Why does the power come back on for a few seconds after the power goes out?
In: Technology
The thing with high voltage power lines is that they have a tendency to go out for faults that were there, but are gone again just a few seconds later.
Regulations differ in different places, but the general idea is that the power line must be without power really fast when a fault occurs. 100 milliseconds kind of fast.
And that is difficult, because the breaker itself needs 50-75 of those milliseconds just to physically move; the monitoring electronics need to act before they have enough data to have the full picture of what type of fault they are reacting to.
A really flimsy tree branch seldom stays on the power line. The blast when it hits the line is often enough to toss it off the power line again. But it’s also enough for the controlling electronics to be able to tell that there was something short circuiting the line, and the breaker gets ordered to remove power.
In that case, it makes perfect sense to just wait a bit, and try to reinstate the line.
And that’s exactly what happens. A few seconds later, the central computer at the network operations centre orders the breaker to close again to reinstate the line.
If it fails then, well, then you obviously have a real fault.
Where I live, regulations allow automatic reinstatement within 30 seconds and manual remote reinstatement within 120 seconds. After two reinstatements (one automatic and one ordered by the operator) and 120 seconds, and the line is still not up, regulations demand manual inspection; the line may not be reinstated until staff is at location and looks for a fault.
Why? Well. Theoretically, there could be a car stuck in a fallen power line. Whoever is in that car would certainly not want the power to come back on again and again and again.
Or, that tree could have been a thick one that does not fall off again when the line is reinstated. If you then repeatedly try to reinstate the line, you’ll eventually make the tree catch fire. And that…that is also not a desired outcome.
Most of the outages on high voltage lines in the lower part of the voltage span are because of trees. You have to take into account that what you are looking for is pretty likely to be something that will cause a wildfire if you do dumb things.
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