I think we can rule out any kind of state actor, even if they were acting through a proxy. The risk/reward just isn't there, and this is not a hard attack to plan, so there's no point in carrying out a "test". If the goal were to send a message, well, this just isn't going to cut it. The US military will crush you regardless of the status of a power substation in North Carolina.
However, it could easily have been done by someone with a political agenda in favor of a state actor but not in any way endorsed by them, or by someone with a general political motive. Here's the catch with that: For such a person, there is no point to doing this unless you later claim responsibility and gain notoriety. If your goal is ideological, you'd use the chaos you created to try to spread your ideology. If your goal is to have your demands heard, well, you'd have to make those demands first. (Of course, it could be the attackers meant to do that but chickened out when they saw what an overwhelming response this provoked, or something).
It's possible somebody did it by accident or because they are mentally unstable and their dog told them to, etc. Can never discount that. However, I think the most likely explanation is actually that this is a criminal act that was done with some only proximately related goal in mind - blackmailing an energy executive? Stealing something of value from somewhere in the area of attack? Distract authorities while conducting some other illicit behavior somewhere - like a break-in to some other piece of infrastructure - for whatever reason?
Hopefully we find out someday. (And I mean, hopefully we stop this stuff from happening, but that is gonna take some doing.)