ONAN/ONAN/ONAN? 1

[BLadewig]

Reviewing a transformer spec (230/69/12.47 kV, 167 MVA single rating) and noticed the following verbiage:

"Class... ONAN/ONAN/ONAN or ONAN/ONAF/ONAF at Contractor's option"

Seems curious to me that they would suggest three stages of non-forced oil and air. The engineer that put this together is not available for questions, and I was wondering if anyone out there might suggest reasons why the ONAN/ONAN/ONAN rating was provided. ONAN or ONAN/ONAF/ONAF would seem the more accurate way to put this.

 

[BLadewig]

Thanks in advance, btw.

 

[jbartos]

Suggestion: Contact ONAN for interpretations.

http://www.onan.com/na/pages/en/products/powergeneration/index.cfm

 

[brupp]

Cute answer jbartos.

BLadewig, you may be wrongly assuming that the person who stated the ONAN/ONAN/ONAN was stating it correctly/correctly/correctly. My guess is that they were considering the triple voltage levels as a candidate for three separate cooling ratings. Otherwise there is no reason I can think of to state the cooling levels 3 times. If they had listed it twice, i.e. ONAN/ONAN, that might have been to match the MVA rating at two separate temperature rises, for example 100/112 MVA ONAN/ONAN 55/65 C rise. But since you say it has a single MVA rating I can think of no reason to indicate the ratings as you described. I would write it off to someone not following standard rating conventions.

 

[tgott]

BLadewig: I agree that does seem strange, especially when it is a single MVA rating. Perhaps there is a temperature requirement, ie. 55C / 65C, but I've never seen it written that way. I would suggest its either a typo, its an error from putting a spec together from multiple spec's, or the guy who wrote the spec's really didn't know what he was writing.

 

[BLadewig]

Thanks to all respondents. The answer ended up being that I can't read; this is a 100/133/167 MVA xfmr. Lesson(s) learned.

 

[electricpete]

Ummm... I still don't understand. A rating like 100/133/167 mva usually gives the ratings for the three cooling modes as you indicated originally. If there is 55/65 you may get another batch of 3 more ratings.

The ONAN/ONAN/ONAN part still sounds wacko to me.

 

[jbartos]

Suggestion: The obtained rating 100/133/167MVA is normal or standard rating of transformers of this size. They have 100MVA rating for Ambient Air cooling AA, 133MVA rating for Forced Air cooling FA (meaning the FA is added to AA cooling), and 167MVA rating for Forced Oil cooling FO (meaning that FO is added to FA and AA cooling features). Visit Toshiba and Mitsubishi websites for transformer desciptions, explanations, and excellent pictures.

 

[electricpete]

jartos - I believe the term ONAN (oil natural, air natural circ) has replaced OA, ONAF (oil naturla, air force circ) has replaced FA, OFAF (oil force, air forced) has replaced FOA. The ONAN in question is not referring to the ONAN company that you linked to.

So my question (like the original question) is this: how can we have three stages of naturcal circulation oil/natural circulation air (ONAN). I can understand multiple stages/ratings of forced cooling where the fans or pumps can switch on in banks, but I don't understand multiple stages/ratings of forced cooling.

 

[jbartos]

Suggestion: Reference:
1. IEEE Std 141-1993 IEEE Recommended Practice for Electric Power Distribution for Industrial Plants,
Table 10-11 Classes of Transformer Cooling Systems, on page 509
would state for the above transformer:
OA/FA/FOA = Liquid immersed, self-cooled/forced-air, forced-liquid-cooled/forced-air, forced-liquid cooled, which is difficult to interpret since FOA stands for liquid-immersed, forced-liquid-cooled with forced-air-cooled.

 

[jbartos]

Suggestion: Visit

http://grouper.ieee.org/groups/tran...WG_C57_12_00/PCS_C571200_Proposed_Changes.pdf

for correlations "in progress"

 

[jbartos]

Suggestion: Table from the previous posting:
C57.12.00 Section 5. Rating Data, and Table 2 – Cooling Class
Designations.
Present Designations (2000) Previous Designations
ONAN OA
ONAF FA
ONAN/ONAF/ONAF OA/FA/FA
ONAN/ONAF/OFAF OA/FA/FOA
ONAN/OFAF OA/FOA
ONAN/ODAF/ODAF OA/FOA∗/FOA∗
OFAF FOA
OFWF FOW
ODAF FOA∗
ODWF FOW∗
∗ Indicates directed oil flow per Table 9, Note 2 of IEEE Std. C57.12.00 – 1993.

 

[jbartos]

Comment: The posted link has changed its content very quickly. A new standard has been approved. However, many other standards, many specifications throughout the industry, and many transformer nameplates will retain the old cooling designations for considerable amount of time.

 

[BLadewig]

Finally talked with someone that worked on this spec, and I think it was basically just a case of lazy-brain. The transformer is rated 100/133/167MVA, and one of the cooling class options spec'd is ONAN/ONAF/ONAF. On their own, these designations make sense.

They wanted to also provide the option of building the transformer without any forced cooling, and apparently sort of followed suit with the above format. There are no multiple-temperature rises spec'd. So I don't think it's necessarily wrong to write ONAN/ONAN/ONAN, just maybe not preferable. Out of curiosity, when you have no forced cooling then you really don't have different output stages, right? It wouldn't be a 100/133/167MVA transformer, but rather a 167MVA one.

 

[jbartos]

Suggestion: No force cooling would be 100MVA out of normally presented 100/133/167MVA transformer ratings under different cooling conditions.

 

[brupp]

BLadewig, you are correct that a naturally cooled only transformer would only carry the naturally cooled rating (OA in U.S. parlance, ONAN for the rest of the world). The multiple ratings being discussed here represent the additional capacity that the transformer can carry, staying within its temperature limits, at the various cooling stages. If you don't have forced cooling stages, you don't have additional MVA ratings. EXCEPT, the 12% discussed above is the additional capacity you can get by allowing a higher temperature rise. So a 150 MVA ONAN transformer could also carry a 168 MVA ONAN rating. The first rating is at 55C rise and the second is at 65C rise above ambient.

By the way, the trend for new units is to ignore the 55C rating and only specify a 65C rating. But some people prefer to use only the 55C rating and operate their transformers cooler, thereby extending their life (the transformers, not the engineers).

 

[BLadewig]

Helpful, thanks.