OA\FA ONAN\ONAF diferent? 1

[iskar2000]

Please tel me what is the difference between OA\FA and ONAN\ONAF?
What mean when it is said that power ratings are given at OA ta 55oC and FA at 56oC temperature rise?
Where I can read something more about it except ANSI?

 

[electricpete]

OA/FA is the older terminology. The letters are not very descriptive.

ONAN\ONAF is the newer terminology for the same thing. This newer terminology is clearer (IMHO):
ONAN = Oil - Nat circ/ Air - Nat circ
ONAF = Oil - Nat circ/ Air - Forced circ

 

[electricpete]

There may be up to 4 ratings on the transformer.
For example MVA1/MVA2 @55C and MVA3/4 at 65C.

In this case MVA1 and MVA3 would be ONAN ratings and MVA2 and MVA4 would be ONAF

 

[brupp]

Following up on the two previous responses, what I think is being described in the question is the 55C rise OA (ONAN) rating (the base rating) and the 65C rise FA (ONAF) rating, which is the top rating of the transformer. In the terminology of one reply, that is MVA1 and MVA4. Rather than list all four possibilities they have excluded the two middle ones, which tend to be meaningless anyway.

 

[peterb]

brupp -
electricpete is correct. The transformer has an ONAN rating at 55C rise and an ONAN rating at 65C rise, as well as an ONAF rating at each temperature rise. You would use the 55C ratings if you want to extend the life of the transformer, knowing that it can be operated at the 65C ratings at the expense of life expectancy.

 

[iskar2000]

I'm find in internet a table with relations of kVA ratings between different type of cooling and between different temperature rise 55oC and 65oC

http://www.pti.sk.ca/table2.htm

but my problem is that I receive specifications where it are given two ratings example: 2.25MVA at 55oC and 2.677MVA at 65oC and it is wanted transformer to be OA/FA but how you can see the ratio between the two rate is only 16%
Please explane me what it is mean

 

[peterb]

Iskar 2000 -
Looking carefully at that table - and considering the 1000 kVA transformer for example:

Base rating 55C = 1000 kVA ONAN or OA)
Base rating 65C = 1120 kVA (+12% of 55C OA rating
Forced air (FA) rating 55C = 1250 kVA (+25% of OA rating)
Forced air (FA) rating 65C = 1400 kVA (+12% of 55C FA rating)

Note the difference between the ANSI (+25%) and the CSA (+33%) fan cooled ratings.

For the transformer that you are concerned about, what standard is it built to? And is the 2.677 MVA/65C rating with or without fan cooling? By the way, I calculate (2.677/2.25)*100 = 119%.

 

[redtrumpet]

To further confuse things, sometimes the nameplate will give maximum kVA at 0 degrees Celsius ambient temperature.

Good point, Peterb, to clarify the difference between fan-cooled ratings for ANSI and CSA.

I thought ANSI still used the OA/FA designations, and that ONAN/ONAF were purely CSA. Judging by the above posts, it looks like ANSI has switched to the ONAN/ONAF nomenclature.

 

[MarkAthay]

As far as I know operation at the 65 degree C ratings does not in any way reduce the life of the transformer. What you have is the ANSI standards were originally written around transformer insulation that was only rated to 55 degrees C. Newer insuation has since come along, and a long time ago at that, that is rated for 65 degrees C operation.

The temperatureis NOT the temperature of the insulation, but is the temperature rise above a 40 degree C ambient. The actual hot-spot temperature inside the transformer is rated to 105 degrees centigrade. Lower ambient temperatures give a higher operating capacity because of the increased temperature differential across the cooling elements. Increasing the air circulation, cooling the environment, or adding cooler stuff to cool down the transformer such as spraying it with water in an emergency can all help to reduce the temperature of the transformer and effectively increase the transformer capacity without degrading its life expectancy.

BTW most transformers die not from overheating but by some other action, such as mechanical damage from through-faults, oil loss, corrosion, etc.

 

[dpc]

Mark,

I'm not quite sure I agree with you. Everything else being equal, I think a transformer with 65 degree C insulation will last longer if operated within a 55 degree C rise.

I agree that it can be operated at 65 deg C rise and should have a "normal" life expectancy. But it should last longer if operated at a lower temperature - if it doesn't fail of something else first.

Also, isn't the maximum ambient for transformers in the ANSI standards actually a 30 deg C average over a 24 hour period?

 

[MarkAthay]

It IS a 30 degree C over a 24 hour period, and is also refernced as a 40 degree C top temperature during that 24 hour period.

The ANSI life expectancy of a transformer is continuous operation at the rated temperature for 30 years. Since the temperature maxes out for a faiirly short period each year (usually), the life expectancy is much more than 30 years.

 

[jbartos]

Suggestion: Visit

http://www.usbr.gov/power/data/fist/fist1~5/1~5_cont.htm

http://www.usbr.gov/power/data/fist/fist1~5/1~5_fig6.h

http://www.usbr.gov/power/data/fist/fist1~5/1~5_fig7.htm

for more info and compare the peterb posting indicating:
"Note the difference between the ANSI (+25%) and the CSA (+33%) fan cooled ratings."