ONAF Cooling

The transformer has a basic rating of 12 mva. When you add the proper number of fans the rating is increased to 20 mva. Some transformers also have a temperature rating. Example at 55C you have a rating. if it has a 65C rating it has a higher rating. You may want to look at the nameplate to see if you have both a FA rating and a 65C rating. You transformer may be a 12/16/20 mva. That means the base is 12 mva, one set of fans is 16 mva and 2 sets of fans is 20 mva. If you have the 65C rating that 20 mva becomes 22.4 mva.

Good information has been provided by wareagle – another GA.

Here is some more information about the Transformer cooling system designation (ONAN, ONAF, KNAN, KNAF etc.) defined in major standards (IEEE C57.12.00 Section 5.1 [2], CEC etc.):

Each letter in the cooling system designation has its own meaning. For an oil emerged transformer, there are two things related to cooling: (1) Oil which is in direct contact with the transformer winding and carries the internal heat outside of transformer tank for releasing to external media and (2) the method by which the heat is released. The first letter (O, K and L) indicates the type of oil, the second letter (N and F) indicates the method of circulation of oil, the third letter (A and W) indicates the media to which the heat is released and the fourth letter (N and F) indicates flow of the circulation of the media.

First letter: Internal cooling medium in contact with the windings

O - Mineral oil or synthetic insulating liquid with flash point < 300°C

K - Insulating liquid with flash point > 300°C

L - Insulating liquid with no measurable fire point

Second letter: Circulation mechanism for internal cooling medium

N - Natural flow

F - Forced circulation through cooling pumps

Third letter: External cooling medium

A - Air

W - Water

Fourth letter: Circulation mechanism for external cooling medium

N - Natural flow

F - Forced circulation through fans (for air) and pumps (for water)

For example: ONWF means the transformer is mineral oil filled and the flow of oil is natural, the oil released the heat to water (through a heat exchanger) which is circulated using a pump.

Note: There is some other cooling system designation such as OA, FA, OA/FA/FA, OA/FA/FOA etc. but these are the old ANSI system.

One of the major limitations of the KVA rating of the transformer is its winding temperature rise with higher usage of KVA. With releasing more heat from the windings, the winding temperature rise can be limited to certain tolerable range (55ºC or 65ºC) even with allowing more current passes through it, which eventually increases the KVA rating. So, the KVA rating of transformer depends on the cooling system. The better is the cooling system, the higher is the KVA rating. The cooling system ONAF releases more heat than the ONAN system and hence the ONAF rating is higher than the ONAN rating for the same transformer.

See other related threads:

http://cr4.globalspec.com/thread/44878

http://cr4.globalspec.com/thread/43826

http://cr4.globalspec.com/thread/23761

- MS