Thanks for the responses. There aren't any loads being supplied at 5kV. The only published direction from the utility is to have a solidly grounded WYE on their side, 15kV but a new guy wants the WYE WYE interposing transformer.
I've only ever used an interposing transformer on arrangements...
I've done quite a few generator plants that tie into medium voltage utility networks. They ranged in size from 1.5MW to 5MW. The utility side in our area is 15kV grounded WYE. I've always specified the generators with a grounded WYE at either 5kV or 15kV but with a with a neutral grounding...