Yesterday morning our neighborhood experienced a series of power outages. There is a large electrical substation a 1/2-mile down the road from our house; presumably our electricity comes from there. Each time the power went out, it coincided with a loud noise that I assume came from the substation: it was roughly a 1/2-second of noise, at 120 Hz. Same pitch you get from fluorescent lights, just really, really loud. A couple of the outages lasted just a minute or so, but a third one lasted for roughly 90 minutes.
I am aware that there are automatic circuit breakers that attempt to reconnect power after a short delay (~1 minute) on the assumption that the fault might be a minor/temporary one; the automatic attempt to restore power helps obviate the need for a crew to visit the site unless there’s a real/ongoing problem that needs repair. So my assumption is that the first few outages were followed by an automatic reset, and the longer outage was a deliberate shutdown while utility workers handled a true hardware problem.
Questions:
-What is the loud noise coinciding with the shutdown of power?
-If it’s a short-circuit, why does it (the noise) last a full half-second before power gets interrupted? My household breakers manage to kill power at the first “POP” when I encounter a short; there’s no time to perceive a 120-Hz buzz.
-What kinds of electrical problems can manifest themselves intermittently such that the automatic-reset function is useful?
In your home, the allowable current levels are tiny compared to power transmission, and making a breaker that can hold 15 amps and trip immediately on 20 or more is not difficult. How many thousands or tens of thousands of amps can safely pass through a substation? If you’re looking to provide overcurrent protection on a line that’s rated for 10,000 amps, do you want it to trip out instantly if it’s passing 10,001 amps? I’m hoping these things are all derated to allow a safe “slow-blow” on overloads.
Wayward critters, tree branches and wind would cause lots of nuisance outages without reclosers.
What you were hearing could have been a arc. Some residential areas at least used to have fuses that let loose with a bang. Power stayed off until somebody came by and replaced the fuse. Maybe automatic resets are more common now.
While the resets save money and shorten power outages, the off, on, off, on… shortens the life of electrical devices.
Without being able to hear the noise, I’m guessing that it was some kind of arc, as thelabdude mentioned. There are a lot of things that can cause arcing. The insulation in a transformer can wear out and start arcing, which can short out the transformer and cause a fire that causes even more damage to the transformer. Some transformers have oil inside of them. When they get old, sometimes they spring a leak, and when enough oil leaks out the transformer can start arcing and sometimes can even explode (it’s a pretty impressive BOOM).
As gotpasswords said, critters can cause all sorts of problems. They get in between conductors and short things out, again causing that same arcing sound. If enough of the critter burns away the arcing may stop, otherwise they can sit there and arc (and burn with a horrible smell) for quite some time.
Substations contain switches to turn lines on and off, and those can fail as well. Any piece of electrical equipment can fail and produce arcing.
Was it this kind of noise? This is an arc.
Ignore the title and comments. That’s not a transformer arcing. It’s a three phase switch opening and drawing an arc while it opens.
It may have taken some time for the fault to cause enough damage to trip the protective device. For example, if the windings in a transformer start to short out, you may not immediately have a fault condition (for example you may not get enough current to flow to trip the breaker). Eventually the windings fail completely or they fail in such a way as to draw enough current to trip the breaker and the power goes out. Some failures do happen pretty much immediately, though. They don’t all take time to trip.
The same sort of slow failure can happen in your house. If you have a frayed extension cord, it can arc out and cause a fire without tripping your breaker. That’s why they invented arc fault circuit interrupters (AFCIs), which have only been in common use for the last decade or so.
Most electrical faults are transitory. Wind blows power lines together or blows a tree into a line. A critter gets someplace where he shouldn’t and causes a short. Automatic reclosers are typically programmed to retry once or twice in a very short amount of time (a second or two). They then typically wait some longer period of time (two minutes maybe) and try again. If that fails, they stop trying and the power company has to send out someone to figure out what the problem is.
The events in the video you linked to above are explained in great detail here. (lots of other fascinating videos and pics there, too.)
I heard the same 120-Hz fundamental, but without so much crackle/hiss (but then if the noise was coming from the substation as I suspect, it was a half-mile away, so clarity would be diminished).
This may be veering off topic, but I saw a power line arc last summer. The sound really freaked me out. It was a surround sound, as opposed to coming from a point source. I was in the house and went to look outside and was almost afraid to touch the aluminium storm door for fear it might be energized, too!
The arc started in some electrical equipment about 1/2 mile down the road, and travelled along the power line to a point about 1/4 mile from my house. It stayed there long enough for me to grab my camera and get a picture.
An electrician friend told me that the massive current draw may have caused the power lines to vibrate (he described it as an electromagnetic field trying to bend the wires into a sine-wave). So I took that to mean the power lines for miles around had become loud-speakers for a few seconds.
He related several other very impressive electrical phenomena that he had seen during his career, but that would be veering even farther off topic. I just wanted to say that I can relate to the sound you heard!
