How can there be a zero sequence current in a single line to ground fault with an ungrounded neutral
The parasitic phase-ground capacitance is a load in all three sequence networks. Even though your zero sequence source impedance is infinite (delta), the capacitance provides a path for sequence current to flow.  In the figure below i show the 3 sequence networks for your system.  The red lines show how they are interconnected for an A-phase to ground fault.  This should help.



If you calculate the B &amp; C phase currents in the parasitic capacitance (from the sequence currents you calculate with the above circuit) you will see the phase current path clearer.  



UPDATE:  Added further explanation on calculating the sequence components and phase currents in the fault, in the ph-ground capacitance, and from the transformer. Note the reference arrows i chose (if you use opposite direction just flip phase angle by 180):