The majority of the primary distribution systems in my part of the country (ND, SD, MN, IA, MT) are grounded wye variants (singlephase, "open wye", or 4-wire wye).

However, here and there, I do find true delta distribution systems as well as the rare delta-connected transformer or transformer bank on wye-connected lines, and those will be linked via the keywords down below:

2400V / 7200V / 7620V / 12.5kV / 13.2kV / 13.8kV / 14.4kV