I need to install a step-up transformer from a 240 panel to get 480 volts for 3-phase equipment. No 277 required. However, in order to avoid the debate about grounded v. ungrounded 480 v. 3 phase 3 wire delta, I'm thinking about specifying a 277/480Y secondary and grounding the neutral, but not using it.
If you're worried, why not just use a delta-wye transformer, ground the secondary neutral, and just not run a 4th wire? If you don't have to worry about phase shift, it's really the way to go.
Though the coils in a delta secondary aren't grounded, the core is, and magnetic core conductance and reactance will hold a delta secondary near equal potential, especially if the phases are balanced. Think of it like a wye-wye with a high impedance ground on the neutral.
I hesitate to corner ground the delta because 480 to ground concerns me. However, I'm not sure I want to float the delta either, hence my question.
However, a delta wye step-up xfmr seems to be special order and costs a premium. I will let the customer decide.
The other question...
The xfmr rep. says that although their step-down xfmr can be connected backwards, the inrush will be 10X primary current. A true step-up xfmr inrush would be 4-5X primary current, but a true step-up xfmr would cost more.
I am concerned about tripping primary OCPD if I connect backwards.
If you don't want to ground a delta secondary solidly but are concerned about just letting stray capacitances balance out the voltage to ground on the phases, how about using a bank of three resistors to provide a ground reference?