HI! I am working to implement zero sequence blocking transformer as well.
If we consider PowerFactory simulation:
1. There is no zero sequence model for vector groups, not having Y grounded in the secondary, It can be confirmed by changing the zero sequence impedance values. The results won't change.
2. From PowerFactory's version for unbalanced load flow, zero sequence current, rather flows in the secondary side and not the primary side.
If you are still working with Unbalanced Load Flow or just dealing with Zero sequence blocking Transformer Simulation. You are welcome to collaborate with us at pandapower, an open source power flow software developed by scientist of University of Kassel anf Fraunhofer IEE.