while studying the purpose of tertiary winding in the this 3-winding transformer , I'hv gone through a statement that " the zero sequence component caused by line-to-neutral load on secondary side cannot be balanced by primary currents as the zero sequence currents cannot flow in isolated neutral star connected primary". Now first of all I wanted to know that the currents in secondary are balanced by primary as this is the basic thing I am not getting this and why can't primary currents balance the zero sequence component ?? And secondly why can't the zero sequence current flow in star-primary?? I am missing some basics , so please elaborate it..??