What is meant by ONAN fullform in power transformers?

Greetings and WELCOME to the site, seshaiah!

I moved your question out of Introductions to this forum topic area so that it will be more likely to be noticed and addressed by those who are best able to answer it. Your question will get noticed much more here. I was unable to find another section that might be more appropriate then this one.

Introductions is for people to introduce themselves only, and we try to not ask questions there. We would appreciate it if you would return to Introductions sometime to tell us a little bit about yourself, if you would be willing to do that.

Concerning your question, it's my understanding, that ONAN means, Oil Natural (thermal syphon action) circulation with Air Naturally circulated for cooling. Or, in short, Oil Natural Air Natural.

Is that the information that you were wanting?

Thanks!