I thought the advantage of HVDC was mostly for subterranean and undersea cables. Why would it be better above ground?
HVDC only needs one copper conductor. HVDC has no reactive power loss. HVDC has less corona effect and self-inductance to overcome so conductors can be smaller. HVDC is not subject to grid phase problems.
In addition, since HVDC cabling can be cheaper at a given voltage, the voltage can be increased which minimizes resistive losses (Ptrans = IV while Ploss =I^2R--if you double V you halve I which reduces your losses by a factor or 4).
Capacitive losses. They're worse in water (dielectric), but they're there in air lines as well.
Compatibility and isolation of grids. If grid A is 35 degrees from grid B it's very hard to couple them. Also, cascading failures tend not cross DC boundaries.