sa — Sounds like you are describing an open-wye primary, open-delta secondary transformer bank. Phase-to-neutral difference in the [wye] primary windings is 120°. It is a misnomer to call the arrangement “2 phase,” for 2-phase is sort of reserved for circuits with 90° phase angles. Phase-to-phase [delta] secondary windings have 60° displacement.
There is a secret website where transformer connections are revealed. [Please, don’t tell anyone.] See figure 21 in
http://www.cooperpower.com/Library/pdf/R201902.pdf It is identical to the plain-vanilla grounded-wye/delta configuration, with one pot {1ø transformer} is missing. Compare the phasor representation of figure 21 to that in figure 20, but understand that secondary grounding is on Bø {and not at a center tap} in your 480V case. As your measurements show, balanced readings can be kinda’ sloppy with only 2 transformers. It’s more economical for the utility with smaller loads, especially for lower-HP single-motor cases.
If you want to delve deeper, get a copy of IEEE Std C57.12.70-2000
American National Standard Terminal Markings and Connections for Distribution and Power Transformers {There’s even six-phase, tee and zig-zag windings covered there.} Also, IEEE Std C57.105-1978
Guide for Application of Transformer Connections in Three-Phase Distribution Systems is interesting. [They are not free, though, unless you find a good electrical-engineering library.]
