Suppose I want to design a Generator for a 210MW power plant.Should we have to fix voltage first,Any thumb rule for it.How much addition we have to design,Is it 30% addition of 210mw?
you do not design this. You specify this in a design contract you let out for bids to Brown Boveri, Westinghouse, etc and other large generator makers.
You start with the power you need now and the power you will need in 50 years assuming the growth and you get the design company work out a few scenarios where you build the building and pads for the 50 year plan and populate it with equipment in 10 year phases