I not saying it’s not possible, but it is not going to be easy. Power plants have upgraded their Distributed Control Systems to systems that are physically isolated from utility networks. They can not be accessed from the outside, not even from corporate networks. This is by government edict. I am not familiar with the Transmission and Distribution side of the equation. If there is going to be a cyber attack, that is where it could come from, since systems are so interconnected.
The utilities that were my customers were absolutely paranoid about this. The testing was extensive and relentless. Audits are done once the systems were installed, and the systems automatically logged any changes in the software.
Once again nothing is impossible, though the safeguards around these systems are huge. Hope it gives everyone some level of comfort.