A large distribution grid can work like this. There're several power station each outputting 50 Hz AC. Each power station feeds energy into a substation next to it which raises the voltage and then feeds energy into a powerline and that powerline goes to a substation close to the customers. The deal is customers don't really want to depend on a single station and that single powerline so they've crafted a distribution ring - there's a chain of high voltage substations connected to each other and each power station feeds energy into that ring - each substation in the ring is connected to two of its neighbors and also to the power station. The more advanced design is to have each power station connected to two of those substations via a separate powerline.

Now electricity "moves" at the speed of light which is high but still finite and over the distance of dozens and even hundreds of kilometers the phase difference between the different power stations will be notable and because of that phase difference different power stations should partially cancel each other out.

If several power stations were connected to a single point they could have been synchronized appropriately but in the described setup there's no single point - there're multiple substations separated by long powerlines forming a closed ring and so it looks like something will always be out of phase with something else.

How is synchronization possible in such conditions?

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    \$\begingroup\$ Many cities have a distribution ring around their metro area. It furnishes more than one route for power delivery, it's called "the Loop". That is why many downtown's are referred to as "the loop". The substation automatically corrects phase. If lagging, its generators run as a motor and draw power, and allow for its speed to increase, bringing it into phase. If leading, its generators will slow from the load, bringing it into phase. When three phase power lines are down from a storm, the linemen connect a line to the line with the least voltage difference. Does this make sense. \$\endgroup\$ Commented Dec 2, 2013 at 13:48
  • \$\begingroup\$ @Optionparty, The downtowns I know of called the "loop" or something similar (Chicago, Washington DC) are called that because of a transportation loop (Chicago's "El train", DC's "Beltway"), not an electrical power loop. \$\endgroup\$
    – The Photon
    Commented Dec 2, 2013 at 17:16
  • \$\begingroup\$ A Quadrature booster can be used to correct phase angles between nets. \$\endgroup\$
    – jippie
    Commented Dec 12, 2013 at 6:34
  • \$\begingroup\$ Electricity in a wire does NOT move by the speed of light. In Copper wires electricity travels with about 1/2 the speed of light, which is (to my knowledge) as fast as you can get with electricity. \$\endgroup\$
    – Gewure
    Commented Jun 8, 2017 at 12:23

3 Answers 3


Let's say that your hypothetical ring system is exactly 1/2 wavelength in circumference.

enter image description here

Now, let's suppose that you start each generator by synchronising it to the local frequency, which is how it's done in real life. (In really olden days, using three lightbulbs, in olden days, using a synchroscope; these days using a auto-synchroniser.)

enter image description here

The clock faces represent phase, as measured against a global reference (say GPS time.) We do, indeed, have a problem. At the open breaker there is a 180 degree phase difference, and closing the last breaker is likely to make something explode.

The trick is that generators are started in synchronisation with whatever the local frequency is, but once they are running, they are slowly adjusted to be in synchronism with a common phase reference - say GPS time.

enter image description here

Now you only have a 45 degree phase difference across the open breaker, which is more manageable.

In practice, no part of a ring system like this would be a eighth-wavelength long (1000km?) so the phase difference would be less than 45 degrees.

In practice, I have never heard of this being an issue. Possibly because real world networks aren't long enough; or GPS phase synchronisation is implemented as above; or possibly because transmission networks are not built as rings, but as rather more densely connected meshes where there are many short interlinks between nodes, which act to equalise frequency within the local "neighbourhood" of substations.

For more detail on check sync relays and auto-synchronisers, see §22.8 Power System Measurements - Synchronisers in the Areva/Alstom/Schneider book Network Protection and Automation Guide, 2011 edition (NPAG). Alstom NPAG page. NPAG 2011 on Scribd.

Update: While cleaning up my reference files, I found a document Fundamentals and Advancements in Generator Synchronising Systems by Michael J. Thompson of SEL Inc, a well regarded manufacturer of power systems protection and control equipment.

The document is very interesting in general and also includes some guidelines regarding the tolerances in voltage, frequency, phase, when synchronising:

enter image description here


Firstly, if you're going to have such a ring you need to make sure that it's sized to a whole number of wavelengths. Having done that, fire up a single power station and apply it to the ring. Each distribution point will have a signal at a particular phase.

It's then obvious that you don't need to be in phase with the system, just your attachment point to the system. Measure the local phase and start up the generators in phase with that.

A smaller version of this applies to grid-tie inverters used with solar panels. They sense the mains phase and apply power additively to it. As a result they usually turn off when disconnected from the mains, to prevent issues when trying to reconnect at a different phase.

  • \$\begingroup\$ In your example all the points where there isn't a power station connected yet there will be two lines coming to the station and they will have different phases, won't they? \$\endgroup\$
    – sharptooth
    Commented Dec 2, 2013 at 10:33
  • \$\begingroup\$ Why will they be different phases? (assuming the ring is an integer number of wavelengths) \$\endgroup\$
    – pjc50
    Commented Dec 2, 2013 at 10:37
  • \$\begingroup\$ I completely missed how that part of the answer was important. Yes, this makes sense. And it's kinda funny for layman's perspective. "Johnny, we need to cut this half meter of cable otherwise the whole thing won't work". \$\endgroup\$
    – sharptooth
    Commented Dec 2, 2013 at 10:43
  • \$\begingroup\$ Bit longer than that in this case: one wavelength ~= 6000km (source www.ukerc.ac.uk/support/tiki-download_file.php?fileId=1055 ); if you're using three-phase then you can pick the nearest 2000km. Alternatively if your ring is much smaller than 6000km then the phase differences will be small, and can be compensated with inductor/capacitor systems (sometimes called "reactors", confusingly) \$\endgroup\$
    – pjc50
    Commented Dec 2, 2013 at 10:50
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    \$\begingroup\$ They have it at "approximately zero" wavelengths (which is probably anything up to a hundred km), at which point the phase difference is close enough to zero. \$\endgroup\$
    – pjc50
    Commented Dec 2, 2013 at 13:37

Power stations do not need to be exactly synchronised with the grid - a small phase angle difference is acceptable, and is in fact required to push power into the grid.

The flow of real power depends on the phase difference between the source and destination. See https://en.wikipedia.org/wiki/Quadrature_booster

The flow of reactive power depends on the voltage difference between the source and destination. See https://en.wikipedia.org/wiki/Static_VAR_compensator

One of the advantages of HVDC transmission systems is that they eliminate any stability issues caused by phase difference between source and destination ends.


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