Can One Can Do Substation Designing Course Online?

But, the real course is on the field.

To my knowledge, no (it is a very specialised type of training). This is the sort of thing you need to go to University (not online) and do (likely by getting an Electrical engineering degree specialising in power transmission and distribution, then possibly a separate specialised course in substation design if it is available).

From my experience a substation designer is generally just a person with an Electrical engineering degree (specialising in power transmission and distribution preferably) who has at least some experience and has a good grasp of the associated electrical and civil standards relating to substation construction.

Another name for a substation designer is "electrical consultant".