What happens if an entire country turns off all lights and electric devices during peak consumption hours? [duplicate]

I've been reading about smart grids but I studied Computer Engineering and my already limited knowledge of power systems is very rusty. I keep hearing that traditional power stations have to adjust production to match demand, but I don't understand how this is possible. What happens when there is more demand than production? Who runs out of power? What if there's more production than demand? I read that power stations look at historical demand levels to increase production during peak consumption hours, but what if they increase production and everybody turns everything off, driving demand to zero? What happens?

EDIT

Just to clarify, I want to know what happens if demand goes up and down while production remains constant. What happens if demand goes to zero in a second? Does the power plant break down? How? What if the demand doubles in a second? Do the lights in my house lose half of their brightness?

• youtu.be/5uz6xOFWi4A you may find this interesting (from memory, I think it addresses some of your questions - but it's a while since I watched it!) – Jonathan R Swift Feb 15 at 10:57
• Power companies even track tv shows to know when breaks happen such as half-time in football matches... check out Dinorwig (electric mountain) if you want to see how quickly they can respond to changing demand. – Solar Mike Feb 15 at 11:25
• There's a Tom Scott video for that, too youtu.be/6Jx_bJgIFhI – Jonathan R Swift Feb 15 at 11:28
• "What happens when there is more demand than production?" Parts of the grid are switched off to reduce load. "What if there's more production than demand?" Reduce generation. – Transistor Feb 15 at 12:18

The power companies have both automatic and manual regulation mechanisms inside their power plants which sense how big the demand for output (electrical power) is at any moment. Small changes in load are accommodated by adjusting the steam flow rates through their turbines, the water flow rates through their dams, or the reactivity of the core in a reactor. Large changes are managed by turning on or off entire power plants, a process that requires days to accomplish for big steam plants. In dams like those on the Columbia river up here, where there are many separate turbines in the dam structure, big chunks of power can be quickly modulated by opening or closing the gates to one turbine at a time.

• They tend to run nuclear at full output and vary hydro as it is easier to control. But depends on which country... – Solar Mike Feb 15 at 20:20
• What happens if there's nobody there to adjust these parameters though? Does the power plant "overheat and explode" or "break down" if there's more production than demand? Sort of like a cyclist's bike might break down if you go down a hill too fast with no breaks? What if there's less production than demand? The voltage in my sockets will decrease and the lights in my house will go dim? – Ariel Feb 16 at 9:15
• the power plant's generators and transformers would overheat if stressed with too much demand. in times of high demand, the plant operators call other plant operators and ask for them to add capacity to the grid and switch it into their portion of the grid. this is called load balancing and is routine. when demand goes down, the excess power is similarly shunted to other places where it is needed at that moment. when high demand exists everywhere, the supply voltage is turned down at the plant(s) to prevent overheating and the lights will go dim, which is called a brownout. – niels nielsen Feb 16 at 20:42

Note: I'm not a power grid engineer. Someone with more detailed information may edit or comment on this answer.

What happens when there is more demand than production?

Then they turn up the production. It can be automatic or manual, as niels nielsen says. Large adjustments (like starting up a power plant) need a human to decide.

The balance of supply and demand is measured by the grid frequency. On average it's 60Hz (in the USA) but there are small fluctuations. If it goes above 60Hz, generators automatically lower their power. If it goes above 60Hz, generators automatically increase their power.

This is because most of the power grid consists of big spinning generators and motors, and they have lots of inertia. If the frequency drops to 59.98Hz, then a motor that's spinning at 59.99Hz is now a generator feeding power back onto the grid, resisting the frequency drop. And vice versa.

What happens if they can't turn up the production? If all the power plants are already running full throttle.

Then the whole system collapses, basically. The frequency goes down and down despite everyone's best efforts to keep it up. At some point, power plants start tripping off (disconnecting from) the grid and doing emergency shutdowns, as the control systems realize they can't keep everything running. That obviously worsens the problem, and within a matter of minutes, everything grinds to a (metaphorical) screeching halt. Everything is dark. No power anywhere.

Who runs out of power?

Now, obviously, the system operator (the company that runs the grid) doesn't want the whole grid to shut down. Partly because it means they're a failure and should feel ashamed of themselves, partly because it's an annoying process to start up the power plants again without any power. So what they will do, before this happens, is make sure there is enough spare production that they could increase production if they needed to. They may start up power plants and let them run idle, for example. If they still don't have enough, they will start disconnecting people from the grid, called a "rolling blackout" or more euphemistically "load shedding". Apparently, that is what you now have in Texas.

What if there's more production than demand? I read that power stations look at historical demand levels to increase production during peak consumption hours, but what if they increase production and everybody turns everything off, driving demand to zero? What happens?

Well they reduce production. Most power plants can shut down, and the ones that can't have to have safety procedures anyway (in case of a trip or emergency shutdown). So they can go all the way down to 0%. If demand decreases faster than they're expecting, they might have to vent hot steam to the atmosphere or something, instead of using it to spin a generator. Wasteful, but not destructive.

When you turn everything back on at once, though, there will be a blackout though and the system operator will be really annoyed.