Why do pole pigs tend to explode during storm conditions?

They do?

In our area we have had TWO major storms (by major, I mean that power was out for DAYS) and NO "pole pigs" blew up.

Have you seen them explode? Is it the BOOM sound you might have heard? The BOOM is usually just an expulsion fuse blowing. A pole in my back yard has three of them. They've gone off several times since I have lived here. They do sound big and powerful, like a cannon when blowing. They don't outwardly appear damaged after I've seen them blow. Sometimes it takes the utility workers a couple tries replacing the fuse before the fault is cleared. I think they just try a fuse (squirrel factor) and then search for a fault if the new one pops. John

We had one blow here but it was a rusty old thing that probably just shorted out internally or it may have been that the oil just boiled a little when the fuse blew. There was some oil coming out tho.

A failure mode for transformers is created when the utility pole is not properly earthed. Lightning strikes wires highest on pole - high voltage or primary wires. Lightning seeks earth ground. If transformer is not properly earthed, then lightning may seek earth ground via your home.

Lightn> I've always wondered about this. It seems that whenever a heavy storm

| I've always wondered about this. It seems that whenever a heavy storm | hits, pole pigs explode like bombs. It's hard to believe that the | standard failure mode for a utility transformer is exploding, especially | when you consider how dangerous it may be to people nearby. | | Why do pole pigs explode like this? Why can't they implement internal | protection, like an overcurrent or temperature controlled fuse? Not to | mention, replacing a fuse or other protective device would probably be | less expensive than replacing the whole pole pig...

I was watching CNN coverage of Hurrican Frances. In a few segments they were covering some video of various power faults. In many cases the talking heads referred to "transformers exploding". In one video when they said that, there was no transformer at all; it was a tree limb entangled in the wires which appears to have line to line voltage in the 12,470 to 14,400 volt range. In another segment, there was a transformer, but the arc was below it. It looked like a broken primary (7,200 to 8,300 volts from line to ground) dangling down the side of the pole.

High voltage high current electrical faults can be very spectacular and very noisy. This is what makes people think something exploded. Though expulsion fuses can mimic an explosion, they are generally not that big.

If a transformer were to explode, it would be due to the oil rapidly overheating to the point of bursting the containment. You'd see flames and fire, in addition to electrical arcs, which would last until the oil has burned up. If it's on a pole, that oil would fall to the ground and you'd see flames all around the pole. Such things happen, but they are a very small fraction of the kinds of outages that occur in storms.

Most outages are lines blown down, or poles knocked down by vehicles slipping on the road, or faults due to tree branches or other objects falling on the lines.

Here is an MPEG video of a substation transformer exploding. In this you will see at first just an electrical fault arc (which produces a lot of heat). Eventually there will be a big spray that looks like steam being ejected, but this is the oil. It soon ignites. In the middle of that flame you will see a burst of light for a couple frames. That was the high voltage expulsion fuse finally blowing a bit too late.

Here is another MPEG video of a substation involving some kind of fault on the wires. If you step through frame by frame you can see that the flash nearly floods one of the frames. Electrical arcs can be very explosive.

And here is another fun MPEG video.

Can you really tell the voltage range that accurately by the sparks and arcs?

What exactly happened in that video?

We had a dry-core 4160/480 transformer fail one time. When all tagged and we opened the enclosure, the apprentice asked which of the three cores had failed. Without any hesitation I pointed to one and said, "That one." He wondered how I could tell, so I showed him. The coils had deformed enough that the wooden spacers between windings had actually fallen out the bottom end. Told him, "That's the blown-transformer indicating stick." For a long time he believed that was a deliberate "feature" of large dry transformers, much like some fuses have a popup. ;-)

daestrom

|> I was watching CNN coverage of Hurrican Frances. In a few segments |> they were covering some video of various power faults. In many cases |> the talking heads referred to "transformers exploding". In one video |> when they said that, there was no transformer at all; it was a tree |> limb entangled in the wires which appears to have line to line voltage |> in the 12,470 to 14,400 volt range. In another segment, there was a |> transformer, but the arc was below it. It looked like a broken primary |> (7,200 to 8,300 volts from line to ground) dangling down the side of |> the pole. | | Can you really tell the voltage range that accurately by the sparks and | arcs?

It's the same range, just a different path. One was line to line and the other was line to ground. They could be plus or minus a few times but this is just the typical voltage.

My favorite video they showed was the one where one line was dangling more loosely next to another, being whipped in the wind, and the arcs were traveling down the line as the point of least distance moved.

|>> Here is another MPEG video of a substation involving some kind of fault |>> on the wires. If you step through frame by frame you can see that the |>> flash nearly floods one of the frames. Electrical arcs can be very |>> explosive. |>>

|>>

| | What exactly happened in that video?

I really don't know for sure. Something was being tested and either a fault was present or a connection was loose. It may have been an intentional fault test (the camera was way too conveniently present).