HVDC line grounding system question

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 right. Additionally, multiple conductors (6) run from the ROW to the circle. The street view at https://goo.gl/maps/K94ZceiRfUL2ePaU7 shows the huge towers as well as 4 grounding leads, two at the top of the towers and two others on wooden poles. They are on rather substantial insulators.
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 carry current during normal operation, perhaps with the line at half power with one side out of service and the grounding (neutral?) carrying the return current?
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???? 3/1/2021 8:03 ?.?., ?/? Michael Moroney ??????:

the ground wires on top of the poles are lightning arresters. they normally carry no current. 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. I read that in HVDC feeds there are no fround wires and the ground current flows through, the well, ground, from the starting point of the transmission line to the end point. and there are cases like submerged feeds where the current has to flow through earth (and I suppose the cable's shield, but that would be very thick for 1200+ amperes).
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I don't think they are lightning arrestors. Near the south end, this line has an insulated conductor and a pair of (uninsulated) lightning arrestor lines. https://goo.gl/maps/LGNiEYMcC7rrtjMQA

I followed these lines to the circular ground point using Google Satellite View zoomed in. 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.
They are not the feed for a town. I have never seen a delta distribution system using the ground as a conductor in the US (although this is Canada)
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On 1/3/21 12:03 AM, Michael Moroney wrote:

You might try the sci.electronics.design group. There are still some knowledgeable people there.
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wrote:

In a DC system I don't think either conductor is grounded. It also looks like the other line on the wood poles is another DC system since there are only 2 conductors, both ungrounded. I suppose it could be a delta distribution but they usually run all 3 legs on a delta unless it is just a run down a residential street. I don't see much medium voltage single phase derived from delta but I assume someone might do it. It is usually wye (one hot and a neutral) in Florida
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