Could a state level actor take down portions of the power grid?
Possibly, although they would have to have extremely detailed knowledge of the system to do so, including planned outages and on-going maintenance schedules.
Could the put down major portions or all of the US power grid?
No, they could not.
[li]Much of the system is still electromechanical instead of electronic - That means that it has mechanical and electrical as well as electronic portions to it. Unless someone was physically present to shut down the mechanical portions, they would remain in service.[/li][li]Much of the power system is on INTRAnets - They have no access or exposure to the Internet and as such they would have to accessed in person rather than at a distance.[/li][li]The outages would be noticed as they were occurring - All of the major systems in the US have people monitoring them 24-7. That means if one or more went down, multiple people would see it or hear the alarms and would respond accordingly. Any attack would have to compensate for this, making it far less effective.[/li][/ol]
While a Carrington event (a massive solar storm episode like one in 1859) would probably knock out a great deal of a major country’s electrical grid, very few cyber attacks (even those by state level actors) could do more than knock out a few cities or at most, a metropolitan area. Certainly a disaster, but not the “armageddon” that cyber security experts are won’t to sell to the public.