Why this combination is the default
The wye secondary gives a neutral, and a neutral is what a building needs. From a 480Y/277 secondary you get 480 volts between any two phases for equipment and 277 volts from any phase to neutral for lighting. The delta primary has no neutral and does not need one, because the utility side is a three wire system.
The delta traps harmonics
Third harmonic currents and their multiples, called triplen harmonics, are in phase on all three conductors. In a wye they add up in the neutral. In a delta they circulate around the closed triangle and never leave it. Putting the delta on the primary keeps those currents out of the utility system, which is the second reason this arrangement won.
The 30 degree shift
Delta to wye introduces a 30 degree phase shift between primary and secondary. It does not matter on a single transformer feeding a building. It matters enormously if you are paralleling two transformers, because units with different vector groups cannot be paralleled. Check the vector group, normally Dyn1 or Dyn11, before assuming two units can share a bus.
How to read the terminals
H1, H2 and H3 are the high voltage terminals, on the corners of the delta. X1, X2 and X3 are the low voltage terminals, at the ends of the wye legs. X0 is the neutral where the three wye windings meet, and on a pad mounted transformer it is bonded to the tank and to the grounding electrode.
Common questions
What is a delta wye transformer
A transformer with its primary windings connected in a closed triangle and its secondary windings connected with one end of each joined at a common neutral point. It is the standard connection for distribution transformers because the wye secondary provides the neutral a building needs.
Why is delta wye used for distribution
Two reasons. The wye secondary gives a neutral, so you get both phase to phase and phase to neutral voltage. And the delta primary circulates triplen harmonics inside itself rather than passing them back into the utility system.
What is the phase shift in a delta wye transformer
30 degrees between primary and secondary. It is irrelevant on a single transformer but it prevents paralleling with a unit of a different vector group, so check for Dyn1 or Dyn11 on the nameplate before paralleling.
What do H1 H2 H3 and X1 X2 X3 mean
H terminals are the high voltage side, X terminals the low voltage side. On a delta wye unit the H terminals sit at the corners of the delta and the X terminals at the outer ends of the wye legs. X0 is the neutral point.