My latest publication is Underground HVDC Supergrid can work in Europe in the March issue of Natural Gas & Electricity, a Wiley publication:(credit as follows: "Faulkner, Roger W.Natural Gas & Electricity30/8, ©2014 Wiley Periodicals, Inc., a Wiley company.").
This article specifically discusses a European supergrid based on HVDC loops.
Over the years that I have pursued the vision of a continental scale supergrid, I have always proposed designs based on HVDC loops with multiple terminals. In the beginning I did not yet fully appreciate that loops are intrinsically redundant, as long as there are main loop circuit breakers between neighboring AC/DC converters linked to the main loop. All the currently installed HVDC schemes rely on the AC grid to supply redundancy in case a sudden generator or transmission line failure occurs. It is this fact that sets the maximum capacity that can be carried by an individual line, and at a much lower level than is feasible for the HVDC links per se.
Creating a true DC grid based on intersecting loops ("meshed grid") requires a DC circuit breaker that can interrupt main loop fault currents, which could go as high as hundreds of GW during a short circuit. To avoid that, I have come to think that superconducting fault current limiters (SCFCLs) are vital in the mix of technologies to protect the DC supergrid. In effect, the SCFCLs protect the circuit breakers by allowing them to be designed for lower (but still very large) fault currents.
The other technology that will be needed for a supergrid in addition to circuit breakers and fault current limiters are flow regulators, which work by adjusting the voltage drop through the regulator. In effect, flow regulators work like throttling valves on a water distribution system, evening out the flow through the various parallel connections of the supergrid. In order to not be lossy, the flow regulator must be hooked up to the AC grid or to energy storage, so that the inline load imposed to create a voltage drop on an HVDC main line is exported to the local AC grid directly or via an intermediate energy buffer storage. (This last bit was not discussed on the Wiley article.)