65 Deg C vs. 55/65 Deg C Power Transformer Ratings

[ThePunisher]

HI all,

We have specified ratings as per ANSI at 65 Deg C Rise and our client asked us to specify dual 55/65 Deg C rises to take advantage of the extra capacity.

For example, taking a 15/20 MVA ONAN/ONAF transformer with a 65C rise and changing it to a 55/65 would change the rating to a 15/20/22.4 MVA. This is still ONAN/ONAF with dual ONAF capability right? OR is it ONAN/ONAF1/ONAF2?

I looked into ANSI C57 but am not able to find a direct rating comparison for both 65 and 55/65 rises and is giving me some confusion.

I have two other transformers with standard 65 Deg C rise rating: 25/33.3/41.6 MVA and 1/1.33 MVA. if I will apply the same factor, will the 55/65 deg C counter-part be 25/33.3/37.5 MVA (or 25/37.5 MVA) and 1/1.5 MVA respectively?

Thank you all for your guidance.

 

[ThePunisher]

And by the way, it is for Canadian application and hence, the ONAF will always by +33% of the ONAN 65 Deg C rating.

 

[dpc]

It's going to be whatever the manufacturer puts on the nameplate. If it has one stage of cooling fans, it should have four ratings - 55 deg C rise and 65 deg C rise ONAN and likewise for ONAF.

 

[prc]

The 55/65 C average winding rise is a US legacy. The difference in kVA rating will be 5-9 %.C57 completely changed over to 65 C rise in 1977 as Industry practice. In US 65 C rise came in to use in early 1950's with the use of thermally upgraded paper for conductor covering. But Europe went ahead with 65 C rise with normal kraft paper covering.

 

[111R]

The standard applied factor is 1.12 to convert from load with 55°C rise allowed to load with 65°C rise. This is done for every cooling class rating of the transformer. 15/20/25 at 55°C becomes 16.8/22.4/28.0 at 65°C.