High voltage three phase transmission lines

Question

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?

Answer 1

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.

Answer 2

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.

See also this wiki article on power transmission skin effect.

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.