Transformer Overload Capacity

[Humble2000]

Could anyone refer me to a source which I can find info on xfrms overload capacity (especially liquid fill) based on their temperature rise capabilities?

I am told that 55deg xfrm overload is different from 65Deg and
55/65 deg transformer.
one is 112% the other one is 133%.
For long time pick up setting of an overcurrent protection on the secondary side of a transformer.

Thank you

 

[Marmite]

It depends on a lot of things, such as ambient temperature, load cycle, initial loading, type of insulation, size of transformer, method of cooling ie forced or natural, how much life expectancy you want to sacrifice etc. As a first pass have a look for the specification the transformer was built to, or ask the manufacturer. GE Power produce a handy little guide called the Distribution Transformer Manual GET-2485R which has lots of information on overloading transformers. I'm not sure if it's available electronically though.
Regards
Marmite

 

[jghrist]

ANSI/IEEE Std C57.91, IEEE Guide for Loading Mineral-Oil-Immersed Transformers

 

[DanDel]

You can find requirements that will allow you to provide protection up to 250% of the transformer's FLA.

At one time, I had a pamphlet from ABB (I think) regarding their recommendations for protection of their transformers. It showed the short-time overload ratings near that value, but for only several minutes or so. My suggestion is to find out your transformer manufacturer's recommendations.

Unless you have overtemp protection, your overload protection should be much closer to the FLA unless you want to void any manufacturer's warranty or disregard their recommendations.