The secondary usually has to be grounded somewhere too, with a few limited exceptions. Where you ground this can make a lot of difference what the phase to ground will be.
I think this is why they use 3p wye a lot on 480. It keeps your L/G down to 277.
If you corner ground a delta you get 480-480-0 but if you centertapped it you get 240-240 and a 415v "wild leg".
