I recommend you look up the CLH Pipeline System which was the UKs secret fuel pipeline to distribute jet fuel across the country during WW2.
Of course, the CLH did not have to deal with electromagnetic waves, and while it's not possible to block magnetic fields, it's possible to redirect magnetic field lines through conductive materials. Modern cables are often shielded by wrapping them in copper foil which creates a Faraday cage to prevent signals from interfering with each other. If you were to wrap your cables in several of such shielding layers, little of the magnetic field would still be detectable.
Someone else suggested that you can't really bury the cables without making it public, as people will damage them from just digging out their yards and stuff. However, you can bury cables deep (tens of meters) to prevent most accidental damage from property owners digging out cellars or whatever excavations are commonly done. Things that require deeper digging, like high rise buildings or mines, require government approval. In addition, this further helps with masking the magnetic fields. While this is expensive, it doesn't seem unreasonable in war situations.
In addition, you can make your power grid redundant by connecting each city to several power plants and create multiple power lines from each plant to each city. While this sounds outlandish, this is what we actually did here - not for power but for information. 548
It's called the internet, although in it's early stages it was called ARPANET and one of its goals was to continue functioning, even if large parts of the network are taken out.
Those suggestions should be enough to not only make your power grid hard to detect, but also make it hard to destroy... except that at some point your power stations have become the weakest link. A well-placed air strike, sabotage or a full attack might easily take out your power dams... and create a fair bit of mayhem once the broken dam floods the surroundings. In 1979 the Teton Dam broke and damaged Fort Hall, a city that is more than 130 km far away (by car, I assume the flood followed the river which takes an even less direct route). Usually cities are built near water sources and to me, it seems that your power requirements seem to be bigger, as you have few centralized power generators. Thus the dams are a way more interesting target than the power grid itself, even if the grid is somewhat vulnerable.
So in your case, I would care less about the grid, but more about the few power generators.
* They can not be hidden.
* They're vulnerable to (high altitude) air strikes and carpet bombing.
* They're costly to create and repair.
* Repairing them will take much longer than simply replacing a cable.
* If you take them out, there will be a lot of secondary damage.
* If you take them out, you might take out power for several cities.
So my final suggestion is... go with decentralized power distribution. Maybe everyone uses solar or wind power, or there's a small thorium reactor in every neighborhood. 284