Insulation resistance

Dear,

This is the justification you have obtained about the insulating material behaviour. This is what it shows that the insulating material's resistivity is not only depending upon temperature, moisture absorption capacity, but also upon the operating voltage - where, the molecules of the material are polarised gradually.

So, there is an index called Polarisation Index, which you may study well in any IS standards / IEC standards.

If you are having motarised Megger, try this ....

Polarisation Index = (Megger value after 15 miniutes /initial Megger value )

I had tried this for transformers with 2.5 KV Megger.

For new transformers, may be 1.5 - 2 ( If less, needs maintenance )

For in service transformers may be 4 - 6 ( If less, needs maintenance )

It isn't surprising.