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As far as i know the following are true:

  1. for a power grid to work properly, the voltage supplied by all the generators has to be relatively synchronised at every point in the grid, especially at the generators themselves.

  2. changes in electric fields travel at a finite speed $c$ in any reference frame.

This seems to me to give a contradiction for the proper working of grid power for a sufficiently large power grid. Since 2 generators A and B at a distance $d$ would experience a lag of $d/c$ between them, so say A is synced to the voltage it receives from B, then B receives the voltage from A with a phase delay of $2d/c$. If then $d=c/4f=1500km$ at 50Hz, The voltage from A would be opposite that of B causing a short circuit at B. Since $O(1500km)$ is a scale on which some power grids operate, how come light delay and the resulting interference are not an issue?

Qmechanic
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jucom
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3 Answers3

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You are correct, so what does the industry do?

Grid Design and Control:

Power grids are engineered to account for phase differences and signal propagation times. They use various devices, such as phase-shifting transformers and reactive power compensators to manage and adjust phase relationships.

Generators locally and across the entire grid use sophisticated control systems accounting for delays in signal propagation such as phasor measurement units (PMUs) to monitor/adjust voltage Phase at different points in the grid in real-time.

Strategic segmentation of the grid also adds to the prevention of the kind of destructive interference that you described.

DrJay
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Even worse than that - any piece of wire has its own inductance and capacitance, which are non-negligible when the line is long. Essentially, a power line acts as a waveguide, which sustains its own electromagnetic modes, which may completely distort whatever signal is sent via the line. The problem had been recognized more than a century ago, and appropriate measures are taken to offset these effects or correct them - you may read more about it in connection to Telegrapher equations.

As for the direct question in the Q.: obviously the phase of generators should be chosen in such a way that the currents are in phase at each point, not the generators themselves (which may be far apart.)

Community
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Roger V.
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A related problem is instability. I don't think the current means to control synchronization issues using dynamic reactive compensators (as mentioned above) is going to be good enough when there are a large number of green-energy sources (when more and more people on the grid can supply energy to the grid through solar and wind energy). It's been suggested that smart grids are a solution, which would mean that there would have to be additional communication systems in place for the green energy sources. Whether these can prevent all modes of instability (without adding new ones), I really don't know. See, https://www.hivepower.tech/blog/grid-stability-issues-with-renewable-energy-how-they-can-be-solved

Andrew_UELtd
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