how do I calculate potential gradient in 132kv substation?

Question

to limit corona discharge

Answer 1

I think you have asked the wrong question . To try to eliminate it make the part of the insulator nearest earth a larger diameter.
You cant eliminate anything by calculation. You do your calculation and modify your design if necessary.
If you have just one disc shape then the voltage gradient is uniform across the thickness of the disc. if you have two discs then 2/3 of the voltage is dropped across the first disc. So to improve the gradient keep adding disc and increase the diameter.
Most insulators tend to be roundish.

Answer 2

Wow, if you don't know, this is not the place to ask.