Twisted pair overhead power lines? Why?

[Paintedcow]

The media likes to exaggerate things. Apparently to get more people to watch their coverage and thus increase their ratings. This seems to be the name of the game lately.

Just recently they were predicting a major blizzard, with white-out conditions, high winds, extreme drifting, and more..... They said to stay off the roads. This was predicted to occur at a certain time. The end result occurred 6 hours later than their predicted time. It snowed, and was fairly heavy at times, but no where close to a "white-out". It was windy and of course there was some drifting as a result. I happened to be driving home at the time, and had no problem driving. In the end, it was just a common winter snow storm, with some wind and drifting. No biggie!

In the past, I have seen REAL Blizzards, as well as "white-outs", and drifts so high you could not even walk thru them. I still recall getting trapped on a storm like that, while driving and not only did I have to park my car and wait over two hours for the storm to lighten, but also had to turn around and head home, knowing I'd never get to my destination. Then it took almost 3 hours to drive about 10 miles.

BIG DIFFERENCE !!!

Exaggeration seems to be the name of the game, when it comes to media coverage these days. Just like last year there was a local house fire. The news said "it burned to the ground". I saw the house the next day. The entire house was still standing, roof intact, but noticable charred siding from all the windows on one side of the house. "Burned to the Ground" means NOTHING IS LEFT! There was an entire house still standing, but with serious damage. (Since then, this house has been repaired. It was resided, new windows, and I suppose the whole interior was gutted and replaced). Obviously the structure remained sound, or they would have demolished it....

[clare]

Yes there is a big difference - BUT - they put out warnings on worst case scenario so people don't end up in the situation you did. You'd really be upset if they said there was a chance of light drifting snow with moderate winds (what you got when the heavy storm was forcast) and you ended up with the full-blown blizzard. It's a case of CYA

[Jon Danniken]

I used to rely on NOAA for my weather forcasts, but I consistently found myself planning for rain when it never happened. Nowadays I still check there (great source of satellite and dopplar imaging), but I find that the spot predictions from weather underground suit my activities a lot better, even breaking it down by hour as to when it is going to rain.

Jon

[Micky]

You know, I've seen this myself I think, and from the ground, I thought the cables were different, one for electricity and one to keep the cable up, like you suggest. But it seems maybe not.

It does keep them from picking up stray signals.

This is not about power lines specifically.

[Dick]

Twisted pair balanced lines use two-phase (antiphase) signals to drive transformers that have excellent common-mode impedance matching between "legs" but do not provide assymmetric signals. Since these conductors path from source to destination, any induced dereference is allowing the inductance rejection of reactive noise. This leads to twisted, braided co-jacketed cables when used in symmetric balanced signal re-transmission.

[clare]

When it comes to power lines, the twisting has NOTHING to do with signals. The "Arial triplex" or "arial quadriplex"cable is a means of keeping the wires from flapping against themselves in the wind and from sagging.

It is "officially" called ACSR - for" Aluminum Cable Steel Reinforced" Generally one of the cables is bare and acts as the Neutral as well as the main suspension cable..

Secondary distribution cable is generally all aluminum instead of steel reinforced - this runs from the transformer or pole to the electrical service of the building at "user voltage" while the steel core is primary distribution cable at higher voltage (generally speaking) In rural applications the steel re-inforced cable is often used from the transformer at the road in to the central distribution pole of, for instance, a farm - at "user voltage"

[Dean Hoffman]

Naw. This is close to what I was talking about.

One of the wires is ground, the other three current carrying. I think the hot lines are usually at 7200 volts. Each one of those four would be a twisted pair. Both of the twisted pair have voltage. Power for a building site would be supplied by a single transformer taking power from one of those lines. That would be the typical 240/120 city folk see. Three phase for irrigation would have three transformers usually supplying 480 volts. Philo and dpb answered my question.

[gfretwell]

7200 volts is medium voltage and would not be used for anything but the most local distribution. We have 13kv to ground here for the local distribution and the main trunks going between towns is 48kv or 250kv

The wires on my street

Those wires you see on the big structures out in the country are at least 48kv and might be closer to a million.

[gfretwell]

They do twist long distance power lines to cut down on radiating power but it is on the order of about once a mile. Still a fraction of a wave length at 60 hz. There is a twist on ther 250kv line behind my house about a quarter of a mile away. I may have a picture but it is not on my web site so I can't link it right now.

[hubops]

Transposing the three phases perhaps ? < rather that twisting >

John T.

[gfretwell]

That is exactly what we are talking about but when you look at the line, it looks like a twist is going on.

[Franciszek Anioł]

There are practical reasons for overhead line systems to rotate their lines. If you have a following system on a pole near a transformer: A(1) B(2) C(3) [three line system (1 line per phase)] and connect that to a following pole, you would connect at the front ('input of the pole') the same A(1) B(2) C(3) configuration and rotate it by '120°' you would get the following configuration at the end ('output of the pole') A(3) B(1) C(2) and that would be repeated till you arrive at the starting configuration. What does that achieve? It mitigates the difference in geometric dependent capacity towards literal ground, which would be present if nothing would be done. (read

at medium line model. Which in Germany gets called 'pi-equivalent', because of the form.) Thanks to that each line will behave comparably equal over longer distances (10 km and more)

Yes, mechanical advantages can be integrated in to that concept. These aren't the main focus of this expense though.

Cable twisting in power line cables has more to do with production processes, and contributes more to its mechanical integrity than its resulting dielectric properties. (The same is effect is achieved in ropes. Comparable to climbing ropes, which also have a core and a shell desing.)