I note that directional drilling is now very advanced, and gas wells are routinely drilled more than 2 km deep, and can turn through a desirable rock formation at depth (normally, this is for "fracking"). Such a well can be drilled for less than $1 million in most cases, and I think it would be highly desirable to use this technology for HVDC ground electrodes. I envision a ground electrode which is ELECTRICALLY INSULATED from the ground down to about one km, and which turns horizontal in an underground saline aquifer (which are found nearly everywhere on Earth at some depth). This method of grounding would eliminate ground currents near the surface, and might well enable a more economical implementation of HVDC projects. Has this been tried anywhere in the world as far as you know? Is there some issue holding this up technically, or is this simply a case of technology momentum?
The larger converter stations > 400MW will still be bipoles, but with grounded neutrals. This allows an imbalance of the two poles, which will be used to re-balance the current in the two major bipoles at the point where the +/- 800kV tap into the main loop occurs.
I had this informative interaction with Stig Nilsson on this topic:
From: Roger Faulkner