Power Transformer 55/65 Degree C Rating

[kingjoey]

Please explain what the meaning of insulation temperature rating of a power transformer.

I am confused as to what a 55 Degree C, a 55/65 Degree C, and a 65 Degree C designation really means.

Also, it is my understanding that the lower the temperature rise the lower the rated full load capacity of a power transformer. Why?

Thank You.

 

[jghrist]

Transformer insulation used to have a limit of 55°C. About 40 years ago (I think), insulation was improved so that it would withstand higher temperatures. ANSI changed the limiting temperature from 55°C to 65°C to reflect the change in technology. Some users prefer to keep a 55°C rating to be more conservative.

If a transformer has a 55°C rating, this is the loading that will produce a 55°C hot spot temperature. This is less than the loading that will produce a 65°C hot spot temperature because higher loading produces higher temperatures. Some users want to know how much more they could load the transformers if they took advantage of the newer insulation technology, so require both a 55°C and a 65°C rating.

 

[kingjoey]

Thank you jghrist. Two more questions.
If a transformer is rated with a 55/65 Degree C designation, what is the actual insulation rating? Is it 55 or 65 Degrees C?

What is the actual KVA of the transformer? Is it the 55 Degree C KVA rating or the 65 degree C KVA rating?

Thanks,
KingJoey

 

[dpc]

It's 65 deg C insulation. Also, just to be clear, this is a temperature RISE above ambient, not a actual temperature. In the US, ANSI-rated transformer kVA ratings are based on a maximum average ambient temperature of 30 deg C.

One rating is as real as the other. Generally, I'd use the 65 deg C rating in describing the transformer capacity.

 

[jghrist]

dpc is correct. Also, the 65°C limit is top-oil rise, not hot-spot rise. The hot-spot temperature is 15°C higher than the top-oil temperature at the full 65°C rated load. I should have said that the temperature limit of insulation is 110°C (30+65+15). I also prefer to specify a 65°C rating only to avoid confusion. If you want to be conservative in the loading, then load to less than 100% of the 65°C rating.