Transformer Selection

2 X 4MVA to give redundancy to the system.

the cost of those transformers should be take in account! 4MVA covers the peak load and you get 0.5MVA extra (is not much but you can in the future plug a few more machines) 2 transformers will rise in others costs as well, such construction and maintenance of facilities, between others still. thats my choice- nº1

Since 3.5 MVA is peak load, you can also opt for say 2x2.5 MVA or 2x2.0 MVA depending on base continuous load. Such an arrangement ensures base loads are met, in the event of failure of one of the transformers.

be careful in choosing your KV and MVA...you might end up with high fault current level...choose the right kV and MVA...

since you do not wish/need to use the transformers in parallel - having 2 is better to limit your fault levels . . . you should still have a bus coupler so if you are operating at less than half load you may save on the fixed losses of one transformer . . .

Even though the initial expenditure will be more I strongly recommend for 2 MVA transformers with bus coupler arrangement as it will be much more convenient and maintenance . There will be saving in recurring losses and can avoid power outing for preventive maintenance and all the more essential if a continues process industry which needs un interrupted power supply..