Greenpeace

Let's assume that this technology is practical as stated. It may not be but let us assume that they can transmit the amount of power they say under normal conditions.
Jet engines sucessfully power planes every day of the week but let a few birds get sucked in and all the suddend your landing in the Hudson river.
Probably the most important question to ask is what happens when someone accidently breaks a cable with a trenching device or a terrorist's bombing attack?
The second question to ask is what happens if the supply of liquid nitrogen is accidently interupted or if there is an overcurrent which causes the superconductive properties to revert to normal resistive properties?
With the current technology if a line is broken the damage is usually confined to the immediate area because the curcuit breakers trip before enough heat is generated that would permanently damage the whole cable. (During an overcurent on a cable feeding Cleveland the curcuit breaker's or opperator's malfunctioned resulting in a cable sag.)
Given that this is a new technology would this also be true or would the whole cable need replacement?
Question three is what dangers are involved in the manufacture of liquid nitrogen? Would it be done on site or trucked in? Yes, it is all around us in 78 percent of the air but any leak in the manufacturing process in a closed area can result in rapid suffocation if enough oxygen that is in the ordinary air is driven out of the area. (Certain death penalty advocates suggest nitrogen as a quicker and painless way to kill someone vs. leathal injection.)
The last question I have is what happens if the liquid nitrogen begins leaking underground and how does having something so cold underground affect the soil and enviroment around it?
The AC-DC transmission lines with electronic conversion are already in use. That doesn't rule out some problem with that technology in the future but so far I have not heard of one.