I understand what three-phase power is. But when I look at some pictures of a double-circuit-three-phase-power-line I see two or three lines close together? What is the purpose of these lines close together? (the wires are attached by smaller wires or connectors)
Is there a separate alternator for the second group of three-phase?
It has 2 lines instead of 1 line for one of the phases. So instead of having two sets of A, B, C it has AA, BB, CC? Or is that second line just a neutral line?
The pairs of lines are the same phase and at the same voltage - they are really just a single thick wire split into two thinner ones.
It is easier to install two smaller wires to double the current capacity than a single thicker wire. It is easier to handle the lighter cable and you can stock just a single gauge of wire and handling equipment. It also provides some redundancy if one wire fails.
There is an effect with AC electricity that the current mostly flows near the surface of the conductor. A number of thinner wires have more area-near-the-surface and so a larger effective cross section area than a single thick one. At the 50/60Hz frequencies used by AC transmission this only affects wires more than a cm thick. See Does electricity flow on the surface of a wire or in the interior?
You are right to identify that each 'bundle' of conductors is a phase conductor. There are several advantages to bundling conductors, generally these are linked to lower overall power loss in transmission. They include (quoted and paraphrased from here):
- Lower corona losses. A lower voltage gradient in the vicinity of theline. This reduces the possibility of corona discharge. At extra high voltage, the electric field gradient at the surface of a single conductor is high enough to ionize air, which wastes power, generates unwanted audible noise and interferes with communication systems.
- Higher capacitance. This can help in improving power factor.
- Lower losses due to skin effect. (as pointed out by @Martin_Beckett)
- Lower inductive reactance, compared to a single conductor.
- More efficient cooling due to the increased surface area of the conductors, (therefore lower line losses).
Also, the first picture shows a conductor at the top. This is at earth potential and is mainly for lightning protection (as well as reduction of fault impedance and communication). Lightning is more likely to strike this conductor and be dissipated through the towers, resulting in lower probability of an outage. A fibre optic communication link is often included in the centre of the lightning conductor.
