Question: how to select vcb rating?
Answer: A 500 kVA transformer will draw at full load nearly 700 Amps per Phase at 400 Volts. The primary current at 11 kV is 52.5 Amps / Phase.
To allow for overload a factor 1.6 has to be taken into account.
The VCB has to be rated for 100 Amps/11 kV minimum.
An electrical engineer will do the exact calculations for you.