Power generation and distribution
Do you know that all electric power generators in your country are turning 100% synchronously? I mean really, totally and absolutely 100% synchronously.
Yes, this is true for all the generators connected to your national power grid (except those few that are connected through an invertor). In fact it is likely that most surrounding countries also have generators fully synchronized to yours as countries mutually share power grid.
This is why they call them synchronous generators :)
It is not as tricky as it seems to have them all revolve synchronously. This is in fact how synchronous generators do, and they cant do any other way. When you interconnect synchronous generators by power wires this is almost as if you connected them physically on the same shaft. If you try to change turning speed of one of them, they all follow.
The synchronous generator
When a synchronous generator is connected to power grid the grid itself will drag the generator into the synchronous speed (ok, you have to make it revolving at least somewhere near the synchronous speed prior to connection because otherwise you may blow something up). If you now leave generator shaft fee-rotating, it will neither spend nor produce electric power (ok, it will spend a little because of internal loses).
What happens if you now put some constant torque on generator shaft in order to speed it up? The shaft will advance just for a small angle (depending on how much torque you applied) and then will stop advancing resuming again good old synchronous speed. The generator will start introducing power into power grid the power produced is proportional to applied torque.
Well, actually the generator will resume rotating at a bit greater speed than before you applied torque, but all the generators in the network will follow this speed-up. This increase in rotation speed will be rather small (hard to measure) because power produced by your generator is probably quite small compared to combined power produced by all other generators connected to the power grid.
Similar things happen if you try to slow down your generator shaft. The shaft will retread for a small angle and will resume at synchronous speed that will be a tiny bit slower than before. All generators will follow. Your generator will start spending energy from the power grid and will become synchronous motor.
In fact, whenever you turn on any of your household appliances it introduces a small (quite non-measurable) slow-down in synchronous speed of the power grid across your hole country (and probably wider). All generators in all power plants are starting to turn a bit slower. This is not a big deal if you turned a toaster on. It however my become a big thing if all the people decided to turn theirs toasters on at around the same time.
Workers and automatic machines in power plants will always maintain the synchronous speed (50Hz in Europe, or 60Hz in USA). If at any moment they feel that their generators are turning too slow, meaning that people are spending lots of electricity, they will apply a bit more torque on their generator shafts (allowing more steam to steam turbines, or more water to water turbines) to bring synchronous speed back to tolerances.
Automatic machines can promptly correct output power of a power plant but only for very small percentage of nominal power. It is not possible to change current power generation of a power plant for, say, 30% in a fraction of a second. That is why a power plant must know at least coarsely how much energy will be needed in future hours. For every power plant energetic needs forecasts are produced for every day (just as weather forecasts).
In addition, there is a governing body that decides how much energy will be produced by each power plant each day (it means, how much torque will an power plant be allowed to apply to their generator shafts). In normal circumstances they keep cheep-energy producing power plants (like nuclear ones) at full load all the time, while expensive-energy producing plants (like natural gas ones) are used only at peek needs.
BTW, private-owned power plants are tending to produce more power than allowed (because they get paid for produced energy). The problem is that it creates speed-up of the power grid synchronous speed. A governing body monitors such conditions and threatens violators (the worst threat would be an immediate shutoff ).
Danijel Gorupec, 2006