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Comment by photochemsyn | Hacker News Reader

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#Fre...

Cascading power grid failures might be another good example:

https://www.science.org/doi/full/10.1126/science.aan3184

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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_analogie...

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

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

A specific analogy doesn't have to be used forever. You can drop it when it starts being problematic.

Asked a different way, what didn't work for the purposes of this article? It's at a high enough level that "becomes an impediment" isn't really an issue. Nor are "p-n silicon junctions". Nor the speed of transmission.

That's the concern being raised. It's indeed problematic to use a water bucket/pipe analogy in an AC grid when talking about transmission and generation of power, and related things like spinning reserves. (It's less problematic when discussing energy.)

The water analogy is perfectly fine to use to talk about the bulk AC transmission grid (which is low-frequency alternating current).

photochemsyn speaks about how the hydraulic phenomena is not adequate to describe some electronic aspects (e.g., modeling p-n silicon junctions). These issues are irrelevant to how well the hydraulic analogy works to describe the workings of the AC transmission grid.

Yep, exactly my point. For the purposes of this explanation in the article the analogy works fine, hence why I'm asking what exactly didn't work.

That it doesn't work in all possible cases talking about A/C transmission doesn't really impact its use as an analogy in a high-level view of the grid.

There are many aspects where the analogy is flawed, when talking about the electrical grid, e.g. you will have different types of loads (capacitive, resistive, inductive) and those loads will change the way your grid behaves. This is not something that you have in fluid dynamics at all (to my knowledge) yet any grid operator who would ignore such things would have their gear just explode.

A load being inductive, capacitive, or reactive has to do with its impedance, which is a concept related to the steady-state analysis of AC networks.

The hydraulic analogy easily extends here - just imagine water flowing back and forth.

This is well studied [1] and power engineering books make use of this analogy.

[1] https://en.wikipedia.org/wiki/Hydraulic_analogy.

I had a great teacher try to explain thusly:

The grid is a heavy rope that you are trying to keep tension on so that it doesn't touch the ground. One side of the rope is the power station, the other end splits and frays into individual strands that connect to every meter on that grid.

Strands are continually, almost randomly, disconnected (let go), reconnected (picked up), and the amount of "pull" on the rope each strand creates will vary (usage fluctuations). This means the power station needs to continually be adjusting how much it "pulls" on the rope to maintain tension and keep it from dropping.

I like this one because it not only avoids water and pipes and cuts to the core of the balance issue that is going on. It's not "thing A flows from point B to point C" but rather "this is an active system and careful balancing act".

That being said I recognize that the water pipe models are well studied, even in how they successfully model some aspects (and fail to capture others) of electrical grids.

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