Transformer Question

Without knowing the ratings, impedances, and vector group of the transformer and the system/protective relaying to which it will be connected; it's only a guess.

Sorry not to give some details

size 30/40/50 MVA

Z%=7.5

DY1

solidly grounded

The best way to determine that is to contact some transformer manufacturers. You don't say where you are located in the world, but there are multiple manufacturers of large power transformers, and they probably have done something like that. You also don't tell what rating the transformer must be (kVA or MVA size) and if the windings should be delta or wye configuration. I have seen smaller power transformers that have reconfigurable secondary windings that allow multiple voltages. However, there may be capacity limitations at some of the voltage taps - that's something to discuss with the manufacturer. I suspect it mostly depends on how much you are willing to pay...

You can maybe just add a step-up transformer....

You said:

"Currently we will use 13.8 and in the future upgrade the cables and use 25kv."

It seems to me you have already your 138/13.8 kV transformer. For the future if you'll need any way another transformer order a standard 138/25 kV one. This will be cheaper I think.

If the future means over 10-15 years then you will need a new transformer, probably.