To add to what some have said about working with the power company, the relationship can be complex. I work for a power company somewhere in the US and we have several large customers that draw power in the range you mention.
At least here, it is not normal for the power provider to be the same as the power supplier. We do not produce our own power. We buy it from, “the grid,” which is another company that is tied into the national grid.
Not only are we concerned about the needs of a particular plant from our distribution network, we are concerned about having sufficient power into our network. Electrical energy in this quantity is difficult to store. Usually, it must be purchased live.
Given this, we monitor live load constantly. I have never asked but I would presume that some of our engineers could tell you when each of the plants on our network has shift changes and their load dips. Small or scheduled dips, or changes from network-insignificant loads (one house at a time) are either planned or come out in the wash. Aggregate network-wide changes like winter heating at night or summer air conditioning during the day take excessive planning and often cause headaches anyway.
A plant stopping causes loud alarms and usually phone calls or meetings to decide what to do next. There are many people with a lot of experience who know how to anticipate, communicate and mitigate situations like this.
Similarly, when a plant like this turns on, either as its original start or after some stop event, there is a great deal of communication and planning. We would be likely to have seen detailed plans and load specs well in advance in order to be able to adjust as necessary. If you’d like to turn on your AC during the day in the middle of the winter, we won’t pay much attention. If you’re running a GW plant, we know this (because we built you an expensive substation that can push a GW) and we’ll probably call you frequently to ask what you’re planning.