4 ms·

Just a question: Why does demand exceeding supply cause the frequency to drop? In my mind it should cause the voltage to drop? If this question is too long t

Just a question: Why does demand exceeding supply cause the frequency to drop? In my mind it should cause the voltage to drop? If this question is too long to answer on HN, are there any good resources that explain this?

blendergeek 6y ago
Well, if anyone sees my comment, I found some (I think) great answers on Reddit. So if anyone else has the same question here is (as far as my non-electrical engineer mind can tell) some great answers. https://old.reddit.com/r/AskEngineers/comments/1tnbob/why_does_the_frequency_drop_when_demand_increases/ https://old.reddit.com/r/AskEngineers/comments/1tnbob/why_do...
i_am_proteus 6y ago
This is a gross simplification but hopefully makes sense: Grids using multiple generators use "frequency droop," deliberately governing the generators so that as power goes up, frequency goes down. This makes it easier to load-balance between generators connected in parallel. If one generator were to be "overloaded," its frequency would go down, increasing power output from other generators. When connecting a new machine, it's frequency is typically slightly higher than the grid; after it is connected it "assumes" some load and its frequency drops to match the grid's frequency. https://en.m.wikipedia.org/wiki/Droop_speed_control https://en.m.wikipedia.org/wiki/Droop_speed_control
- anamexis 6y ago
  And to simplify even more: Generators are just spinning magnets. When load goes up, magnetic resistance goes up, so given the same input mechanical power, the spinning will slow down.
  - xattt 6y ago
    A coal power plant in Thunder Bay, Ontario used to drive an otherwise decommissioned generator for the sake of having inertial mass to stabilize the frequency of the local supply. I would imagine this is common practice in power generation in general.
    - PhantomGremlin 6y ago
      Yes, it is common practice: https://business.directenergy.com/blog/2016/may/reactive-power https://business.directenergy.com/blog/2016/may/reactive-pow... https://openei.org/wiki/Definition:Reactive_Power https://openei.org/wiki/Definition:Reactive_Power
    - Johnythree 6y ago
      Yes, it's called a "Synchronous Condenser" https://en.wikipedia.org/wiki/Synchronous_condenser https://en.wikipedia.org/wiki/Synchronous_condenser
xtian 6y ago
In an AC grid, increased or decreased load affects resistance which impacts the rate at which the mechanical source of power generation can rotate and therefore the frequency of the current: https://physics.stackexchange.com/a/2661 https://physics.stackexchange.com/a/2661
tgsovlerkhgsel 6y ago
If you add load to a generator, it increases the amount of force needed to turn it. (There's a nice experiment with a hand crank generator that has a switched load - it's very noticeable, and logical - the electrical power you use needs to come from somewhere). If the amount of force that's available doesn't increase, the generator slows down. The generators are synchronized to the grid. Of course, as the frequency drops, power plants regulate by applying more force to the generators (e.g. increasing the amount of gas or water fed to a turbine).
- econcon 6y ago
  If you've a small load dynamo spins faster as there is much less force required to make it spin. So frequency should go up, not down. As the spin is directly proportional to the frequency
  - quietbritishjim 6y ago
    GP comment: Why does increasing load make the frequency go down? Parent comment: Increasing load makes the frequency go down because ... Your comment: Decreasing load makes the frequency go up Your comment is in agreement with everything before, but you said it in a tone of disagreement. Maybe you misread the parent comment?
    - econcon 6y ago
      ah, yes i did. thanks :)
205guy 6y ago
I first came across this phenomenon with the Kosovo issue mentioned elsewhere. But it immediately made sense to me from driving a gas car (you don’t notice it in an electric): hold your foot on the gas pedal without moving it and go up a hill. What happens? The engine RPM go down and also the car slows down (because it depends on RPM to the wheels). As the load increases, without more power (fuel flow) the engine slows down. Same with an electric generator/motor: if you pull power off more than it’s being driven, each push from the driving field is less than enough for the rotational rate, so the rotation is slower, and thus the frequency too.

blendergeek 6y ago

Well, if anyone sees my comment, I found some (I think) great answers on Reddit. So if anyone else has the same question here is (as far as my non-electrical engineer mind can tell) some great answers. https://old.reddit.com/r/AskEngineers/comments/1tnbob/why_does_the_frequency_drop_when_demand_increases/ https://old.reddit.com/r/AskEngineers/comments/1tnbob/why_do...

i_am_proteus 6y ago

This is a gross simplification but hopefully makes sense: Grids using multiple generators use "frequency droop," deliberately governing the generators so that as power goes up, frequency goes down. This makes it easier to load-balance between generators connected in parallel. If one generator were to be "overloaded," its frequency would go down, increasing power output from other generators. When connecting a new machine, it's frequency is typically slightly higher than the grid; after it is connected it "assumes" some load and its frequency drops to match the grid's frequency. https://en.m.wikipedia.org/wiki/Droop_speed_control https://en.m.wikipedia.org/wiki/Droop_speed_control

anamexis 6y ago
And to simplify even more: Generators are just spinning magnets. When load goes up, magnetic resistance goes up, so given the same input mechanical power, the spinning will slow down.
- xattt 6y ago
  A coal power plant in Thunder Bay, Ontario used to drive an otherwise decommissioned generator for the sake of having inertial mass to stabilize the frequency of the local supply. I would imagine this is common practice in power generation in general.
  - PhantomGremlin 6y ago
    Yes, it is common practice: https://business.directenergy.com/blog/2016/may/reactive-power https://business.directenergy.com/blog/2016/may/reactive-pow... https://openei.org/wiki/Definition:Reactive_Power https://openei.org/wiki/Definition:Reactive_Power
  - Johnythree 6y ago
    Yes, it's called a "Synchronous Condenser" https://en.wikipedia.org/wiki/Synchronous_condenser https://en.wikipedia.org/wiki/Synchronous_condenser

xtian 6y ago

In an AC grid, increased or decreased load affects resistance which impacts the rate at which the mechanical source of power generation can rotate and therefore the frequency of the current: https://physics.stackexchange.com/a/2661 https://physics.stackexchange.com/a/2661

tgsovlerkhgsel 6y ago

If you add load to a generator, it increases the amount of force needed to turn it. (There's a nice experiment with a hand crank generator that has a switched load - it's very noticeable, and logical - the electrical power you use needs to come from somewhere). If the amount of force that's available doesn't increase, the generator slows down. The generators are synchronized to the grid. Of course, as the frequency drops, power plants regulate by applying more force to the generators (e.g. increasing the amount of gas or water fed to a turbine).

econcon 6y ago
If you've a small load dynamo spins faster as there is much less force required to make it spin. So frequency should go up, not down. As the spin is directly proportional to the frequency
- quietbritishjim 6y ago
  GP comment: Why does increasing load make the frequency go down? Parent comment: Increasing load makes the frequency go down because ... Your comment: Decreasing load makes the frequency go up Your comment is in agreement with everything before, but you said it in a tone of disagreement. Maybe you misread the parent comment?
  - econcon 6y ago
    ah, yes i did. thanks :)

205guy 6y ago

I first came across this phenomenon with the Kosovo issue mentioned elsewhere. But it immediately made sense to me from driving a gas car (you don’t notice it in an electric): hold your foot on the gas pedal without moving it and go up a hill. What happens? The engine RPM go down and also the car slows down (because it depends on RPM to the wheels). As the load increases, without more power (fuel flow) the engine slows down. Same with an electric generator/motor: if you pull power off more than it’s being driven, each push from the driving field is less than enough for the rotational rate, so the rotation is slower, and thus the frequency too.