5 ms·

> Water tank and water pipe analogies fail pretty fast when it comes to thinking about electricity. In what ways does this analogy fail, for the purposes of un

> Water tank and water pipe analogies fail pretty fast when it comes to thinking about electricity. In what ways does this analogy fail, for the purposes of understanding power grids?

pixl97 4y ago
Well, for one, if you get low pressure in one set of water pipes the entire water grid doesn't attempt to collapse because it goes out of sync. The grid is a gigantic clock that at least in the US is running at 60 ticks per second. This is pretty easy to manage if you have some massive power source on it like gigantic turbines at a nuclear power plant. All your small clocks aren't going to push that around so much. The problems come in when all you have is small clocks, who sets the phase of the grid?
- chestertn 4y ago
  In the pipes analogy, pressure corresponds to potential energy (voltage). Low voltage conditions does not necessarily trigger frequency stability issues. Grid synchronization is a different issue to the electrical-hydraulic analogy (which, by the way, was the one that Maxwell used and its pretty useful). Grid synchronization comes from the fact that electrical quantities in the network (current, voltage) are alternating periodic. Generators, which are usually mechanical, oscillate and induce such periodic signals. The system must remain synchronized. But this has nothing to do with the network being electrical. [1] https://en.wikipedia.org/wiki/Hydraulic_analogy https://en.wikipedia.org/wiki/Hydraulic_analogy
- ZeroGravitas 4y ago
  https://www.nrel.gov/news/program/2020/inertia-and-the-power-grid-a-guide-without-the-spin.html https://www.nrel.gov/news/program/2020/inertia-and-the-power... > “We find that replacing conventional generators with inverter-based resources, including wind, solar PV, and certain types of energy storage, has two counterbalancing effects,” said Paul Denholm, NREL principal energy analyst and lead author of the guide. “First, it’s true that these resources decrease the amount of inertia available on the system. But second, these resources can reduce the amount of inertia actually needed > “Ultimately, although growth in inverter-based resources will reduce the amount of inertia on the grid, there are multiple existing or possible solutions for maintaining or improving system reliability,” Denholm said. “So, declines in inertia do not pose significant technical or economic barriers to significant growth in wind, solar, and storage to well beyond today’s levels for most of the United States.”
  - throwaway52022 4y ago
    The NREL inertia video explainer felt a little like it was begging the question - "inertia protects the grid because it has inertia and keeps spinning" - it doesn't quite feel like it explains where the extra energy comes from or goes, just that the mass keeps spinning. (I also haven't had a physics class in a long long time so some of this is not obvious to me, except that I understand from just common sense that if something's spinning you had to put a bunch of energy into getting it going in the first place and it's going to keep going if left on its own) Anyway, I was hoping someone could fill in some details for me. Imagine a simplified grid: a dam that sends water through a penstock past a turbine/generator and into an electrical circuit, and a couple of resistance heaters on the other side of the circuit. The energy comes from water flowing through the dam - the dam operator opens up the sluice gate to let water flow through, the generator extracts the mechanical energy and turns it into electrical energy and it goes down the wire to the resistance heater where it gets turned into heat energy. Everything is balanced - the right amount of water is flowing through the dam to turn the turbine at the right speed to balance out all of the energy flowing through the wires and into the resistance heaters (and lost along the way, like losses in the transmission lines, etc). In this setup, there's some measure of pressure that turbine pushes back against the water flowing through the penstock of the dam, which is balanced out by how much pressure is coming from the water behind the dam and the pressure being put on the surface area of the penstock in the dam and the pressure being relieved by the water leaving the dam. I get that thanks to inertia, if the sluice gate accidentally slams shut and all water stops flowing through the dam, the turbine is going to keep spinning for a bit and energy is going to keep going out onto the grid, though it will start to slow down due to friction at the turbine and energy being extracted from the system by the resistance heaters on the other end of the grid. What I'm less clear about is how does inertia help when the water keeps flowing at the regular speed but when demand drops from the grid load. Let's say one of the resistance heaters turns off in a home somewhere - what happens to the energy from the water that was previously flowing into the grid via the turbine? Does the inertia in the spinning of the turbine somehow push back against the water flowing the dam, slowing the water down a bit/building pressure up in the penstock and