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Let's consider a simplified electricity grid - say there are two power plants (A and B) 1000km apart. There's a town (L1) half way between them, and another town (L2) some distance downstream from B. The time taken for the electricity to travel from A to B is about 1/300th of a second, so that would put A's output 1/5th or 1/6 of wavelength out of phase with (itself?) the time it's got to plant B. Does plant B synchronise to the plant A's phase as it is at plant A, or plant A's phase as it is when it gets to plant B? If the former (presumably using a global time source as the basis), then the two would be in phase when they reach L1, but not when they reach L2. If the latter, then they'd be in phase reaching L2, but not reaching L1.

schematic

simulate this circuit – Schematic created using CircuitLab

askvictor
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  • 1/300th of a second? Your calculation is off by several orders of magnitude. Along the same lines, what exactly do you think the wavelength of a 50 Hz sine wave is? – R Drast Aug 02 '17 at 10:25
  • @RDrast What do you mean by that? Possibly out by a factor of two or so, depending on the cable and isolation used. Speed of light over 1000 km. – pipe Aug 02 '17 at 10:30
  • My error on the speed, units error. Still look at the wavelength. – R Drast Aug 02 '17 at 10:44

2 Answers2

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Grids are not synchronised by setting the generators to a specific phase, they are self synchronised by the fact that the generators can only run within a small phase difference of their local connection to the grid.

A generator which is powered enough to overcome its own losses will settle down to be exactly in phase with its local connection. If its driving power is now increased, it will speed up for a moment, advancing its phase, and exporting power into the grid. It will settle down to be slightly in advance of its local connection, where the power it exports into the grid is equal to its input power.

The phase across the grid will be determined by power flows as well as distance. A normal grid will have power being injected at multiple discrete points, and withdrawn (to a first approximation) more or less uniformly over the distribution area.

In your simple illustration, if plant B was supplying both loads, and then plant A came on stream, plant A would initially synchronise to the 1000km delayed version of B's phase. Once A started supplying significant power however, the A-L1 phase shift would be modified by the A-L1 power flow.

As it's power flow that controls phase shift, there are problems when a grid becomes weakened by connections going offline in an unplanned way, say due to storm damage. As only a certain amount of power can flow through a connection, only a certain phase shift can be controlled. If the remaining connections between two regions of the grid cannot support the power flow required to keep them synchronised, then the connections may trip out, and the regions become 'islanded'. Without the self-synchronisation, it can take a long time to re-establish the grid connections.

Neil_UK
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  • That's starting to make sense; can you clarify: if the load at L1 requires power from both A and B, they will ultimately synchronise together, in that case, L2 will only be getting power from B, making the fact that A is out of phase with B at L2 irrelevant, as A's power isn't getting to L2 at all? Is there a more complicated scenario (involving more plants and loads) where this would be a problem, or does it all boil down to my example? – askvictor Aug 02 '17 at 21:48
  • Also, This is all assuming turbine generator loads, and that the grid will drive the generator as a motor to get in sync (do I have that right?) What happens in the case of wind turbines - do they follow wind speed or grid speed? What about solar plants - they have electronics to convert to AC; does phase matching circuitry mimic what happens in a spinning generator? – askvictor Aug 02 '17 at 21:52
  • please help me with this problem https://electronics.stackexchange.com/questions/352783/synchronising-power-synchronising-coefficient – Nikhil Kashyap Jan 29 '18 at 15:27
  • @NikhilKashyap sorry mate, I only do hand waving, not equations. At least, in this field anyway. – Neil_UK Jan 29 '18 at 17:59
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A small power plant can only follow the grid frequency, it can not alter it. But there are powerful power plants that dictate the mains frequency by means of measuring and acting together synchronously. When the load is cut off, then the entire network will start to gain frequency. Same, when the load is increased the frequency begins to drop. A "coalition" power plants do increase/decrease power simultaneously, so they regulate the frequency.

Marko Buršič
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  • Marko please take a look at this problem https://electronics.stackexchange.com/questions/352783/synchronising-power-synchronising-coefficient – Nikhil Kashyap Jan 29 '18 at 15:26