% Loading of ONAN Type Transformer

Between 0 and 100% of its kVA rating?

As per standard we can load maximum upto its full load capacity.!!!

It depends on how long you want the transformer to last.

Assuming regular maintenance and testing, you should be able to attain its design lifetime (30 years? 40 years? Depends on what was specified at design) when operated at up to 100% capacity. You can operate it over its rating, but the associated temperature increase will deteriorate the insulation system more quickly, and you shorten the life of the transformer. How much you shorten it depends on the level of overloading and how well you maintain the transformer during its life. Lower operating loads and temperatures have the opposite effect - there is less deterioration of the insulation (although you cannot eliminate it completely).

With regular maintenance and keeping the load below maximum rating, you should get many years of good service out of a well-designed transformer.

If power quality is not good(harmonics) use K-rating.