Oil filled Transformer Ambient Derating- ANSI C57.91

[cherryg222]

This is regarding Oil filled Transformer derating for ambient higher than 30degC as per IEEE C57.91.

The specified values are 1.5% for each deg C rise above for Self Cooled and 1% for each deg C rise for Forced air Cooled.
If I have a 60/80 MVA ONAN/ONAF Xmer, and the average max is 45degC, does it mean that ONAN rating of 60MVA needs to be derated by (45-30)x1.5=22.5% and the ONAF rating by (45-30)x1=15% ????

Or in case of ONAN/ONAF rated Xmers, both the OA and FA ratings are to be derated by 1% per degC rise?? i.e 15% derating for both 60MVA and 80MVA?

Need clarification on the above. Thanks.

 

[cuky2000]

Cherryg222,

Here are a couple of comments to consider:
The range of trans operating temperature per the standard is: [highlight #CC0000]+40ºC[/highlight] < T < [highlight #3465A4]-20ºC[/highlight]
Temperature Rating: 24hour average = 30º C

The ambient temperature of [highlight #EF2929]45ºC[/highlight] exceeds the standard operating range.

SUGGESTION: Consult directly with the manufacturer to verify if there is any limitation for high temperature and get advice for derating the transformer beyond the standard rating.

 

[magoo2]

In my opinion, you can operate a transformer in a higher ambient than 40C provided you keep the insulation hot spot temperature under 110C. The hot spot temperature is the sum of ambient, average temperature rise and hot spot over average, or most commonly, 30C + 65C + 15C. So if we bump the average ambient up to 45C, we must lower the average winding rise to 50C. This means that we must reduce the loading such that you only get an average rise of 50C in a 45C ambient.

 

[cuky2000]

Hi Magoo2,
What should be the derating factor to determine the max. MVA for ONAN & ONAF modes for an ambient temperature of 45oC?

 

[cherryg222]

magoo2...Isn't that precisely the reason that ANSI/IEEE is derating the transformer at the rate of 1% per 1degC rise above 30degC reference (for OA/FA transformers)? Here for 15degC rise, the derating will be 15%. 60/80MVA @ 30degC ONAN/ONAF will be derated to 51/68MVA @ 45degC.

 

[magoo2]

The transformer derating can be approximated with the use of a computer program based on the ANSI loading guide. This is preferred rather than rely on a rule of thumb like 1% or 1.5% per degree C.

The best bet would be to refer the OP to the transformer manufacturer since the realistic answer depends on the specific transformer design and most transformer manufacturers have the capability of running the loading guide equations for their specific designs. I hate to pass the buck, cuky, but the manufacturer would have the specific design details available to get a more accurate assessment than I would.

 

[magoo2]

cherryg222 - I just saw your post when I submitted my response to cuky2000. If you are comfortable with a rule of thumb approach, then go for it. I really don't like it.

 

[cherryg222]

magoo2 I understand what you are saying. The approximation can be followed but later validated by Transformer vendor. If the derating is within a reasonable margin of 10%. it can easily be accommodated. If the derating factor is more than 15% +10% of 15% = 16.5% then it means that we may have to bump up the ratings. A close check for each rating is required. Thanks to magoo2 and cuky2000 for your help.