# High voltage three phase transmission lines

The high voltage transmission lines in North America looks like this. If a three phase balanced circuit uses three voltage sources, would that mean that this pole in figure one is carrying the currents of 6 three phase voltage sources?

When I look at the transmission lines, some of them has only three "hands" (I do not know the exact term) like the one in figure two , and some of them six like the one in figure one. I always wonder what exactly is the reason behind it.

Another question that I have is that, if you count the cables in the picture, some "hands" have three cables and some have four cables attached to it. Why? The three cables make sense to me because that means the cables are carrying the current corresponding to three voltage sources that are 120 degrees out of phase from each other. But what about the ones with four cables?

• What exactly do you mean by "currents of 6 circuits"? That I can not attach more than 6 TVs to it? I have no idea how power companies distribute things but I would guess that each "level" is one phase. Mar 14, 2016 at 10:44
• The great Mr Tesla invented the 3 phase power distribution system which is shown in your photo and explained here: en.wikipedia.org/wiki/Three-phase_electric_power Mar 14, 2016 at 10:46
• This pole carries three phases - i.e. two cables will carry the same phase, allowing to transport twice the current. In addition, the mechanical load on the pole is more balanced. Mar 14, 2016 at 10:48
• The first photo looks a lot like the UK's pylons.
– user16324
Mar 14, 2016 at 12:04
• More information at the Pylon Appreciation Society. They're real! By the way the devices attached to the wires each side of the insulator are resonance dampers - lead weights, as far as I know, to absorb any resonance running along the wires towards the insulators. Mar 14, 2016 at 18:57

Actually there are many different types of high-voltage transmission line towers in North America, although that is certainly one type that's not too unusual. The really high voltage lines are held with different towers.

I can't even guess where you got the idea that tower is carrying six 3-phase lines. It is clearly carrying two 3-phase lines, one triplet on each side. There is also a lightning catching wire on top.

The individual conductors are made of multiple wires in this case. This has two advantages. First and obvious is that the current carrying-capability scales with the number of wires. Second, from the point of view of the E field a little distance from the bundle of wires, the bundle looks like a single large wire. This large diameter bundle is less "sharp" to the E field, which helps reduce corona losses. Sometimes you see the bundle deliberately arranged in a circle when the voltage is really high.

## Second picture

You have now added a second picture of multiple towers. Here you can see each line of towers carrying one 3-phase transmission line with two lightning catching wires above. If you look closely at the full-res picture, you can see that each phase is made of three wires, and this time they are deliberately held in a circle by spacers at regular intervals.

• A square is the deliberate arrangement of four conductors to look like a circle :-)
– user1844
Mar 14, 2016 at 11:46
• @Will: Yes, and in this case it's a triangle since there are only three conductors. Mar 14, 2016 at 12:02
• The first photo looks very much like a UK 400kV line, which is a four-conductor bundle.
– user1844
Mar 14, 2016 at 14:35
• I think it's pretty obvious how he thought there were 6 three-phase lines. There are 6 arms that each have a set of three wires - it's a very logical conclusion. Well, some look like they might have 4 wires, but it's hard to follow all the parallel and intersecting lines. Mar 14, 2016 at 18:38
• @jph: That doesn't make any sense. There are clearly 6 separate bundles. I can see being confused about the lightning wire above, but there just aren't 18 separate bundles there no matter how you look at it. Each bundle seems to have 4 conductors, so couldn't be confused with 3 phase, and they are shorted together anyway. Mar 14, 2016 at 20:26

If a three phase balanced circuit uses three voltage sources, would that mean that this pole is carrying the currents of 6 circuits?

Here's this quote from wiki electrical power transmission: -

Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close to the surface of the wire. Because of this current limitation, multiple parallel cables (called bundle conductors) are used when higher capacity is needed.

Taking this further, two sets of 3 phase cables sharing the load current will be lighter in weight and cost less than one thicker set.

So, for long haul power transmission it is advantageous to use two 3 phase circuits to reduce power transmission losses. For short haul a tower with three main wires is fine. A fourth wire is also used and this is an earth wire to protect against lightning strikes.

• There's something confusing about this answer which appears to conflate the bundling (to reduce skin effect) and the double-circuit, which is nothing to do with skin effect and is about management.
– user1844
Mar 14, 2016 at 11:44
• @WillDean yes you are correct. I shall amend. Mar 14, 2016 at 11:46
• @WillDean you say it's about management but two separate cables will have less weight for the same overall current handling capacity due to skin effect. I'm not ruling out other reasons of course. Mar 14, 2016 at 11:55
• I understood from somewhere that another reason for the bundling is to approximate a larger diameter cable. Breakdown from a sharp point or a small radius cable starts to become a problem somewhere above 132kV, so higher voltage lines (up to 440kv) need to bundle to avoid this. Am I mistaken?
– user16324
Mar 14, 2016 at 12:08
• @Tom Skin effect applies to all frequencies. At 60Hz, the skin depth is about 8.5mm . Compare to the radius of power line conductors. Aug 10, 2016 at 23:33