I posted this to alt.engineering.electrical and it was suggested I post this here since this group is more active than alt.engineering.electrical and with knowledgeable people.
(somewhat edited)
There is a long HVDC power line from northern Quebec Canada to Ayer, Massachusetts USA. It operates at either +/- 375 kV or +/- 450 kV depending on source. As I understand it, it is grounded at exactly one point, near Saint-Claude, Quebec. You can see it on this Google satellite view:
I don't think the lines on top of the towers are lightning arrestors. They are insulated from the towers while lightning lines aren't. In addition, near the south end of this line (near Ayer, Mass.), this line has an insulated conductor as well as a pair of (uninsulated) lightning arrestor lines.
They are not a feed for a town.
I followed these lines to the circular ground point using Google Satellite View zoomed in. It showed enough detail to see the towers/poles (and their shadows) and the lines themselves. They are two of the 6 conductors which go there. Two others are the two conductors on top of the pylons. It appears the other two are also the two conductors on top of the pylons but from the ones continuing past the point where the lines go to the circle.
See:
Does anyone know any details for this rather odd setup? Why multiple grounding conductors, and any details of the circular structure? Do the grounding conductors sometimes carry current during normal operation, perhaps with the line at half power with one side of the HVDC conductors out of service and the grounding line carries the return current? Meaning it's more like the neutral wire in home electrical wiring, carrying current and connected to the actual ground at only one point?