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Electricity can’t be stored, so when I turn on a light, the power station must immediately produce some extra electricity. How is this possible? From an engineering perspective, how are power stations designed to handle continually varying loads?

Viktor T. Toth, IT pro, part-time physicist

Updated Jan 22 · Upvoted by Furkan Öz., M.Sc. Electrical Engineering & Electromagnetism, Middle East Technical University (2020)

Originally Answered: Electricity can’t be stored, so when I turn on a light, the power station must immediately produce some extra power. How is this possible?

Let me offer an answer using not electricity, but mechanical power as an analog.

But first… have you ever wondered why the almost universal depiction of a stereotypical factory inevitably comes with a smokestack?

When you actually look at a modern factory, it usually has no smokestacks at all. Rather, it is probably just a nondescript warehouse-style building in an industrial park somewhere, indistinguishable from similar buildings that are used as actual warehouses, strip malls, etc. But that’s because in a modern factory, most machinery would be powered by electricity. This wasn’t the case in the 19th century, however. In fact, the typical interior of a factory was like this:

See all those gears and belts and whatnot just under the roof? That’s what powered the machinery. Every factory had its own stationary steam engine (hence the smokestack!) that produced a fair amount of mechanical power, which was transmitted mechanically to the factory floor through gears and shafts. Individual pieces of machinery were connected to shafts under the roof using belts or other mechanisms. Machines were connected or disconnected with clutches, when needed. The steam engine, which might have been housed in its own building actually, powered the shafts at a constant rate of rotation.

Now what happened when you attached a new piece of machinery to the shaft, drawing more power? Obviously, it would slow the shaft down. But the steam engine had a regulator (governor), a mechanical contrivance that sensed when the engine was slowing down or speeding up and responded by increasing or decreasing the flow of steam, opening or closing a valve. So while there would be a brief, transient slowdown in the shaft’s rate of rotation, the regulator would quickly kick in, and through an increase in the flow of steam, restore the nominal rate of rotation. Conversely, if a machine was disconnected from the shaft, there would be excess power and the shaft’s rate of rotation would increase; the regulator would sense this and close the steam valve, reducing power to the steam engine, again restoring the nominal rate of rotation.

Electrical power networks work pretty much the same way. In fact, if the generator happens to be driven by steam, its feedback mechanism may be recognizable to a 19th century engineer! When you turn on a load and increase the power draw on the generator, the generator would start the slow down. As soon as that happens, its governor (be it mechanical, electrical or electronic/computerized) kicks in, increasing its power (e.g., by opening a steam valve if it is a steam turbine) and restoring its nominal rate of rotation, so that electric power continues to be generated at the correct voltage and frequency.

Of course it also helps that modern electricity networks tend to be gigantic (often serving a large part of a continent), so any individual load, even if it is something a lot larger than a light in your room, is dwarfed in comparison.

Moreover, there is plenty of tolerance. The line voltage does not need to be maintained exactly. In fact, while the frequency is supposed to remain stable, significant variations in voltage are permitted by electric utilities. For instance, here is what my server’s UPS (uninterruptible power supply) measured the day before yesterday throughout a 24-hour period:

One can only guess what happened, e.g., around noon when there was a noticeable drop in voltage. Did a major power station go offline here in Eastern Ontario, Canada? Or did a large consumer such as a major data center or manufacturing facility suddenly come online? Or was it just a local event, something that increased the load on the large transformer nearby that serves my neighborhood, reducing its output voltage a little? Whatever the reason, every change in the voltage that you see in this plot is a result of some event of this nature: either a power source or a load getting connected to, or disconnected from, the system.

What these variations in voltage do is quite simple. Suppose your electricity network is generating 100 megawatts and exactly 100 megawatts is being consumed. But now, you turn on a 100 watt light. That would mean 100.0001 megawatts, but that much power is not available. Instead, the voltage drops a little. As a result, all the loads that previously received 100 megawatts of power now receive only 99.9999 megawatts; an almost impercetible 0.0001% drop. Meanwhile, your light comes on and consumes not 100 watts, because the voltage is slightly lower, but only 99.9999 watts. Again, an imperceptible difference. But when you add the two numbers, you notice that the total system consumption remains 100 megawatts, like before, and the balance between power generated and power consumed remains in place.

Sincerely,

James Robert Deal
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James
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