zig zag vs open delta
Zig-zag transformers (also called interconnected-star or interstar) are nearly always used as earthing transformers. These are used to create an artificial neutral point for earthing a delta-connected system (which normally has no neutral point.)

Refer - ABB Transformer Handbook (2e) Section 2.5.6. Earthing Transformers (Neutral Couplers) or the J&amp;P Transformer Handbook 12e Section 6.2, Neutral Earthing.

Open delta transformers might be another way of doing the same thing. I can't find any references to people doing it this way. The disadvantage would be that one phase would be tied to earth, and the other two phases would be at full phase-to-phase voltage relative to earth.