This equivalent circuit is shown below for the D secondary side of the transformer. I splits as shown in the diagram above. From this point, knowing all three winding currents inside the delta, they can be reflected to the primary side by proper turns ratio. Find I A , I B and I C , the three primary line currents. The above phasor diagram is drawn with the convention adopted in page 1 of these notes.
D scheme can supply without thermally overloading the remaining two single-phase units.