This is not the normal situation where a delta-wye isolation transformer is being reverse connected. Rather powerplay is discussing a wye _autotransformer_. As an autotransformer there isn't a separate primary and secondary winding, and it can be operated to buck or boost the supply.
One question: is this connected to an _ungrounded_ supply, or simply the three phase legs of an otherwise grounded supply?
Normally this wye autotransformer would be connected to a wye supply and the neutral point of the autotransformer would be left floating. The wye of the autotransformer would be at approximately neutral voltage, but not exactly. Connecting the neutral of the supply to the neutral of the autotransformer could cause significant circulating currents.
Connected to an _ungrounded_ 480V supply, the result would be an ungrounded 600V output. Not particularly more or less dangerous than any other ungrounded system.
Connecting the autotransformer neutral to ground wouldn't give you a good 'solidly grounded' system, since a wye transformer has relatively high impedance to the neutral. You need a slightly different transformer to operate as a 'grounding transformer'.
One type is a 'zig-zag' wye transformer, and I suppose it would be possible to create a combined zig-zag grounding transformer and boost autotransformer.
Another approach is to have a 'tertiary' delta winding, no connected to any load, but one which permits circulating currents in the event of a ground fault.
-Jon
