This are definitely interesting, and it was detected currents in the ground in Oslo when the subway in Stockholm in 1950 was started. (417 kilometers away and supplied with 750V DC)
Still this are considered as a non existing problem.
I'm a educated as a mechanical engineer, but have dealt pretty much with electro...
When we consider our traditions with IT systems, these problems are not of importance .... ????
I see lots of equal sides to your 277/480V systems, and your 208 delta's
