We're well beyond the scope of the NEC here; perhaps the answer is best found in an NESC forum.
After all, with two generators, how can you monitor for phase loss?
I suspect the customer lacks faith in nhis PoCo because of power quality issues. In this day of varieble frequency drives, induction heating, UPS systems, fluorescent lighting, etc ... I am personally of the opinion that delta systems in general, and ungrounded deltas in particular, is long gone. The guy is experiencing voltage swings because .. he is creating them!
I'm not clear on the theory, but it appears that the neutral of a wye system does far more than just give you 277v lighting.
Full disclosure is in order here. My employer. based in part on my opinion, has committed millions to convert part of out facility from ungrounded delta to grounded wye. Talk to me in two years - I'll either be the smartes kid in town, or unemployed!
