I think the thing to understand is that we cannot prevent a catastrophic failure in which the entire electric grid is brought down. There are over 55,000 electric substations in the US, mostly protected by a chain link fence. You can't just take out any 9 and think that will crash the grid. Now of those 55,000 only about 30 are considered "Highly critical". These are the substations at the "super hubs". If you take out 9 of them, properly chosen, you bring down the whole grid nationwide.
I suspect those 30 are much harder to get to than the others. However terrorists can still bring them down even if they don't attack directly by attacking the flow of electricity creating surges that would trigger a breaker to flip.
Also, in warfare you always cannibalize things to keep other things working. I suspect if you took out a critical substation the US would cannibalize less critical ones.
My point is I don't think the plan is a frontal assault on 9 super hubs, rather I think teams of 3 could take out hundreds if not thousands of the smaller substations that are not guarded. This will also cause a collapse of the grid and make it that much harder for the US to cannibalize other substations to keep the power flowing.
Here is the scary thing, US military bases in the US rely 99% on the domestic grid. There are up to 450 active military bases in the US. My guess is that bringing their power down would be the main target in an initial assault and would require about 1500 men to be in place in the US. Taking out the substations to create blackouts would serve two purposes, it could crash the whole grid and at the same time the substations the military would try to fix first would be destroyed and unrepairable.