It would be a lot easier and cheaper to just shoot out a lot of transformers on the 3 grids at the same time.
You might be right about that — beta testing has already occurred.
The damage doesn’t have to be comprehensive — just wide spread. Whether it’s an EMP attack or bullets placed in transformers; IMHO the effects of any large-scale damage to critical infrastructure would quickly be magnified many times over by the ensuing breakdown in social order. Consider Katrina, tropical storm Sandy, etc. Now, imagine that there’s no help coming from the rest of the country, because a large part of the rest of the country is also affected.