natural. Oil circulates inside transformer by covection. Air circulates outside transformer by convection.
• Oil Natural Air Forced (ONAF): Oil circulation is natural but  air is forced with the help of fans installed near radiators.
• Oil Forced Air Forced (OFAF): Oil is circulated with the help of cooling pumps installed between transformer main tank and radiators. Air is forced with the help of fans installed near radiators.
• Oil Directed Air Forced (ODAF): Oil is circulated with the help of cooling pumps installed between transformer main tank and radiators, and direction of oil is directed to flow inside winding. Air is forced with the help of fans installed near radiators.

For example one transformer may have rating like 100MVA OFAF / 70MVA ONAF / 50MVA ONAN. This means this transformer can deliver 100MVA if both pumps and fans are in service, 70MVA if only fans are in service and 50MVA if no pumps or fans are in service. Generally, starting of fans and pumps is automated with temperature operated switches. These switches are installed for measurement of transformer oil temperature. 

For example, for a typical transformer, if temperature rises above 70⁰C Fans will starts. If temperature rises further above 75⁰C pumps also starts. If temperature further rises above 100⁰C transformer trips. if temperature falls below 70⁰C pumps stops. If temperature falls below 60⁰C fans also stops.