behind the dam - with that pressure buildup being exactly equal to the energy that used to be going into the resistance heater? And that pressure either stays built up from the turbine until someone lowers the sluice gate a bit to cut back on the waterflow through the dam? Or does nothing involving inertia happen here - if the resistance heater gets turned off the overall load is reduced and the turbine spins a bit faster because there's less pushing back on it, and the water can move through the dam a bit faster, and the turbine just spins faster until someone notices it's going a bit too fast and the gate needs to be lowered so it drops back to rotating at 60hz? Similarly, if someone turns on another resistance heater and now more energy is needed on the grid, but the sluice gate isn't opened up immediately, is inertia involved here somehow? If the turbine has to push harder on the grid side because of extra load, presumably the turbine slows down? Or does the turbine get pulled along by the new load somehow (more inertia?), and so more water can push past the turbine, giving it the extra energy it needs (and presumably dropping the water pressure in the penstock in the dam? And the pressure stays low until the sluice gate is opened up a bit more and more water can flow through the dam?) I am using water pressure from a dam here, but I assume this would be equally true in a gas plant generating steam - if more energy is needed, the pressure in the steam drops until someone turns up the burner and creates more steam, etc, or if less steam is needed the pressure just builds up until someone notices and turns down the burner? If anyone can explain how inertia and the grid translates into changes in the actual source of energy, I'd much appreciate it!
photochemsyn 4y ago
The fact that the AC power grid is synchronous is very unlike any water/gas system: > "In a synchronous grid, all the generators naturally lock together electrically and run at the same frequency, and stay very nearly in phase with each other...Small deviations from the nominal system frequency are very important in regulating individual generators and assessing the equilibrium of the grid as a whole. When the grid is heavily loaded, the frequency slows, and governors adjust their generators so that more power is output (droop speed control). When the grid is lightly loaded the grid frequency runs above the nominal frequency, and this is taken as an indication by Automatic Generation Control systems across the network that generators should reduce their output." https://en.wikipedia.org/wiki/Wide_area_synchronous_grid#Frequency https://en.wikipedia.org/wiki/Wide_area_synchronous_grid#Fre... Cascading power grid failures might be another good example: https://www.science.org/doi/full/10.1126/science.aan3184 https://www.science.org/doi/full/10.1126/science.aan3184
- chestertn 4y ago
  We are talking about electrical water analogies. Stability issues are beyond the network being electrical and correspond to the electromechanical dynamics of the grid. These types of analogies are very sound and well studied [1]. Great power engineering books, such as Olle Elgerd's Electric Energy System Theory, make extensive use of these. https://en.wikipedia.org/wiki/Mechanical–electrical_analogies https://en.wikipedia.org/wiki/Mechanical–electrical_analogie...
  - photochemsyn 4y ago
    The water pipe - electrical circuit analogy is not necessarily horrible for DC circuits, but I think it becomes an impediment to learning pretty quickly. There's no water model that works for things like p-n silicon junctions for example. There's also the fact that transmission of energy by fluid in a pipe (sound wave speed I think) isn't anywhere near as fast as transmission of energy in a wire by electric fields, even though the electrons themselves aren't really moving that fast at all in bulk (*drift velocity is low). There's a 'pipe full of marbles' analogy for that effect, but the actual energy is carried by the electric field, not by a flow of electrons as with a flow of water through a water turbine. There's also the Drude model which treats the electrons as something like an ideal gas with electrons scattering off the positive metal ions: https://en.wikipedia.org/wiki/Drude_model https://en.wikipedia.org/wiki/Drude_model However if you get into band theory of solids as a means of explaining conductors, insulators and semiconductors, anything related to a water pipe analogy falls apart: https://en.wikipedia.org/wiki/Electronic_band_structure https://en.wikipedia.org/wiki/Electronic_band_structure
Nasrudith 4y ago
For one the "water" moves near light speed effectively and tanks andand reservoirs are far more expensive, and gravity doesn't meaningfully apply. And there is some leakage with distance. And you cannot use a turbine to generate water from torque. Even if many equation structures are the same as fluid mechanics the assumptions break from differences.
- chestertn 4y ago
  > The "water" moves near light speed effectively This is not how the hydraulic analogy works. The "speed" of the water is irrelevant. The analogy is used to understand losses, energy balance, energy conservation, reactive power, etc. The hydraulic analogy is very sound.

