Fundamentally, the transformer shouldn't need to be oversized, since the VSD (if it has an active regenerative grid-side) should operate with minimal harmonic currents, almost unity power factor, and almost zero unbalance.
However, you might want to overrate the xfmr just a bit. This might be due to
- temperature (what is the operating environment/cooling?)
- grid voltage. If this is a bit low at your site for any reason, then the VSD will still demand up to pu power and this could lead to currents >1pu
- grid voltage/freq. If grid voltage every goes high, or frequency every goes low, then perhaps overrating the xfmr a bit (in voltage, and hence power for the same current rating) is a good idea, to avoid saturation.
Dr. Andrew Roscoehttp://personal.strath.ac.uk/andrew.j.roscoe