This is slightly imprecise. It actually works the other way round: the demand is "communicated" through the network by slight changes in the frequency.
This can be described best for the example of steam-based generators, as they are typical for the majority of coal/gas/oil/nuclear power plants. When demand for electricity grows, for example because a new factory is powered up, the turbines in the power plants are experiencing larger forces attempting to slow them down, starting with the power plant closest to the new factory.
The speed (and thus the frequency) is constantly measured, and if it drops, the steam valve is opened just a tiny bit more to counteract the drop. If this is not possible because the power plant is already at maximum output, either other power plants have to take the load (and the electricity is then routed through the network to the factory) or additional power plants may have to be added to the network.
The important thing is: the frequency change is a means of communication, it originates from demand changes, and because the entire network is kept in sync, it works to communicate demand changes across the entire network. Power plants providing energy to a sub-part of the entire network always attempt to run their turbines with the exact same speed of 50 rotations per second, which should ideally always conform to the frequency in all other sub-networks that a specific sub-network is attached to.
If just one of these sub-networks does not perform its duty of re-establishing 50 Hertz by powering up their own plant output, the rest of the network must either channel enough power into this sub-network to allow its local generators to re-establish 50 rotations per second by taking some of the load off of them or - if that is not possible or decided against - must also deliberately drop their frequency (and thus the rotational speed of their generators) to match the sub-network that is deviating.
The only alternative to this would be to drop off the deviating sub-network entirely, but that is usually only done in extreme cases of deviation.
If the USA's power grid is about 4,000,000 meters wide, and the electric signal propagates at roughly the speed of light (3 x 10^8 m/s) (https://physics.stackexchange.com/questions/358894/speed-of-...), then it will take roughly .013 seconds for the electrical signal to reach from one station to another. This is a lot out of phase! (the period of 60 Hz is .016 sec) How is this managed?
If two generating stations on opposite ends of the country are both contributing to the electrical signal, how is this time lag accounted for? I have two guesses:
1. The power network is "mapped" such that there is one central generating station and all other generating stations are time lagged based on their graph distance from this generating station
2. All stations try to generate simultaneously, with their output signals interfering with eachother to a certain degree. This might not matter much because their local loads dampen the strength of their signal.
Either way seems to introduce inefficiencies.
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:
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.
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.
For example, there was a big power outage in Florida in 2008 that caused a generator to suddenly go offline, and several orgs had a couple dozen power frequency meters running on the grid at the time, so they were able to make an animation of the east coast power grid "ringing" over the course of about 10 seconds as the load changed rapidly throughout the grid.
The animation for that is here: https://www.youtube.com/watch?v=bdBB4byrZ6U
Do you know how the power plant's control loops are tuned? It seems like just tuning your plant to be properly damped isn't enough, because there's also lots of feedback from other power plants.
(And really, the load changes are typically more gradual. Even "everyone just got home from work" is a fairly spread out event, compared to, say, the sudden loss of a few hundred MW of generating capacity...)
https://www.youtube.com/watch?v=slDAvewWfrA https://www.geek.com/geek-cetera/tea-time-in-britain-causes-...
> TV pickups [...] are a surge in demand caused by the flushing of toilets (leading to a surge at the pumping stations) and the opening of fridge doors by millions of people. There is a common misconception that the number one driver of TV pickup is the boiling of kettles. In fact, this only creates a pull on the local network for a short period of time until the water has boiled, and can therefore be managed relatively easily, whereas flushing the toilet causes a longer surge at the water and sewerage pumping stations, and opening the refrigerator lets the chilled air escape, causing the compressor to run.
Personally I'd expect a shortage to result in a lower voltage, not a lower frequency.
I'm also rather surprised that the correct functioning of my clock depends on the political stability of a politically unstable part of the continent.
The opposite is also true, if you suddenly increase the load on the generator, the generator will start spinnging less fast and the frequency drops to say 49.9Hz.
This is all very carefully monitored, and there are multiple automatic (massive) power breakers throughout a power network to detect these changes. Imagine all power lines of a large part or entire country suddenly dropping away. The network outside of that country would suddenly experience a large decrease in load and its frequency would go up, while inside the country the load would have a large increse, reducing the frequency in that country. If the change is so large that the frequency goes above 52.5Hz or below 47.5Hz, then the automatic circuit breakers trigger to prevent damage to the entire network (basically adjusting the load of the network). If it can't adjust enough, more circuit breakers will trip, causing larger outages.
Example of these are the 2003 Italy blackout and 2003 US/Canada Northeast blackout.
Reconnecting a blacked out part to the grid isn't easy either. People tend to leave things on during a blackout, such as airconditioners and TL lights. Now when the power comes back, these things draw a low of power for a short period of time during startup. Normally this isn't bad, but when they all do it at the same time the load is massive (think of a power spikes of 7-8x the normal usage), usually tripping the same huge circuit breaker again.
As for the clock deviation, apparently running below 50Hz for a longer period of time has been condoned by the people in charge of the EU power grid. Maybe for too long. There are two solutions: - generate more power in the EU, but that costs money. They could sell the power to the slackers (which I guess they don't want to pay for) - cut the slackers off (which is very drastic and does not help the EU's idea of looking out for each other).
But a decision has to be made. Looks like more power is currently being generated, as the frequency is now on 50.010Hz.
