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I know in a power distribution system, the power is generated (from a dam or turbine or something) and consumed immediately.

But the power consumed is variable. So how does the generation system take that into account.

For example what happens to the hydro dam turbines (or wind turbines etc) when I turn my light off or unplug my laptop charger?

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This is more of an addition to the previous answer by blacksmith37. I think it might be more appropriate as a comment, or perhaps an edit to the previous answer. With no ability to comment at this time, I'll leave the next step to people smarter than myself. I believe this is a fairly straightforward answer:

  1. How they predict the amount to generate
  2. How they handle excess generation
  3. How they handle shortage of generation

The extent of my knowledge is coming from personal research and experience working in a coal power plant in the Midwest US.

  1. Energy consumption is regulated by independent service operators (ISO). Essentially, each day they (ISO) take a bid from all the sources willing to sell their electricity. ISO's are very, very good at predicting the amount of power needed by the public by considering the weather, time of day, and other factors. The power plants, turbines, panels, etc. make enough electricity to meet this demand each day.

  2. The typical variability from the expected power consumption is small. If there is excess power generated, you can look at this question on the electrical engineering stack exchange.

  3. If there is not enough power being generated, "peak" plants that can ramp up quickly are used, as described in blacksmith57's answer. While coal power plants ramp up slower, they don't usually run max capacity. What this means is that several coal plants can all run at 80% capacity (arbitrary number) and each plant can slowly boost their output during "peak" time, to at least reduce the demand. Faster reacting gas plants are much better operating as "peakers".

The relationship between the ISO's and the power plants is highly regulated following market manipulation at the turn of the century.

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  • $\begingroup$ Oh okay I see. I'm curious about the really short term effects of changes in power consumption rather than changes over hours. For example if a reactor is producing 100MW, and a factory which uses 10MW suddenly shuts down, do all other consumers in the network get increased power at that instant? If not how does the reactor adjust the amount of power produced in that instant? $\endgroup$ Feb 17 '18 at 19:55
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gas and steam turbines, nuclear power plants, and hydro plants can be throttled to match their power output with the demand on an hour-by-hour basis. when demand is high, they are ramped up to full rated power; when it is low, they are trimmed back to lower power levels or the resulting excess generating capacity is shunted over to another portion of the power grid where the power demand is high at that moment.

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If the system is not producing enough power voltage will drop so they know to produce more power. If voltage is too high then producers drop power. Distribution can bleed off power if required to not over power the consumer but the need for that is rare.

In the US there is a grid west of the Mississippi and a grid and east of the Mississippi.

There are 3 type of sources:

  1. Have almost no control. For example wind.
  2. Have control at a variable cost. For example coal or natural gas. Nuclear also has variable cost but it does not vary much. Nuclear is typically just run at design capacity.
  3. Hydro is unique in you have a finite source. There is only so much water behind the dam. It does not cost more to let out more water but when you let out all the water you cannot produce more.

First you fire up the cheap power (wind and nuclear). Then fill in the variable power with coal and gas.

For a state like WA that has a lot of hydro they have the luxury of selling off power at a premium during the summer when demand is high in the south. In the winter the reverse is true and WA can typically buy power for a discount from the south when demand is low in the south. A damn like Grand Coulee will not vary output by demand on a hourly or daily basis. They will just have like monthly flow rates based on water level. Since hydro is cheap power it is consumed first.

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  • $\begingroup$ I think it is Hoover Dam ; and wind power is one of the most expensive ( before subsidies ). Wind is controlled , much generation is turned off at night. That is partly why production averages only about 34 % generation of installed capacity. $\endgroup$ Feb 15 '18 at 17:05
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    $\begingroup$ Maybe you meant a dam like Grand Coulee ? $\endgroup$ Feb 15 '18 at 18:04
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It generally does not. After the investment is made in a dam generation site (or nuclear) it is kept running. That is where the distribution system is essential, the power is shipped across the county to where it can be used. Wind and solar are different, they may shut down when you need them, so are generally backed up by gas turbines (they can start up and shut down quickly). Distribution also is very important for wind and solar: but, that is why wind and solar require such large taxpayer subsidies; They basically need a matching gas turbine system to back them up. Coal units can be adjusted, but slowly.

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  • $\begingroup$ Doesn't answer question. $\endgroup$
    – Phil Sweet
    Feb 14 '18 at 2:14

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