Insulation Resistance Test

What do you expect i guess the insulation resistance will increase.kindly tell what is the method used for testing. Oil insulation property decreases with more usage(making and breaking-arc) due to the increase in carbon content.

Insulation resistance will vary with temperature. There are correction factors that reference it to 25C.

Search Google for "Insulation resistance temperature correction" and you will get a lot of hits. Here's the first one I got, with a good nomograph:

http://www.uiitraining.com/b51a/200/eer365.htm

Exactly the same as when the oil is cold.

If the transformer cannot withstand the applied test voltage at both ambient and at normal operating temperatures, then it is faulty.