# Transformer Ratings



## dayrongarcia (Feb 3, 2015)

Hello all,

[Content removed by moderator]

Thanks

[Remember the confidentiality agreement you signed when you took the test. Development of test questions is expensive and both EB and NCEES take that agreement very seriously. This one was a little to close to the line.]


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## PwrEngr (Feb 3, 2015)

Hey pal, I would suggest that you don't make reference to the exam when asking your question(s). Try to be general while making your point, just saying.

Thank you...


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## Ken PE 3.1 (Feb 3, 2015)

Different ratings are based on the type of cooling used, I.e. oa/fa.

And what pwrengr said.


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## Kovz (Feb 3, 2015)

Like Ken said, it represents different cooling ratings. So a transformer with forced air (FA) cooling will be able to have a higher rating than a naturally cooled transformer.

Do some research on Transformer Cooling methods to get a better understanding.


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## eksor_PE (Feb 3, 2015)

Different stages of cooling methods have different KVA ratings. Look at IEEE Standard C57.12.10 page 4 for reference.


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## kduff70 (Feb 4, 2015)

Since Transformer Rating of Apparent has come up as topic

I understand that there is cooling classes sets for transformer(example OA/FA/FOA) and each transformer will have a cooling rating set that goes with a set of apparent power and a voltage rating. But I'm not sure how the transformer cooling rating applies to a set of( Apparent Power rating) MVA So if I have cooling set of rated transformer with a 21/28/35 MVA how would I get the true rating of the transformer base on a given set of OA/FA/FOA the information does really elaborate on rating other than an explanation of what the letter mean. Should there be some sort of table to go with the classification?



Just curious to understand how work transfromer rating


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## Flyer_PE (Feb 4, 2015)

^Unless stated otherwise, the impedance values are given on the OA rating of the transformer.


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## PwrEngr (Feb 5, 2015)

In lign with eksor_PE suggestion, IEEE Std C57.12.36, Pages 4, 5, ... gives further details.


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## kduff70 (Feb 5, 2015)

PwrEngr

Do you know where I can get a copy of that section without having to purchase the whole IEEE book?


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## Kovz (Feb 5, 2015)

Here is a few pages I scanned in. I have access to the electronic version through work.

IEEE Std C57.12.pdf


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## kduff70 (Feb 5, 2015)

thank you kovz


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## MannyEE (Feb 5, 2015)

Thanks Kovz!


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## Kovz (Feb 6, 2015)

:thumbs:

I just hope all this good karma helps me pass the exam in April, lol


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## kduff70 (Feb 6, 2015)

you and me both i wish you well


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## PwrEngr (Feb 9, 2015)

Nice job from you Kovz, I missed his request.


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## ksunny (Jul 15, 2022)

I am still not sure how to solve this ? If someone could post the solution that would be great. Also for fault current do we use OA rating ?


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## ksunny (Jul 15, 2022)

I am still not sure how to solve this ? If someone could post the solution that would be great. Also for fault current do we use OA rating or ?


kduff70 said:


> Since Transformer Rating of Apparent has come up as topic
> 
> I understand that there is cooling classes sets for transformer(example OA/FA/FOA) and each transformer will have a cooling rating set that goes with a set of apparent power and a voltage rating. But I'm not sure how the transformer cooling rating applies to a set of( Apparent Power rating) MVA So if I have cooling set of rated transformer with a 21/28/35 MVA how would I get the true rating of the transformer base on a given set of OA/FA/FOA the information does really elaborate on rating other than an explanation of what the letter mean. Should there be some sort of table to go with the classification?
> 
> ...


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## Warrior PE (Jul 18, 2022)

ksunny said:


> I am still not sure how to solve this ? If someone could post the solution that would be great. Also for fault current do we use OA rating ?



It doesn't matter what rating you choose for fault current analysis OA/FA or FOA. I mean it doesn't matter you choose first stage, second stage or third stage rating because Impedance of the transformer is directly proportional MVA rating. Your fault current PU value will remain same for transformer.


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## Warrior PE (Jul 18, 2022)

kduff70 said:


> Since Transformer Rating of Apparent has come up as topic
> 
> I understand that there is cooling classes sets for transformer(example OA/FA/FOA) and each transformer will have a cooling rating set that goes with a set of apparent power and a voltage rating. But I'm not sure how the transformer cooling rating applies to a set of( Apparent Power rating) MVA So if I have cooling set of rated transformer with a 21/28/35 MVA how would I get the true rating of the transformer base on a given set of OA/FA/FOA the information does really elaborate on rating other than an explanation of what the letter mean. Should there be some sort of table to go with the classification?
> 
> ...


The rating of the transformer is limited by temperature rating of winding and core (after MVA & voltage). hence, you can apply cooling methods to load your transformer more and more. With the cooling designs we have at the present, you can add two stages of cooling making the rating OA/FA/FA or FOA. I am not sure what you mean by true rating of transformer. If you can elaborate on that, may be i can answer your question.


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## ksunny (Jul 19, 2022)

Fault current PU value will remain same but if I multiply PU value with the base current value (I_base=MVA rating/(1.732xV)) for amperage value the result will be different depending on the MVA rating being used .So in this situation should I use OA rating ? 

By true rating I meant rating that can be used to find missing values i.e current on primary or secondary side of transformer.


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## ksunny (Jul 19, 2022)

Fault current PU value will remain same but if I multiply PU value with the base current value (I_base=MVA rating/(1.732xV)) for amperage value the result will be different depending on the MVA rating being used .So in this situation should I use OA rating ? 

By true rating I meant rating that can be used to find missing values i.e current on primary or secondary side of transformer.


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## Warrior PE (Jul 20, 2022)

I might be wrong here but i think fault MVA and transformer MVA are two separate things when it comes to fault calculation. Fault MVA determines fault current, same fault current will flow through the transformer.


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