Section 11.2.5.2 of the Program Application Guide: Volume 1 (for PSSE 33.9) gives an in-depth explanation of the effect of neglecting transformer phase shifts.
"Neglect of the inherent 30° phase shift of wye-delta transformers results in the following:
- Calculated sequence and phase currents that are correct in all branches on the fault side of the transformer.
- Calculated sequence currents that are correct in amplitude but erroneous in phase in all branches that are removed from the fault by a transformer.
- Erroneous values of phase current in all branches separated from the fault by a wye-delta transformer.