I know that power/electricity generated (from conventional power plants or renewables) is generally instantaneously consumed, with grid operators constantly ramping generation to equal demand. My question is, what happens to the excess power, assuming insufficient storage? Regardless if it is a minor excess from an imperfect generation/demand alignment or if its from intermittent solar/wind power that doesn't have sufficient temporary storage, what happens to power that has no home? Is it just dissipated in some waste resistor to be drawn off as heat? Where does the excess power go?
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2 Answers
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First off, energy storage doesn't really come into play in grid control. (Grid-scale energy storage is basically an experimental future technology that doesn't have a practical impact yet.) So the grid is basically about managing things such that supply=demand within very close tolerances. So:
- This is mostly done by throttling natural gas plants up and down, because they can change power quickly and most of their cost is fuel, so it makes sense to throttle them. (By contrast, for nuclear and hydroelectric most of the cost is initial construction, so they're always run at 100% power rather than throttled up and down. They're known as "base load" plants, as opposed to "peaking" plants.)
Smaller supply-demand mismatches can be handled in a couple ways:
A lot of loads will naturally draw more power if line voltage is higher (motors will run slightly faster, heaters will be slightly hotter). So, if generation is slightly too high, the voltage of the power lines will increase slightly, which causes more energy to be dissipated. The U.S. power grid is designed to deliver 120V +/- 5% to your house, and devices are designed to handle that slop.
Some larger industrial loads need a lot of power but are not time-critical. Utilities will make deals with these industries to give them cheaper power in exchange for being able to turn their power on/off to help balance the load. This is called "virtual generation" or "virtual demand". (For example, some oil fields turn off their pumps during times of the day when consumers are running their air conditioners.)
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$\begingroup$ Just a side note since you asked about renewables: obviously wind/solar energy input is not controlled (other than that it can be turned off on demand). So the amount of "peaking" ability on the grid limits the percentage of wind/solar power the grid can use: you always need to maintain peaking plants equal to your wind/solar generation capacity, so you can turn them on when it's not windy/sunny. From what I've read, the practical limit (where the peaking requirement causes severely diminishing economic returns from wind) is about 20% of total grid power. $\endgroup$– LukeCommented Oct 13, 2020 at 23:41
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The generators have governors that control the speed of the machines. When generation exceeds load the machine speed up and the governor action slows back down (e.g. to 60Hz reference).
In addition, many plants are on AGC which is a wide area approach to controlling frequency (and thus the generation-load balance).
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$\begingroup$ You're welcomed. Note - I didn't mean to insult your intelligence or experience level. However, on a first reading of your reply, I noted that it was brief to the point that I assumed that it came from someone with much less experience than you. My apologies. $\endgroup$ Commented Oct 14, 2020 at 1:45
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2$\begingroup$ This is the correct answer, imbalance in power shows up as excess frequency, not as excess voltage. $\endgroup$ Commented Oct 14, 2020 at 6:26