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
There’s a whole series of things to keep supply and demand very closely matched. As /u/gnonthgol said, there’s inherent stability. But that’s fairly small on the scale of the system, and only holds it together on timescales of seconds or less.
Next up are powerplants, and these days batteries, that respond to automatic controls on the seconds -to minutes scale. After that, you’ve got plants on 10- and 30- minute standby in case something. Underlying the whole thing is a room full of operators scheduling powerplants based on forecasts over everything from the next 5 minutes to about a week out.
There’s also some natural and artificial stability from consumption. If the grid is underpowered, it falls below 60hz, and that naturally causes some things to consume less power. Grid operators also play some large consumers or automaticly shut off if frequency falls below some threshold, usually 59.9 Hz.
Take a look at [this] (https://dataviewer.pjm.com/dataviewer/pages/public/ace.jsf). It shows how far out of balance the biggest grid in the U.S. is. At the moment, they’re serving about 70,000MW of load, and they’re keeping it mostly within +/-300MW.
For more, see the Balancing Fundamentals chapter here: https://www.nerc.com/comm/OC/BAL0031_Supporting_Documents_2017_DL/NERC%20Balancing%20and%20Frequency%20Control%20040520111.pdf
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