If the problem is that the frequency is too high, i.e. power load is too low in that coubtry, why would tripping a break help? Wouldn't that very suddenly aggravate the problem and create even higher frequencies?
"On a level road it is easy to maintain speed. On reaching a gradient, however, the rider needs to make more effort to achieve the same speed. Going downhill, the rider needs to apply the brakes to keep the same speed.
In the entire European network the electrical generators are set up in such a way that they automatically and immediately respond to a change in grid frequency. Depending on the level of consumption they increase or lower their capacity. This ensures that the frequency remains stable. This automatic adjustment can be compared with the cruise control in a car."
0. https://www.swissgrid.ch/swissgrid/en/home/experts/topics/fr...
And very interesting thoughts come up, especially when you add newer developments like non-rotational-mass-based power sources (solar power, batteries, stuff like that) into the mix. Because at the moment with the high number of traditional steam-powered generators, the rotating mass of the turbines and their inertia serve as a kind of buffer against sudden swings in electricity demand with an ultra-quick response time of "basically no time". As a second buffer stage we have the steam supplies in the plants, which can be quickly accessed just by opening a valve, thus the response time is non-zero, but still quite short.
Now replace all those generators with some other power sources without those inherent balancing capabilities (that probably weren't explicitly built into them, but that just happen to be there because of their construction) and you'll get a network that is much more vulnerable to sudden changes on the demand side. Now, explicit mechanisms of storing energy in a way that can be accessed in literally zero time need to be introduced into the network, just to keep it stable and to provide other regulation mechanisms (of which a lot of them also have to be re-thought) with enough time to do their job.
I wouldn't wonder if there were already plans for building large rotating masses with motors on them that are kept at network frequency, with the motors switching over to serve as generators when the frequency drops.
The question of "how can we build a large and stable grid without thousands of tons of rotating mass?" is an area of very active research I've been told.
(It should be noted that other motors, including asynchronous machines, contribute as well)
AIUI the HPR has reaction times on the order of a few seconds.
Flywheels? Yes: https://en.wikipedia.org/wiki/Flywheel_energy_storage#Grid_e....
It was also built to be able to do a black start (startup after a total grid outage).
Aren't all stations?
Some coal plants had gas turbines[0] fitted solely to allow a black start, others a ton of diesel generators. Some plants need power from the grid to kick off - nuclear for instance. Only some percentage of the grid needs cold start capability, and the rest of the grid is bootstrapped from that.
[0] (edit) Think aero engine. Rolls Royce Avons were used at Didcot A, and the industrial Avon is still available after 60 years. That's the same basic engine found in the Hawker Hunter and EE Lightning.
For that reason, all the equipment in the generator building ran on 48V DC, supported by a large bank of batteries.
Somewhere there will be a manual for the unfortunate operators who will have to restart the whole grid in the correct order, relying on the telephone company's backup generators to communicate.
Now that a bunch of wind and solar is being added to the grid with completely different control schemes, there’s a lot of concern about whether the system will continue to stay reliably synced.
Only when people started interconnecting these generators in a grid did the need to settle on a single frequency arrise.
How does the demand affect the frequency ? Is there a logical unit, somewhere, responsible of "communicating" through the network ? Or is it a physical effect ?
> When a factory is powered up, the turbines in the power plants are experiencing larger forces attempting to slow them down.
How does something like a higher demand, a drop in frequency, translate to a larger force attempting to slow down the turbines ?
If this was not the case, we could basically generate arbitrary amounts of power from nowhere, as there was no resistance and we could thus just spin a generator forever with zero power necessary (except for the little bit required to overcome friction) once it is set in motion. The Lorentz force makes sure that's not the case by slowing down the movement, and it slows it down more if more electric power is pulled from the generator.
There is websites such as https://www.swissgrid.ch/swissgrid/en/home/experts/topics/fr... that track and display the deviation.
When the alarm went off the following morning, I proceeded to get ready for lectures (This clock was my only means of telling the time, there were no phones or computers in my room) and after having showered I switched on the TV (which could only receive 3 channels at the time!) where I was met with unfamiliar programmes. This confused me for a long while until I realised the time was about 40 minutes later than the clock told me. I missed my first lecture.
Looking at other comments, it seems that we're a little weird down under and most places don't track the drift.
It scrapes the swissgrid.ch page (with something like "curl | grep | sed > csv" in a loop), so the data is only as accurate/current as theirs…
Edit: here's the actual sheet in case you want to "fork" it – https://docs.google.com/spreadsheets/d/1i9cLeKb5Eq1IoZFOUoOk...
Apparently to match the demand at given instance electricity plants rotate their turbines a bit faster or a bit slower instead of switching a complete plant on and off[edit: this is not exactly right, see the reply below], which results in slight deviations from the 50Hz standard but it is fine as long as it stays between the limits.
At the end of the day, as the demand increases and decreases, the average frequency would be 50Hz and engineers took advantage of that fact to create clocks that may not be accurate to the second but accurate on average. How do they do that? They count the change in the electricity and assume that 50 changes are exactly 1 second.
Unfortunately, due to political issues in the Balkans, the grid was undersupplied or oversupplied for a prolonged period and this created a deviation from the average of 50Hz and the clocks that depend on this average to be 50Hz also lost accuracy that currently amounts to 6 minutes.