I think I see the line you are describing. Not a single 1500MW line but three parallel circuits due to the power limitations of available technology. Still a single functioning line, just not what I was imagining.
As I linked in post 49, they used High-Pressure Fluid Flow (“HPFF”) 345 kV transmission line technology. All three phase cables (per circuit) are installed inside a
single steel conduit. The conduit is filled with a pressurized dielectric fluid. It requires cooling systems with fluid flow / heat exchangers to dissipate heat.
We have a similar concept but lower power system here operating between Houston and Galveston, but it is 69kV in an oil bath. I’ve been in the building for the Houston end of the line.
These type installations would be significant complicated through some of the more mountainous terrain required for parts the Northern Pass Line.
Examples:
http://northernpass.us/assets/south%20kinsman%20south%207.03.13%20optimized.pdf
http://northernpass.us/assets/community-pdfs/bald%20peak%20south%20%20optimized.pdf
If they cam run a gas line from Nova Scota I don't think a power line would be that difficult if the telemetry is in place.