What happens if the attacker generated a (e.g. periodic) pattern of load changes that excites control mechanisms at their resonant frequency?
Maybe the book "Blackout" [1] (which I haven't read myself) has more (fictional) details about what can go wrong.
Wouldn't that behave identically to a sudden loss of generation? The power grids I know of have schemes to deal with that, by automatically shedding large blocks of load in several stages.
If just 10% of a nationwide grid is down, there's a good chance the phone network won't work, internet will be down, trains won't run, credit card/payment systems won't work, etc. All those things have primary and backup systems, but somewhere in the chain of dependencies there will be both a primary and backup that have been shed, and the system designers thought "these two data centers are 500 miles apart, so won't fail together".
A major infrastructure cyberattack seems effective either as an opening salvo in a traditional military war or to multiply the chaos after a terrorist attack. Taking out the power grid would worsen traffic congestion and create increased demand on emergency services in the short term, and have major economic impacts and high visibility in the long term.
If you can make power usage unexpectedly go up by more than ~10% within a minute, most power grids will fail.
I'm struggling to think of any companies who could do that though... Someone with malicious access to teslas servers couldn't even do that... For example, instruct all plugged in tesla cars to start charging all at once. Assume 400k tesla cars are plugged in overnight, with an average 10kw charge ability. That means tesla could add 4 gigawatts to the grid demand instantly, which is only 0.4% of the USA's ~1TW generation capacity.