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-   -   OT: How do High Voltage Lines work? (http://www.diychatroom.com/f18/ot-how-do-high-voltage-lines-work-50703/)

jamiedolan 08-11-2009 01:55 PM

OT: How do High Voltage Lines work?
 
HI;

I know this is a bit off topic for DIY, but I am hoping someone will be kind enough to indulge my curiosity.

I was traveling this weekend and saw a number of large power poles, the kind that feed sub stations, (the ones that are large metal towers that are too high to make drops to individual properties).

I assume most of these would be lines at high voltage, like 34,000v that go to a sub station to get reduced. My thinking was that power would come in on 3 phases to the sub station, then each phase would get dropped down in voltage and sent back out.

However, many of these towers had 5 or 8 lines between them. I could see 1 line being a bonding connection between the towers, but then you would have 4 wires (3 phases plus bonding) or if two sets of 3 phase wires 7 wires (2 sets of 3 phases plus a bonding wire). Where does the 5 or 8 come from? For example the one with 5 wires would have 2 wires at the very top, then maybe 10 feet lower there would be 3 wires. Or does power actually come into a sub station on one line and the sub station transformer there makes it into 3 phase somehow?

If you have a web site / resource that you could point me to if this isn't a simple question to answer, that would be very much appreciated.

Thanks

Jamie

wirenut1110 08-11-2009 02:08 PM

Power distribuition grid
This has some pretty good information, quite a few pages of info.

AllanJ 08-11-2009 04:19 PM

I have seen transmission towers with what look like two ground wires at the very top. Below were 3 (one set)
or 6 (two sets) of 3 phase line.

On the poles you saw, were both of the lines at the very top strung with no insulators? That would mean
they are ground wires. Directly above the phase lines as opposed to being in the middle must offer better
lightning protection.

junkcollector 08-11-2009 05:36 PM

Yah, sometimes, you will see multiple circuits running on the same structure. (Depends on the population density in your area) Practically all subtransmission lines (above 34.5 kV) are 3 wire. The source transformer is usually wye connected, but they run just three wires and the line feeds transformers that are delta connected. Like others mentioned, there are usually one or two static wires ran along with the circuit for lightning protection.

Its funny, most of this stuff is pretty similiar in theory as smaller wiring, but in a larger scale.

Power lines and distribution have been fascinating to me since I was a little kid. A good book to read about power distribution is called, "The Lineman's and Cablemans handbook."

WFO 08-11-2009 06:15 PM

Large transmission lines (138 Kv, 345 Kv and up) often have double circuits with two statics, which would be your 8 wires. As mentioned, single circuit with two statics would be 5.
Most distribution voltages (in Texas) are 12.5, 13.9 or 24.9 Kv. 35 Kv circuits are sometimes called sub-transmission, but they can also be distribution. Of course, there are variations on these voltages all over the country, some still having 4.16 Kv.
Most Transmission is defined as above 60 Kv (phase to phase).

Off topic (since you mentioned the transformer changing the number of phases) they do actually have these, though not in the way you mentioned. The one I tested was a six phase unit, with three windings wound conventionally, and three windings wound in reverse (i.e., 180 degrees out). The net result was 6 phases each 60 degrees out of phase with the others. If I remember right, the application was a huge rectifier and the six phases helped smooth out the ripple effect.

frenchelectrican 08-11-2009 07:56 PM

James.,

We have bunch of 245KV lines near Appleton area and yes there is a 500 KV transmmison line run near here.

And Not too far from my home/ shop have few 134KV line as well that is transmmison and hevey local spur to one industrail customer { a chesse factory in Hollandtown }

The Forest Junction Sub station it was expanded about 2 or 3 years ago they allready more than double up the sizewise IMO almost triple of it and we have one 245 KV line came from Pullman plant { Green Bay steam turbine base load plant } that allready have larger conductor on it.

But we have few 134KV transmmison spurs to couple towns as well.

Merci,Marc

Red Squirrel 08-11-2009 09:41 PM

I just read that article out of curiosity and I never realized the neutral was actually one of the bare wires. What would happen if you were to cut this wire and hold both ends, wouldn't that pretty much kill you? Is touching it normally safe because of the fact that it's grounded, even though it's carrying power? ex: no potential difference between that wire, and your body.

I went for a trip at a cottage we rented and the wires were very low and pretty much could be reached by hand and one thing I did notice is that there was only 2 insulated wires which I found odd, but now it makes sense.

