First: This is not a question about electronics, it's about large (three-phase) power systems where the components have ratings in kV and MVA. I'm mainly interested in symmetrical components, and how the zero-sequence network is modeled. Although the terminology and simulation tools might differ, the physics should be pretty much the same for small and large components.
I have a three-winding transformer connected YNynd, where the primary side is solidly grounded, the secondary is resistance grounded, and the tertiary winding is delta connected.
In the simulation tool I'm using (PSS/E), three-winding transformers can be modeled, but not transformer with the parameters described aboce. I'm wondering if the following will be a physically correct way to model the system.
Disregard this paragraph (I'll let it be, as this was the original proposal:
If I'm modelling this as a YNdd transformer with solid ground on the primary side, and delta windings with angle 30 deg on the secondary and tertiary sides. Between the secondary winding and the bus on the primary side, I insert a zero-impedance 1:1 two-winding transformer with vector group Dyn where the secondary side is resistance grounded, and the angle reverses the 30 degrees from the three-winding transformer.
New proposal (after Lewis' comments):
Model the three-winding transformer as YNynd1, transformer, with both primary and secondary solidly grounded. Then, I add a 1:1 transformer with the primary side solidly grounded, and the secondary resistance grounded, as shown in the last figure. I'm not sure if the intermediate windings (secondary of three-winding and primary on two-winding should be grounded or not). I can't wrap my head around the zero-sequence equivalent of this (a few years since my university days).
I'm mainly interested in simulating line-ground faults on the secondary side of the transformer, but I obviously want the model to be correct for all other cases. I'm not interested in internal faults in the transformer.