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:
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?