> Water tank and water pipe analogies fail pretty fast when it comes to thinking about electricity. In what ways does this analogy fail, for the purposes of understanding power grids?

pixl97 4y ago

Well, for one, if you get low pressure in one set of water pipes the entire water grid doesn't attempt to collapse because it goes out of sync. The grid is a gigantic clock that at least in the US is running at 60 ticks per second. This is pretty easy to manage if you have some massive power source on it like gigantic turbines at a nuclear power plant. All your small clocks aren't going to push that around so much. The problems come in when all you have is small clocks, who sets the phase of the grid?

chestertn 4y ago
In the pipes analogy, pressure corresponds to potential energy (voltage). Low voltage conditions does not necessarily trigger frequency stability issues. Grid synchronization is a different issue to the electrical-hydraulic analogy (which, by the way, was the one that Maxwell used and its pretty useful). Grid synchronization comes from the fact that electrical quantities in the network (current, voltage) are alternating periodic. Generators, which are usually mechanical, oscillate and induce such periodic signals. The system must remain synchronized. But this has nothing to do with the network being electrical. [1] https://en.wikipedia.org/wiki/Hydraulic_analogy https://en.wikipedia.org/wiki/Hydraulic_analogy
ZeroGravitas 4y ago
https://www.nrel.gov/news/program/2020/inertia-and-the-power-grid-a-guide-without-the-spin.html https://www.nrel.gov/news/program/2020/inertia-and-the-power... > “We find that replacing conventional generators with inverter-based resources, including wind, solar PV, and certain types of energy storage, has two counterbalancing effects,” said Paul Denholm, NREL principal energy analyst and lead author of the guide. “First, it’s true that these resources decrease the amount of inertia available on the system. But second, these resources can reduce the amount of inertia actually needed > “Ultimately, although growth in inverter-based resources will reduce the amount of inertia on the grid, there are multiple existing or possible solutions for maintaining or improving system reliability,” Denholm said. “So, declines in inertia do not pose significant technical or economic barriers to significant growth in wind, solar, and storage to well beyond today’s levels for most of the United States.”
- throwaway52022 4y ago
  The NREL inertia video explainer felt a little like it was begging the question - "inertia protects the grid because it has inertia and keeps spinning" - it doesn't quite feel like it explains where the extra energy comes from or goes, just that the mass keeps spinning. (I also haven't had a physics class in a long long time so some of this is not obvious to me, except that I understand from just common sense that if something's spinning you had to put a bunch of energy into getting it going in the first place and it's going to keep going if left on its own) Anyway, I was hoping someone could fill in some details for me. Imagine a simplified grid: a dam that sends water through a penstock past a turbine/generator and into an electrical circuit, and a couple of resistance heaters on the other side of the circuit. The energy comes from water flowing through the dam - the dam operator opens up the sluice gate to let water flow through, the generator extracts the mechanical energy and turns it into electrical energy and it goes down the wire to the resistance heater where it gets turned into heat energy. Everything is balanced - the right amount of water is flowing through the dam to turn the turbine at the right speed to balance out all of the energy flowing through the wires and into the resistance heaters (and lost along the way, like losses in the transmission lines, etc). In this setup, there's some measure of pressure that turbine pushes back against the water flowing through the penstock of the dam, which is balanced out by how much pressure is coming from the water behind the dam and the pressure being put on the surface area of the penstock in the dam and the pressure being relieved by the water leaving the dam. I get that thanks to inertia, if the sluice gate accidentally slams shut and all water stops flowing through the dam, the turbine is going to keep spinning for a bit and energy is going to keep going out onto the grid, though it will start to slow down due to friction at the turbine and energy being extracted from the system by the resistance heaters on the other end of the grid. What I'm less clear about is how does inertia help when the water keeps flowing at the regular speed but when demand drops from the grid load. Let's say one of the resistance heaters turns off in a home somewhere - what happens to the energy from the water that was previously flowing into the grid via the turbine? Does the inertia in the spinning of the turbine somehow push back against the water flowing the dam, slowing the water down a bit/building pressure up in the penstock and behind the dam - with that pressure buildup being exactly equal to the energy that used to be going into the resistance heater? And that pressure either stays built up from the turbine until someone lowers the sluice gate a bit to cut back on the waterflow through the dam? Or does nothing involving inertia happen here - if the resistance heater gets turned off the overall load is reduced and the turbine spins a bit faster because there's less pushing back on it, and the water can move through the dam a bit faster, and the turbine just spins faster until someone notices it's going a bit too fast and the gate needs to be lowered so it drops back to rotating at 60hz? Similarly, if someone turns on another resistance heater and now more energy is needed on the grid, but the sluice gate isn't opened up immediately, is inertia involved here somehow? If the turbine has to push harder on the grid side because of extra load, presumably the turbine slows down? Or does the turbine get pulled along by the new load somehow (more inertia?), and so more water can push past the turbine, giving it the extra energy it needs (and presumably dropping the water pressure in the penstock in the dam? And the pressure stays low until the sluice gate is opened up a bit more and more water can flow through the dam?) I am using water pressure from a dam here, but I assume this would be equally true in a gas plant generating steam - if more energy is needed, the pressure in the steam drops until someone turns up the burner and creates more steam, etc, or if less steam is needed the pressure just builds up until someone notices and turns down the burner? If anyone can explain how inertia and the grid translates into changes in the actual source of energy, I'd much appreciate it!

