The "power grid" is nothing more or less than the high voltage, medium voltage, and low voltage power lines that carry the electric power from the machinery that generates it to the point of use (homes and businesses). 60 Hz in the US and 50 Hz in many other places.
Now, the power companies do need to have communication equipment to monitor the state of that grid and to control the various elements (generators, switches, etc.) I would suspect that they use multiple means of communication: microwave, satellite, plain old radio, probably even land (telephone) lines. But, of course even the telephone lines use things like microwave and satellite for long, cross country connections.
But, another thing that I am fairly sure is done is to transmit digital data using those very power transmission lines that form the power grid, or at least parts of it. When I drive under the some local semi-HV power lines my AM radio picks up a lot of static that sounds a lot like digital chatter to me. So the logical conclusion is that those power lines are also carrying digital communication signals that are piggy-backed on the 60 Hz, AC power. This superimposed data signal is probably carrying the monitoring and control signals that the power company uses, as I mentioned above.
Thus, I think that there is no simple answer to the OP's question.
As far as attack vectors, that would be tricky. I mean, these digital signals would not pass through AC transformers very easily. They would need to be stripped of the 60/50 Hz power on one side of the transformer and re-inserted on the other. If the power company had not provided that path, then the first transformer that any malicious hacker's connection encountered would be a dead end. So one could not just connect to a household or business power connection (230 VAC) and expect to receive, alter, and re-transmit those monitoring or control signals.
A hacker would be far more likely to interfere with the control of the power grid by hacking into any internet connections that the power companies are using. One can only hope their engineers are wise enough to guard against any such intrusion. And do consider the fact that the power lines that carry that power for any appreciable distance (like over a kilometer) are virtually all high voltage lines. A hacker attempting to make a connection to them would need a fair amount of equipment to do so. Things like an insulated bucket truck and the equipment that makes the connection would need to be designed for those high voltages. And, of course, the training to properly use them.
Of course anything is possible where technology is involved. Just ask the Germans or Japanese officials who thought their WWII codes were completely secure. I am sure there are malevolent people who are trying to find the weak points in these systems for possible use when they wish.