Long Distance Power Grids Are Virtually Impossible To Defend Against Terrorism Or Insurgencies
Even a very timid and small insurgency or terrorist group determined to disrupt the kind of power grid you envision is virtually unstoppable.
A group with fifty active members with a million dollar budget could shut down the power of a civilization with hundreds of millions or billions of people with virtually guaranteed success, without needing any terribly expensive or advanced technology, even if the defenders devotes a hundred billion dollars of resources and millions of people to the cause of defending the grid.
Lawrence of Arabia did something similar to disrupt rail transportation in North Africa.
This is a problem inherent in the nature of a long distance power grid across desolate areas that is pretty much impossible to overcome. No technological solution can solve this efficiently. If you have this security problem, you have only one viable choice: abandon the long distance power grid.
Even if the location of the cables could be perfectly hidden, there are so many people at a fairly low level in any bureaucracy with a need to know (e.g. planning commissions, maintenance crews), that any organization with a mildly motivated and halfway competent spy or two could easily learn the location of a host of vulnerable locations from power company records or word of mouth from power company employees.
The only reason that this isn't a problem in places like the United States or Canada is that nobody has a terrorist agenda devoted to taking down the power grid, and that the power grids are predominantly more decentralized and localized.
Option One: Move Energy Via Fuel Cells Rather Than Using Cables
One alternative is to ditch the idea of power transmission over long distance cables entirely.
If you could built a very efficient, high energy density fuel cells, and had a low energy cost way to move those fuel cells (e.g. by ship or barge over a major oceans and/or river systems), you charge your fuel cells at the generator, deliver them to the destination (with a fuel cell inventory cushion for foreseeable interruptions in deliveries), and you power the grid where the power is needed with the fuel cells. Once a fuel cell is out of juice, you can ship the empty cells back on the return trip of the ships/barges.
This leaves you with no immobile cables or transformer stations in the middle of nowhere that are hard to defend in the event of a conflict.
All you need to protect is the generation location, the local grid at the destination, and ships or barges carrying the fuel cells while they are en route (and the incentive for anyone to attack those ships or barges is going to be much lower on the return trip than on the trip from the generator to the place where the power is used). The ships and barges can be protected are small and mobile, and can be protected with point-defense strategies similar to those of modern warships. Also, while the route may be effectively fixed in a river system (although the timing can be made unpredictable if desired), once you are on an ocean or sea you can use an unpredictable and varied route rather than the absolutely most efficient route, to make it harder to target the ships.
A Variation: Use Ship Based Rechargeable Fuel Cells With Short Cables Connecting To Port Cities
In a slight variation on this plan, you could have a short power cable running from the power generation site to the nearest port town at which fuel cells would be charged (perhaps even while still on the ships returning with empty fuel cells).
Similarly, at the destination, a much shorter distance power cable could deliver energy from the fuel cells that would stay on the ship at all times to the local power grid until they were depleted and the fuel cell ship returned to the port near the generator to recharge.
As long as neither the generation site nor the destination are too far inland, this could be a very workable alternative to putting the generator and the destination grid on the same power grid over many 1000s of kilometers of mostly deserted territory where the security of that grid in deserted territory could not be assured.