Since, and unless electricity has properties I’m not aware of, it’s not possible for electric power plants to produce only and EXACTLY the amount of electricity being drawn at an given time, and not having enough electricity for everyone is a VERY bad thing, I’m assuming the power plants produce enough electricity to meet a predicted average need plus a little extra margin. So, if this understanding is correct, where does that little extra margin go? And what kind of margin are we talking about?
In: Engineering
If you hold a pencil in your left fist and twist it with your right hand you can get an idea of what’s happening in a generator. Tighten your fist and it’s harder to spin the pencil. We can increase the torque on the rotating shaft (3600 rpm) to produce more power. To increase or decrease torque we vary the strength of the magnetic field around generator. Increasing or decreasing steam flow to the turbine lines up with demands from the grid. Daytime is tight fist and night is loose fist. I won’t go into vars.
I did a quick study of Dinorwig Power Station in the UK for a university module. It’s a fascinating arrangement with two reservoirs, one 100m higher than the other, to allow for extremely quick power generation in the event of power ‘shortages’ by allowing water from the upper reservoir to drop to the lower, spinning a turbine, and pumping water back to the upper reservoir during off-peak hours where it’s cheaper for it to do.
https://en.m.wikipedia.org/wiki/Dinorwig_Power_Station
The National Grid in the UK will have several of these surge power stations (although of differing types of power generation) to balance the needs of the nation.
Electricity isn’t really “produced” as a commodity, like treated water or goods from a factory. All the electrons are in the wires already. The power plant creates pressure in the wires, known as voltage. This pressure from the power plant, transmitted through wires, pushes electrons through lights, electric motors, etc. Anything that runs on electricity causes resistance, which lowers the voltage.
So if a bunch of stuff gets shut off at the same time, and the power plant keeps producing the same, voltage in the wires would go up. This can cause bad things like melting the insulation on wires and frying electronics. So power plants very carefully monitor how much power is being used, and vary their output accordingly.
There’s also an opposite scenario to what you’re asking. What if there are more lights and motors than the plants can provide power for? Voltage in the grid drops, lights stop working or only light up dimly, motors run slowly or with less strength or not at all. These are called brown-outs.
Think of the electrical network as a tightly wound bundle of strings. If you wind faster than what’s connected can turn then the strings coil up a bit but the extra winding pushes back against you, so you slow down winding because it’s harder. It’s not a perfect analogy but it sort of works.
Everybody needs the merry go round to spin at the right speed. If someone speeds it up, people will get thrown off. If it slows down, it’s not fun for the kids. There’s a sweet spot between fun and getting thrown off, and here that is 60hz. It is the equivalent of a Buddhist tone: it doesn’t waver; it just is.
This is a reason why we pay these people big bucks to keep the merry go round spinning
Let’s simplify the grid down to a single power station which is just a spiny thing with magnets and one light switch. When the light switch is off no electricity happens the spiny thing spins with no resistance beyond friction. When you turn the light switch on the spiny thing feels extra resistance and slows down (we then burn coal to keep it spinning at the same speed under load) when you turn the switch off the extra resistance goes away. Electricity is just an intermediary between those two things and doesn’t exist when there is no load. So extra doesn’t go anywhere it was never there to begin with
Electricity, which is a form of energy, can be stored as a different form of energy. In New Jersey, there are 2 sites, I’m not sure if they are still in use, I visited them as a teenager back in the 80’s. During low demand hours (overnight), excess electricity is used to pump water from a lower reservoir into an upper reservoir where it is stored as potential energy. During high demand hours (during the day) they can release the water to the lower reservoir and turning a generator creating electricity to meet the increased demand. When demand goes down the cycle starts over. Like I said, not sure if they are still in use, but it was a relatively simple solution. There may be more efficient ways if doing it now.
When more electricity is produced than consumed, the voltage of the grid rises.
As the grid voltage rises, power generators, like solar panels, wind turbines, and thermal power plants, will produce less power, allowing the voltage of the grid to decrease.
Excesses power can also be stored with pumped hydro and grid scale batteries.
Pumped hydro can store energy for days or months, while batteries are good for minutes to hours.
Thermal power generators have spinning components which act as flywheels and can store a few seconds worth of power.
>it’s not possible for electric power plants to produce only and EXACTLY the amount of electricity being drawn at an given time
Traditionally, this is exactly what happened. Now we have a small amount of energy storage that can act as a buffer, but it is still pretty miniscule when compared to peak load.
Generators don’t know exactly what the load is at any given time, however they are able to monitor their rotational speed (directly correlated to frequency for synchronous machines). When additional load is added, it essentially “bogs down” the grid and lowers grid frequency, which can be immediately seen and responded to by synchronous generators.
Generators have Primary Frequency Response built into their governor, so that a frequency drop or rise will bias the generator output in the right direction. PFR should kick in within a few seconds and last about a minute. Secondary Frequency Response is an automated system located at a central dispatch location that then kicks in to regulate the units up and down as necessary for longer duration correction, and tertiary frequency response would be dispatchers working with the sites to start up or shut down additional generators.
curtailment is a common practice where excess generation is simply switched off the grid and goes no where.
this practice is wasteful when fuel is used to generate the electricity (like natural gas or coal) but it is very helpful when renewable sources like wind or solar are used
a renewable power plant can switch on excess production when demand is high and switch it off when demand is low an in that way avoid the need for storage in batteries or other means.
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