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This is a continuation of this question. Suppose we have a grid which distributes say one gigawatt total power and all this power is perfectly fully consumed by electrical devices.

However all consumer devices are regulated - each either has a switch-mode power supply or it's powered through a switch-mode voltage stabilizer. So it can be a good old perfectly resistive tea kettle but it would be powered via a high-power switch-mode voltage stabilizer which tries its best to output exactly 230 volts (I'm using European mains voltage here).

Suppose at some point of time a small fraction of consumers disconnect their devices from the grid. So now generation exceeds consumption and grid voltage rises a bit. However consumer devices are regulated and they refuse to drain more power. Those fully resistive tea kettles are connected via switch-mode voltage stabilizers which will not pass the higher voltage through.

Where will excess power go?

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  • \$\begingroup\$ I don't understand... Do you think a 1 gigawatt generator MUST always deliver 1 gigawatt? Or a 5 kW generator MUST ALWAYS deliver 5 kW? If the load drops, the demands on the prime mover lessens. \$\endgroup\$ – R Drast May 20 at 14:41
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Where will excess power go?

The excess power goes to speed up all the generators connectors to the same synchronised grid. That's a lot of spinning inertia, the speed up will be quite slow. Control equipment spots the speed-up, and reduces the power input into the turbines driving the generators. That control action will be sufficient for small excesses of power.

Sometimes, a generator will have to deal with very large excesses of power. Consider a power station tied to the grid by a large feeder, which fails. The power station has now lost 100% of its load, and the speed up on the turbines will be very quick indeed, too fast to be controlled by closing steam valves. To maintain safety, these power stations are equipped with load dump resistors. Think of something the size of a football field, arrayed with a few 100s of MW of electric heaters. They would get switched on line for a few 10s of seconds to load the generators for the time that it took to reduce the steam input.

At least that's the way it used to work when the grid was 100% spinning iron. With the increased penetration of solar, wind and battery storage all driving very quickly responding inverters, some of the power can be turned off very quickly.

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