WFO 08-11-2009 09:59 PM

Quote:

Originally Posted by Red Squirrel (Post 313269)
I just read that article out of curiosity and I never realized the neutral was actually one of the bare wires. What would happen if you were to cut this wire and hold both ends, wouldn't that pretty much kill you?

Yes, pretty much dead.

CDH 08-11-2009 10:22 PM

Quote:

Originally Posted by Red Squirrel (Post 313269)
I just read that article out of curiosity and I never realized the neutral was actually one of the bare wires. What would happen if you were to cut this wire and hold both ends, wouldn't that pretty much kill you? Is touching it normally safe because of the fact that it's grounded, even though it's carrying power? ex: no potential difference between that wire, and your body.

I went for a trip at a cottage we rented and the wires were very low and pretty much could be reached by hand and one thing I did notice is that there was only 2 insulated wires which I found odd, but now it makes sense.

Transmission lines don't have a neutral but rather a static or shielding wire mostly for lightning protection, distribution feeders have a neutral for return and if you cut it you have the possibility of the same voltage as the feeder circuit, if you cut it as stated earlier you would become the return and there is a better than average chance it would kill you, that's why us lineman mack it out or jumper it prior to cutting it, I should also mention that I am talking about a wye primary distribution circuit not a delta, that is for another discussion. Hope this helps.

Red Squirrel 08-11-2009 10:53 PM

Guess cutting is dangerous insulated or not, so guess that does not matter. Just touching it is safe though right as it has potential of 0 to ground? just seems odd there would be a bare wire as those same wires could potentially come in contact with the house soffits or other structure, not to mention rain.

CDH 08-12-2009 04:58 AM

Quote:

Originally Posted by Red Squirrel (Post 313294)
Guess cutting is dangerous insulated or not, so guess that does not matter. Just touching it is safe though right as it has potential of 0 to ground? just seems odd there would be a bare wire as those same wires could potentially come in contact with the house soffits or other structure, not to mention rain.

I think you are referring to a tri-plex house service which does have the bare wire neutral,the insulation is really pretty durable and if shaped properly will pose no problem to soffits etc,the one thing that you have to watch for is squirrels love to knaw the insulation off of the hotlegs, kind of ironic is'nt it Red Squirrel LOL.:laughing:

AllanJ 08-12-2009 08:42 AM

The big advantage of a 3 phase system over single phase is that four instead of six wires are needed to
handle that amount of power divided three ways (wye system). The fourth wire is the neutral and the
load is normally balanced so that this fourth wire can be much thinner than the other three. It takes
some advanced math to show how the neutral carries the difference in current which difference is
zero if the loads on the three phases are equal.

In practice the neutral and ground and (if any) static shield are one and the same wire. Or two wires
up top as the OP and others mentioned but with no difference in the way they are strung, attached,
and connected. If the ground wire is not uppermost (as in distribution systems) it doesn't work as a s
tatic shield.

The delta system uses three wires instead of four and any single phase loads (single phase transformers)
are connected phase to phase as opposed to phase to neutral.

Even though the neutral/ground is grounded at each pole, there can still be enough current (if the
phases or legs are not perfectly balanced load wise) so if you cut the neutral/ground wire between
poles and held the loose ends in your arms respectively, you can be electrocuted. While the soil and
earth overall may be a good conductor, each individual contact between the ground wire/rod at each
pole and the earth does not make for that good a contact and the ground wire ovehead is the
primary neutral conductor.

J. V. 08-12-2009 11:27 AM

Jamie. Next time, count the insulating spacers between the pole and the line. I read somewhere (I think it was a quote from Tom Henry) that each individual insulator was rated at 10,000 volts. So 10 insulators would be 100,000 volts. Now, thats some serious voltage.

WFO 08-12-2009 08:41 PM

Quote:

Originally Posted by AllanJ (Post 313385)
While the soil and earth overall may be a good conductor, each individual contact between the ground wire/rod at each pole and the earth does not make for that good a contact and the ground wire ovehead is the primary neutral conductor.

And there are all types of soil. The sugar sand we have around here (when it's dry) will often have so high a resistance that the distribution lines sometimes don't draw enough fault current to trip. The 7200 volt just lies there melting a little pile of sand into a puddle of glass. Very deadly.

Go back a few months when the cost of copper was high and all the idiots were stealing grounds out of substations and were killing themselves left and right. The one that comes to mind is the guy that literally had his arm burned off cutting the neutral of a power bank. Fatality.

Gigs 08-13-2009 07:36 AM

Went to the Hoover Dam while I was in Vegas... talk about a lot of HV... It's like entire fields full of towers:

http://www.gigstaggart.com/pics/IMG00068.jpg


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