photochemsyn 4y ago

The fact that the AC power grid is synchronous is very unlike any water/gas system: > "In a synchronous grid, all the generators naturally lock together electrically and run at the same frequency, and stay very nearly in phase with each other...Small deviations from the nominal system frequency are very important in regulating individual generators and assessing the equilibrium of the grid as a whole. When the grid is heavily loaded, the frequency slows, and governors adjust their generators so that more power is output (droop speed control). When the grid is lightly loaded the grid frequency runs above the nominal frequency, and this is taken as an indication by Automatic Generation Control systems across the network that generators should reduce their output." https://en.wikipedia.org/wiki/Wide_area_synchronous_grid#Frequency https://en.wikipedia.org/wiki/Wide_area_synchronous_grid#Fre... Cascading power grid failures might be another good example: https://www.science.org/doi/full/10.1126/science.aan3184 https://www.science.org/doi/full/10.1126/science.aan3184

chestertn 4y ago
We are talking about electrical water analogies. Stability issues are beyond the network being electrical and correspond to the electromechanical dynamics of the grid. These types of analogies are very sound and well studied [1]. Great power engineering books, such as Olle Elgerd's Electric Energy System Theory, make extensive use of these. https://en.wikipedia.org/wiki/Mechanical–electrical_analogies https://en.wikipedia.org/wiki/Mechanical–electrical_analogie...
- photochemsyn 4y ago
  The water pipe - electrical circuit analogy is not necessarily horrible for DC circuits, but I think it becomes an impediment to learning pretty quickly. There's no water model that works for things like p-n silicon junctions for example. There's also the fact that transmission of energy by fluid in a pipe (sound wave speed I think) isn't anywhere near as fast as transmission of energy in a wire by electric fields, even though the electrons themselves aren't really moving that fast at all in bulk (*drift velocity is low). There's a 'pipe full of marbles' analogy for that effect, but the actual energy is carried by the electric field, not by a flow of electrons as with a flow of water through a water turbine. There's also the Drude model which treats the electrons as something like an ideal gas with electrons scattering off the positive metal ions: https://en.wikipedia.org/wiki/Drude_model https://en.wikipedia.org/wiki/Drude_model However if you get into band theory of solids as a means of explaining conductors, insulators and semiconductors, anything related to a water pipe analogy falls apart: https://en.wikipedia.org/wiki/Electronic_band_structure https://en.wikipedia.org/wiki/Electronic_band_structure

Nasrudith 4y ago

For one the "water" moves near light speed effectively and tanks andand reservoirs are far more expensive, and gravity doesn't meaningfully apply. And there is some leakage with distance. And you cannot use a turbine to generate water from torque. Even if many equation structures are the same as fluid mechanics the assumptions break from differences.

chestertn 4y ago
> The "water" moves near light speed effectively This is not how the hydraulic analogy works. The "speed" of the water is irrelevant. The analogy is used to understand losses, energy balance, energy conservation, reactive power, etc. The hydraulic analogy is very sound.