The question is sort of a can of worms. There are no easy answers for there are many electric utilities in North America, and almost as many standards. One “committee” description is ANSI/IEEE
…Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution described at
http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.92.4-1991_desc.html Abstract: “The neutral grounding of single- and three-phase ac electric utility primary distribution systems with nominal voltages in the range of 2.4–34.5 kV is addressed. Classes of distribution systems grounding are defined. Basic considerations in distribution system grounding—concerning economics, control of temporary overvoltages, control of ground-fault currents, and ground relaying—are addressed. Also considered are use of grounding transformers, grounding of high-voltage neutral of wye-delta distribution transformers, and interconnection of primary and secondary neutrals of distribution transformers.”
SCE is a good example of a utility having various distribution-grounding implementations. For years, there has been a lot of ungrounded-delta 4800V, but also plenty of more modern 12,000V and 16,500V {and up} solidly-grounded wye circuits. Systems that are solidly grounded do not automatically have phase-to-neutral-connected distribution transformers.
