Dispelling some vicious rumours, regardless of which way power flows through it, by ANSI convention the higher-voltage windings remain with “H” terminals and lower-voltage windings remain having “X” terminal designations.

A problem with 480V ungrounded-delta systems can be potentially disruptive ground-overvoltage occurrences. If 480Y/277V service is needed, there are 208∆ - 480Y/277 drytypes available in 15-750kVA that are intended for stepup service. As opposed to solidly-grounded 480Y, a high-resistance-ground system may also be employed, but it may not serve and ø-n loads, and temporary excursion to 480V on unfaulted phases must not cause undesirable operation in the served equipment. It is crucial that surge-protective device ratings accommodate neutral shift that may occur for insulation degradation at any place in the local 480V system. Remember that transient or resonant ground overvoltages can cause temporary misoperation and sustained insulation damage if allowed to persist. Ther is a very good reason why 480Y/277V systems have been recommended in the US since 1955.

       [Linked Image from 6l6.net]
[Note in ∆Y-stepup operation—delta “X” terminals and wye “H” terminals.]

If a 208Y winding is used on the transformer ‘source’ side as with a more common 3ø drytype, the XO terminal is usually best left unconnected to minimize undesirable neutral circulating current.




[This message has been edited by Bjarney (edited 03-10-2004).]