Where do you ground a delta delta transformer?

Answer 1

In a delta-delta transformer, none of the actual conductors are grounded, directly or indirectly.

You do, however, always ground the casing. This is protective earth ground, or PE ground, as opposed to neutral, which is grounded at the distribution panel, but which is a current carrying conductor. PE ground is not a current carrying conductor - it is a safety ground. Some texts use the term grounded conductor to refer to neutral, and grounding conductor to refer to PE ground. Simplest designation is neutral and ground.

In summary, for a delta-delta transformer, there is no neutral, but there is always a ground. There is always a ground in any circuit, even when there is a neutral, and, except in a few special cases, those are two different connections.

Answer 2

First and foremost, at the frame.

Second, since there is no ground path in open delta, due to Kirchoff's Current Law, it sort of does not matter where you ground the secondary. It makes sense to pick a point where the maximum hot to ground potential is minimized, and that would be from the center tap of one of the windings, if one was provided; otherwise, you could ground one of the hot legs. That sounds odd, however, so maybe you could construct a wye network with resistors or three inductors and ground the center point of the phase triangle. It is dependent on whether or not you are using a wye-to delta start-run sequence. I would say, that in that case, the ground point is clearly the center of the wye.