Spark...
Have a couple Qs per your Scenario...
Q #1: Is there a Three Phase System available to drive the Primary, or only Single Phase?
Q #2: Will the Transformer be "Reverse-Connected", with the Delta Winding being the Secondary; or will the Transformer be connected Delta Primary / Wye Secondary?
Q #3: Will there be only (1) Single Phase L-L Connected Load, or is it possible to make Two (or possibly Three) individual Single Phase L-L Load Connections?
--------------------------------------------------------
Per "Q #1":
If the Currently Active System is 3 Phase 3 Wire, I suggest to connect the Primary to all Three Lines, even if the Secondary side will connect via Two of the Three Windings.
If only Single Phase is available, suggest either using another Transformer, or attempt to reconfigure the Transformer to appear as Open Delta - provided the Windings are on individual Cores, as opposed to being wound to a Single common Core. The latter of these is most common.
(I need to do some thinking on this part - the Single Core would be normal for a Split Coil Transformer, but the unused Windings have me concerned).
.........................................................
Per "Q #2":
If the Transformer will be Connected as Wye Primary, Delta Secondary, I was thinking to feed the Primary with all Three Lines, then Corner Ground the Secondary.
Grounding the Secondary is something to consider, in order to keep the Voltage to Ground stable (reduce the issues of Insulation Breakdown).
.........................................................
Per "Q #3":
If there are / will / can be Two or more L-L Load Connections, then these may be split up between the Three output Lines.
--------------------------------------------------------
In my opinion, the most simple way to drive a Single L-L load via the available Transformer would be to connect the Transformer to a Three Phase 3 Wire Circuit, and use Two of the Three Secondary Output Lines to feed the L-L Load.
bond the Wye Connected Secondary Windings' Star Point ("X0") to a Grounding Electrode System, and derive an Equipment Grounding (Bonding) Conductor from the same Star Point.
Driving the Primary via an L-L Connection (as opposed to L-L-L Connection), would create the same Voltage issues as would be found with a Blown fuse in One Phase Line.
Additionally, attempting to reconfigure the Transformer may be Counter-Productive.
Feeding the Primary via a Three Phase L-L-L Branch Circuit will drive the Delta Windings as normal.
Let me contemplate this scenario in more detail, along with verify some points via text manuals.
-- Scott (EE)
