Try the following simplified short cut method:
Isc = Ifl/Zt
Were:
Ifl=KVAtransf/1.73xKV (Transf full load current)
Zt = Z%/100 (Transformer impedance)
Be ware that this method consider an infinte source on the primary side and do not include motor short circuit contribution from the secondary side.
If you should be interested in more detail calc. consuly the IEEE Std 141 and 142.