Big difference between a power plant (station) and a substation.
Not if your goal is to disrupt service to the end user.
Of course, the elephant in the phonebooth is protecting the high power lines that stretch across the continent. Thermite or any number of industrial cutting technologies could knock down a lot of those towers in a hurry, and crews trying to erect new towers would be difficult to guard.
States like Kaliporno can’t provide their own power locally.
The substation is the switching and routing element, and taking that out is just as effective as taking out the generation, and likely considerably easier.