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HVDC line grounding system question

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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: https://goo.gl/maps/bnsYcbv9Q3ewmkG49  where the power 
line ROW runs diagonally on the right side, and the actual grounding
point is the weird circular shape at the upper left.  Additionally,
multiple conductors (6) run from the ROW to the circle.  The street
view at https://goo.gl/maps/K94ZceiRfUL2ePaU7 near the grounding poing 
shows the huge towers as well as 4 conductors, two at the top of the 
towers and two others on wooden poles.  They are on rather substantial 
insulators.

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.  https://goo.gl/maps/LGNiEYMcC7rrtjMQA

> the other two on the wooden poles seem to
>carry too high a potential to being ground wires. they probably could be
>a MV feed for a nearby town.

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: https://goo.gl/maps/UQTnoFfkAh9hP6zu8, the actual HVDC line goes from
top right to the bottom, and 6 individual lines go to the weird circular
area to the top left.

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?

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HVDC line grounding system question moroney@world.std.spaamtrap.com (Michael Moroney) - 2021-01-04 16:09 +0000
  Re: HVDC line grounding system question Joe Gwinn <joegwinn@comcast.net> - 2021-01-04 14:14 -0500
  Re: HVDC line grounding system question legg <legg@nospam.magma.ca> - 2021-01-04 15:01 -0500
  Re: HVDC line grounding system question upsidedown@downunder.com - 2021-01-05 08:13 +0200
    Re: HVDC line grounding system question Cydrome Leader <presence@MUNGEpanix.com> - 2021-01-05 21:38 +0000
      Re: HVDC line grounding system question legg <legg@nospam.magma.ca> - 2021-01-05 18:15 -0500
  Re: HVDC line grounding system question moroney@world.std.spaamtrap.com (Michael Moroney) - 2021-01-06 06:59 +0000

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