Grounding of 3 winding transformer

[jlazucena]

I have a 3winding-50MVA wye,wye,delta power transformer.

Ho is grounded through a surge arrester, Xo is grounded through a ground resistor and one point of the delta is solid grounded. The delta winding is only for grounding.

I would like to solid ground Ho
How can I model this transformer(Specially the delta grounded)to calculate short circuit currents?

Any comments about it?

Thank you.