Analysis of three phase star star connected transformer
No current (other than that allowed by the magnetization branches as Andy points out) flows through your load because of the lack of neutral on the primary.  With b &amp; c phases on the secondary open, they have infinite load impedance.  Reflecting that back to the primary windings of B &amp; C and you see you have infinite impedance in series with each.  So, there is no path for A phase current alone without a neutral present.
If you want to look at this from a symmetrical components perspective, the open neutral isolates the primary side zero sequence network from the secondary.  So, only positive and/or negative sequence currents can flow through the transformer.