Transformer rated power in Powersystem
Sn is the nominal apparent power. It is given by the nominal voltage Vn multiplied by the nominal current In. That would be multiplied by the square root of 3 for a three-phase transformer. You have Vn for the primary and secondary. The transformer rating plate or purchase specifications would state Sn, Vn, In and other parameters that state the operating conditions for which Vn, Sn and In are valid. There may be more than one  value stated for Sn etc. for different operating conditions.