Comment posted to The Ecomomist story on ABB's new HVDC circuit breaker

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A continental scale supergrid must be DC, in that I agree. The option that gets most of the press in the US are superconducting lines, which have a maximum feasible voltage of ~130kV; this limitation is due to the difficulty of electrically insulating the transition back to conventional wires that is necessary to get power onto and off of the superconducting system. Because of the voltage limitation on superconducting lines, any superconducting system must be an entirely new system; it cannot "play in the same sandbox: as HVDC. The reason superconductivity gets all the press in the US is that there are powerful advocates in the US Department of Energy, American Superconductor, and SuperPower for example. Most of the attention has gone to type II superconductors "high temperature superconductors" (HTS), which are generally ceramics that are barely conductive at all above their curie temperature; HTS has the advantage that it can be cooled in liquid nitrogen rather than liquid helium (as is required for most Type I metallic superconductors). These entities always point to the steep decline in cost of HTS superconducting cables and claim that cost parity is right around the corner. This conveniently ignores the fact that cost parity in terms of cable cost/kW-km is only one of the critical factors, and not the most difficult one either. Not being able to adopt a common voltage with conventional HVDC is another problem, and the other two are real killers: any high value transmission asset should be highly reliable and readily repairable; in these critical properties, superconducting lines are worse even than underground HVDC cables, which are themselves much worse than overhead lines. The three R's of electrical transmission are reliability, redundancy, and repairability. Both superconductors and underground or undersea HVDC cables suffer from poor repairability.

I have been working on a new, deceptively simple technology for continental scale transmission, elpipes. Elpipes are literally HVDC electricity pipelines (www.elpipe.com) which answer several critical problems; first, elpipes allow much higher transmission capacity of ~30 GW than either overhead lines (for which maximum capacity is ~9 GW/circuit), subsea cables (for which maximum capacity is ~2 GW/cable pair), or land-based HVDC cables (for which maximum capacity is only ~0.2 GW/cable pair). There is no magic here; elpipes do that by using a lot more conductor than can be used in an overhead line.

Elpipes combine features of a powerline, a pipeline, and a train. In effect, an elpipe is a slow train that is also a powerline, and runs inside a pipeline which is in effect the track. The ability of the elpipe to move makes it conveniently repairable and one can also do preventive maintenance, which is utterly impossible for a cable. The movable feature also allows all the critical electrical joints between the rigid "cars" of the train to be made in a controlled clean room environment at one end of the line; not having to make any field splices is expected to improve electrical joint reliability tremendously (this is needed, because the segmented nature of an elpipe implies far more electrical joints than are needed in a cable). The fact that elpipes are installed in conduits that are essentially identical to gas pipelines de-risks the cost of installation compared to overhead lines, which are often delayed due to public opposition. The conduit cost shrinks to small part of total cost for large elpipes > 6 GW capacity. The resistance of elpipes/meter is at least ten times lower than the largest prior powerlines, which enables continental scale transmission.

At present, the elpipe languishes in patent pending status, due to a lack of investment. Meanwhile, a tremendous investment has flowed into development of HVDC and HTS superconducting cables, neither of which is capable of matching the economics and rapid repairability of overhead lines. There is a prejudice at the US Department of Energy in favor of "sexy" solutions like HTS superconducting lines, and the major players in power transmission are too conservative to take such a leap. What is needed here is a powerful visionary; elpipes are the last missing piece of the puzzle needed to enable a supergrid.

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