Wye-Delta transformers are natural grounding transformers, provided that central star point is connected to ground. Wye windings sould be connected to side to be grounded. Zero sequence currents to ground are looped by the opposite delta windings.
Zig-zag transformers need only one winding to play as grounding ones, in such a way that in some cases they are not strictly transformers, but three-phase coils Julian
The wye-delta connection for grounding is deceptively simple. The delta winding is left floating (or grounded at one bushing only). The wye side is the “ground source” the same way that a zigzag would be. [Imbalanced] currents at the wye-side neutral point will cause a circulation of current in the delta ‘ring.’ In some cases this ‘feature’ becomes a ‘bug’ of sorts; for instance where a distribution circuit serves has multiple transformer banks with grounded-wye primaries. Both become sources for zero-sequence current but, IIRC the zigzag configuration uses less copper and steel for an otherwise equal rating.
