Last night a monster storm front rolled through southern Michigan. Around 11:30 there was a pretty good lightning strike, and the power flickered for a second or two before shutting down completely. It was out for maybe 30 seconds, and then came back on.
What’s the sequence of events upstream of my wall outlet? Why did the power flicker for a couple of seconds before going out completely? How does lightning cause a power failure that can be recovered from so quickly?
I don’t know the exact details of your outage or how exactly power is distributed in your area, but if I may?
I was an electronics technician at a LORAN station. Besides 4 timer/receiver sets and a rubidium frequency standard set, I was also responsible for the care, feeding, and operation of two 180 KW transmitters. Yeah, I know, not very powerful by LORAN standards, but this was Loran A for the Caribbean chain. Anyway, there were 3 strike overload relays built into the transmitter. If they sensed an overload, they’d shut the transmitter down to protect it and after a short time, they’d reapply the power. If there were 3 overloads in a certain period of time, maybe 30 seconds, it wouldn’t reset automatically and you’d have to manually reset it.
So… maybe there was a lightning strike that triggered an overload relay. Maybe that started a 30 second delay relay. After the 30 seconds, the relay reset and the power was restored. As for the flickering, maybe after the relay opened, there were some capacitors that were discharging or some automatic switching was attempted to get around the momentary outage?
Oh, if it lasts less than 60 seconds, it’s considered a momentary event.
Well, that’s my SWAG. YMMV.
Electrical distribution systems tend to use gigantic circuit breakers called automatic reclosers. The purpose of a circuit breaker is to protect the equipment from an overload, such as a short circuit. The idea behind reclosers is that most overloads on distribution equipment are temporary, like a squirrel getting inside a transformer and blowing himself up. You don’t want to have to call out a bucket truck to reset the breaker in these situations. So they are designed to trip once, then reclose after a few seconds. If the overload is still there, they trip again and wait a few more seconds. Then they might try a third time and stay off for good if the fault persists.
Lightning striking power lines or equipment will obviously cause an overload situation. Probably a recloser in your area tripped off, then reclosed and saw that the line was still overloaded with residual current from the lightning strike, so it opened the circuit again for a longer period, then reclosed and everything was fine.
Yep, as friedo said, you’re seeing an automatic recloser in action.
Most power line faults are transitory. You get the occasional squirrel, as friedo mentioned (a squirrel took out power to about half of the small town I live in a few months ago), but more often (at least in areas with overhead lines) the fault is caused more by something like power lines blowing together in the wind, or tree branches or other objects falling onto the lines. Once in a while a lightning bolt will cause a huge overcurrent and will trip out the line. It’s enough of a pain to have to walk down into the basement to flip back on a circuit breaker. Now imagine that the circuit breaker is a 15 minute drive away from where you are. Hence, they invented reclosers.
You can think of a recloser as a circuit breaker (like a really big version of the kind that are in your house) that automatically flips itself back on after it has been tripped. Reclosers are programmable, but the way that they are usually programmed is that they try a couple of times fairly quickly to reset the line. If the power doesn’t come back on, they wait a bit for the fault to hopefully clear (typically a minute or two, but 30 seconds isn’t an unreasonable time) and they try again. After that, they usually give up, and that means a lineman has to come out and figure out what’s wrong before the power can come back on.
So that flickering you saw at first was the recloser trying to restore power, but failing. Then it waited a bit (30 seconds or so in your case) and tried again, and that time it succeeded.
That’s pretty typical behavior for a recloser. It’s also why you don’t ever touch a power line, even if it has been knocked down and appears to be dead. Due to reclosers or attempts to restart the line, it could become live again at any moment. If you happen to be touching it at the time, that would be very bad.
The power system is a grid.
Lightning strikes somewhere in grid Causing a overload something blows up, not on your section of the grid. All lights on grid flicker. Breakers open. Reclose lights flicker breakers open but this time the one where the problem stays open and all the other ones close. One zone is now without power while the rest of the grid has power restored.