Imagine you are on half a tandem bicycle. You are in a room with the chain of the tandem vanishing beyond the wall. You can't see, but clearly someone else is pedalling the thing as the pedals are rotating. Getting on it is tricky, as you have to rotate your feet at the right speed, but once you're on you can sit there and let it carry them round - or you can start applying pressure through your feet to do work and accelerate the chain system.

At this point you discover the thing on the other end of the chain is not a person but a 60Hz synchronous motor-generator. Congratulations, you are in sync with the grid and (when pushing on the pedals) contributing power.

(A corollary of this is that if your grid connection is down or the grid is split into two pieces, you can't start up again until you get a grid input to know you're in phase. See "black start" for details on this.)

It's like a very long pipe with standing water in it. AC power is like attaching a pump column at one point and adding/removing pressure at just that point. This creates a /wave/ of energy that propagates, but the actual water (electrons) do not. It's more accurate to say they vibrate.

Continuing the metaphor, power plants increase the amplitude of this sloshing wave, and must be synchronized to the phase of the wave /where they connect/ (in order to be additive instead of subtractive). Consumers of the power are always subtractive, '(active/passive) power factor correction' in switching power supplies is about attempting to keep the consumption timed for efficiency. AC Motors are continuous users and mostly just draw all the time.

In early days of electrification the physical topology usually was spanning tree and in fact this was the motivation for research into minimal spanning tree algorithms at the time.

Think of the phase as a distributed signal that indicates the current state of the local system taking into account all connected generators. It's weird but true!

https://www.scribd.com/doc/118169687/Network-Protection-Auto...

This makes it sound more complex than what I learned when I last tried to figure it out =)

They seem to be describing that where two grids meet they go through a synchronizer that disconnects one or the other if they aren't in sync, so implies manually disconnections throughout the network as needed locally.

If instead of a tandem bicycle, it is a very long rope that someone is moving up and down, then when I try to join some distance away there will seem to be a wave passing me. Even if I join in at the correct phase, I will also be generating a wave traveling back to the first 'generator' and interfering with the existing wave along the way. With the right distance and relative strength then we will produce a standing wave between us. Is there an apparent 'direction' to the wave of electric potential in a large grid?

Then there is the issue of triangular arrangements the sibling comment raised, when the radial distances and relative phases don't align.

Maybe my issue is always trying to think through physical analogues I can 'see'.

The nearest I can think to "direction" is "reactive power", or the phase angle between voltage and current at any particular point.

There very definitely are loops and multiple paths to a particular point, it's not called a "grid" for nothing. This little map of the UK grid is interesting: https://www.nationalgrid.com/uk/about-grid/our-networks-and-...

Edit: Aha, further reading - systems for altering the phase angle: https://en.wikipedia.org/wiki/Quadrature_booster

And https://www.researchgate.net/publication/27341916_Influence_...

I'm also wondering since a couple of years about the same question.

Once the generators are switched in, they'll inherently maintain sync, as even if there's too little motive power being supplied by the station's turbines, the difference will be made up by the power grid itself, which will drive the generators round like an electric motor. (Typically this would happen for a brief period after coming online, after which the station would throttle up and start helping to push the generators round.)

If you think of two generators connected together in a power grid, they need to have the same frequency and the same phase. If one of the generators slows a little bit, it begins drawing current from the other. It in effect becomes a motor. Power flows as a proportion to the difference in phase angle.

Perhaps they're not using just frequency to coordinate (solar/wind perhaps needs a sidechannel?), and someone on their grid in a politically unstable area is using power without reporting on the side-channel. Hence the frequency drops due to the unaccounted load.

But the effect is as you say: something goes bang.

Yes, east coast and west coast are out of phase, but only to a perfect observer who can see phase value on both coasts instantly (without a propagation delay). However in reality information about the phase cannot travel faster than speed of light, so to any real observer anywhere between east and west coasts both sides appear to be in sync.

Going forward it may not be a good idea to rely on the grid's timebase as there's talk as well as an NPRM out there to do away with the standard. https://www.federalregister.gov/documents/2010/03/29/2010-64...

All power grids are connected, so there is just one frequency.

http://www.nerc.com/AboutNERC/keyplayers/PublishingImages/NE...

and here is a commercial example: https://www.siemens.com/global/en/home/products/energy/high-...

This is actually quite handy. When demand locally exceeds supply, the phase will lag (this is how generators work), and power will flow toward the excess demand.

More advanced grid operators use various devices to intentionally shift the phase to control the flow of power. [2]

[1] Actually calculating this is surprisingly complicated. You can integrate the Poynting vector; you can model the inductance and capacitance of the line, calculate the current, and use P=I*V; or you can probably do it in several other ways. You might even be able to model it as little packets of energy moving along at the group velocity.

[2] This ability is important for economic and engineering reasons. Imagine you connect two cities with two parallel, competing transmission lines. One has 100 MVA capacity, and one has 10 MVA capacity, but they have the same impedance. (This is a bit farfetched, but capacity is related to impedance, heat dissipation ability, and the ratings of whatever equipment is at the ends of the lines.) Without some kind of active control, to much power will flow through the 10 MVA line and it will fail.

> The electrical grid that powers mainland North America is divided into multiple regions. The Eastern Interconnection and the Western Interconnection are the largest. Three other regions include the Texas Interconnection, the Quebec Interconnection, and the Alaska Interconnection. Each region delivers 60 Hz electrical power. The regions are not directly connected or synchronized to each other, but there are some HVDC interconnections.

Here’s some visualizations of that (see the linked movies) from a cool project called FNET/GridEye:

http://fnetpublic.utk.edu/sample_events.html

But the answer is contained in the parent comment. Each plant generates more power if the phase lags and produces less if the phase leads. Basically, electricity consumers drag on the phase and the plants use this to control output power, so that the phase matching can be done purely locally.

But it is a lot simpler if you use high voltage DC! If you do that you can connect networks with entirely different phases!

https://en.wikipedia.org/wiki/Continental_U.S._power_transmi...

So that takes care of some of it.

Anyway, synchronization is actively managed, when a generator starts up, it is synchronized to the grid adaptively.

https://en.wikipedia.org/wiki/Synchronization_(alternating_c...

So there must be some accounting for this propagation error, either as losses, or engineering.

What I suspect (totally not my field) is that the high power interconnects between large generators do have this triangular lag (the incoming lines lag from the plant's reference frame), but they use quadrature boosting or similar to match that particular interconnect.

T&D (transmission & distribution) result in ~6% of the net energy being wasted. I imagine phase lag "feels" like impedance, with the real component acting as resistance.

Also, it's worth noting part of the appeal of HVDC, aside from line reactance and skin effect, is you get to choose your output frequency.

https://www.nema.org/Products/Documents/TDEnergyEff.pdf

Maybe active synchronisation solves this, but to my (inexpert) mind the network would still suffer from interferences at various points between power plants.

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

BTW North America actually has eight transnational (USA/Canada) interconnection authorities or "regions" in which power distribution is managed; it's not a unitary entity.