Substation Grounding Evaluation

I asked the question below, but did not tag it "grounding" so I am re-posting it with a pathfinder tag that includes grounding.

Our utility uses a wye-delta-wye at substation transformers. The low side wye is grounded and the hi side wye is not. I believe that since it is a grounded wye on the low side, faults inside the substation have a current division factor (ref IEEE 80) of "0", or Sf=0. For the hi side, our system does not use shield wires nor counterpoise UG wiring along the transmission corridors and so the only path back to the source is the ground. So, for the high side faults the current division factor is 1.0, or Sf=1. Is this correct?

We use ASPEN One-Liner to calculate the anticipated fault currents at the high side of substation transformers. The substation grid resistance was not part of the ASPEN calculations. When we get a figure for the grid resistance of our substation, can we simply add this resistance to the point where the original fault current was determined, let ASPEN recalculate the fault current with this new resistance in series and then multiply this new fault current times the grid resistance to get our grid GPR??