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When I was in Alaska, I saw a high voltage line, but I didn't see the Neutral.

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Is this because the water is conductive enough next to it to use as neutral? Or is it just two Phases.

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Where I live, Colorado, all of the power lines have the two neutral wires on top.

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en.wikipedia.org/wiki/Zigzag_transformer "Its applications are for the creation of a missing neutral connection from an ungrounded 3-phase system" –  jippie Aug 11 at 18:42
    
Probably a duplicate: electronics.stackexchange.com/questions/93936/… –  jippie Aug 11 at 18:45
    
    
ok, I edited it, where I live in colorado they have the extra neutral on top as seen in the photo above –  skyler Aug 11 at 20:08
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As far as I know, the wires on top are not neutral, they are for distributing lightning strikes to multiple towers. –  JYelton Aug 11 at 20:17

6 Answers 6

This transmission is likely 3-phase in the Delta configuration, as opposed to the Wye (Y) configuration. Delta does not have a neutral line, whereas Wye does. Wikipedia link to balanced 3-phase circuits As you can see, the Wye connection has a neutral line, whereas the Delta does not.

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That wikipedia page says the 4th neutral is optional on wye/star. –  gbulmer Aug 12 at 2:35
    
@gbulmer & acegard the neutral can be omitted if all phases (3 in this case) of a star topology network are used with equal load, since in that case the neutral has nothing to do anyways. In household and non-energy industry appliances this cannot be guaranteed, so the neutral is provided as well when a 3 phase connection is offered. In case of distribution networks and enegery-industry several steps are taken to ensure that the load is perfectly balanced, in this case that would happen at the transformator. –  Mark Aug 12 at 6:21
    
@Mark thank you, you are correct. So what you are saying is, in transmission lines such as this, the neutral is omitted because it is certain that the loads are balanced? –  Asa Graf Aug 21 at 3:43
    
@AsaGraf Almost: I'm saying that that is a configuration that allows for the neutral to be omitted under the circumstances listed. So it's either that or the grid in Delta-Formation which doesn't need a neutral in any case (see this answer electronics.stackexchange.com)/a/124845/35490 –  Mark Aug 21 at 4:53

Long distance AC transmission is normally 3-phase without neutral.

At the receiving end, a 3-phase transformer lowers the voltage from the "several kV range" to normal household levels (maybe a bit higher if it feeds an intermediate system). For households, a neutral is taken from the output side of the transformer but, on the long-distance transmission, the transformer primary (that receives the power) doesn't need a neutral.

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In my country, and probably in a lot of other countries, there is a ground wire at the top of each pylon. This ground wire is there for reasons of lightning protection. For the power transmission alone all you need is the three phases, because earth is used as reference potential. For long distance transmission there is also no strict need for a neutral wire. If there is nothing wrong, no current flows ever in the neutral line.

A pylon of type Donaumast

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High voltage power lines (the ones on huge towers with long insulators) are almost without exception 3-phase Delta alternating current, two independent groups if the tower has 6 wires. No neutral or ground needed. The two wires at the top of the tower in Colorado are not related to power distribution.

The only place the power company hangs neutral wires is low voltage circuits to homes and small businesses - these are typically the only consumers of non-3-phase power most people encounter.

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3-Phase power transmission does not need a neutral wire. That is part of its attraction. It reduces the amount of material used to make the transmission lines by 1/4

The voltages are out of phase by 120 degrees, say A leads B by 120, B lead C by 120, C leads A by 120.

The maths of that are that A+B+C add up to 0, and hence do not need a 4th neutral wire.

When I was taught this stuff at school in the 70's, AFAICR the diagrams were all wye/star; maybe we were still excited by the UK's "National Grid" so this may have been taught to 16yo, but I likely covered it in Physics at 17-18yo. So our explanation may have been simplified :-)

wikipedia's three phase power 3 or 4 wire circuits show that neutral is optional in wye/star and unnecessary in delta. That 120 degree phase shift is the technique used, and wikipedia covers the theory in Balanced Circuits

You could demonstrate this if you have a spreadsheet which can draw three simultaneous line graphs, by creating three columns containing sin(x), sin(x+2pi/3), sin(x-2pi/3) (a few pi long), and plotting them.

This school of Physics shows the graphs of the maths which IMHO are clearer than wikipedias diagrams.

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yeah I learnt this in high school too, and it's hard to see a high-voltage line to use 4-wire Y layout –  Lưu Vĩnh Phúc Aug 12 at 11:37

Most long distance high voltage transmission is done in 3-phase Delta configuration, requiring only three cables. This saves the cost of the fourth cable, used for the neutral line. It also avoids problems of imbalanced currents flowing through the fourth cable or the earth, since in Delta configuration any current imbalance on a phase is automatically shared on the other two phases, making for lower losses. If 3-phase Wye configuration with four cables is used, it is generally only over short distances at the generating station and at the end-user's facility. Conversion at each end is generally achieved by Wye-Delta transformers. In the bottom photo in this set the wires at the tops of the towers don't show up well, except one of them has a marker ball affixed to it to alert aviators. These top wires are connected to the tops of the towers without insulators and act to divert lighting strikes into the structure of the nearest towers, and then into the earth via ground rods, rather than striking the power cables and causing current and voltage surges in the grid. Also, in the the bottom photo, the six cables comprise two independent 3-phase Delta transmission lines of three cables each.

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