| The problem of stringing conductors going through a tower "window" exists at | present for many lines - mainly in the 300KV and up range. This line didn't | appear to be at that level.
The apparent sizing on that picture suggest to me that the line was merely in the MV range, not the HV range, and thus more suited for distribution.
| For a given line to line voltage- one could consider the conductors places | around the circumference of a circle. For 3 phase, 6 phase, 3n phases, this | circle's diameter will be the same if not affected by conductor radius. This | would be the case for the lines pictured. In that case, there is really no | advantage to a more compact structure as the diameter of the "window" would | be determined by line to neutral voltage. The structure for 6 or higher | phases will involve more complex support and more insulator strings. There | are also mechanical limits on the minimum conductor size. | If the voltage is low- then one can get a more compact structure with 6 | phases but the benefit is not likely to extend to use of more phases. | Practical limit of any advantage - possibly 240KV if right of way costs are | high. | At higher voltages, considerations of surface fields would require larger | conductors (or bundles of 2 to 4 conductors per phase, spaced 30 to 50cm | apart) than what current considerations would require so that is a | disadvantage. Line capacitance and inductance will be affected adversely in | comparison to 3 phase.
Is this why the very HV stuff is constructed with 3 conductors all equal distant from ground, as opposed to a triangle with 1 conductor above the other 2 conductors? Or is it just too impractical to have so much height (e.g. protection wires then have to higher and the whole assembly has to be taller and more subject to tipping over)?
I remember seeing a picture of one of those large transmission lines that have the V-shaped tower going up to a wide cross brace holding up 3-inline conductors, possibly in the 765kV range ... with about 20 such towers in the picture, all tipped over and iced up. The entirety of what was in the picture was affected, suggesting the possibility that much more of that line was affected. Just how stable is that tower design?
| On the other hand, for distribution, spans are short and voltages are low so | a 6 phase line may have 7 conductors (neutral in the centre) and insulation | requirements low enough that an insulating yoke holding the conductors is | practical and additional yokes can be placed as spacers at different points | along the span so that swing effects are eliminated (you have correctly hit | upon a serious consideration). Possibly such a yoke could be used on longer | spans at higher voltages just as spacers are used with bundled conductors | but there are cons as well as pros for that at transmission voltage levels. | Another problem with swing is that, for conductor held fairly rigidly at the | towers, there are side stresses on the conductors. This may not be a problem | as many lines now use V strings to support conductors and these limit | movement at the tower (that is why they are used- reduces the need to allow | for insulator swing) and aren't falling down.
There will still be some swing, maximum at the mid point between 2 towers. Given a specific conductor size and tower spacing, the conductors will have a specific required sag, and that looseness would give a specific amount of mid-air swing. Too many phases around a circle would quickly bring them into contact or arc distance. The N-phase design would have to be more rigid to be practical, I'd think. You could scale up N to such an extreme (disregarding the complexity of creating all the phases) that the wires would just end up bumping into each other or being too thin.
Anyway, I can't see any advantage to more than 6 phase lines and clearly see that in transmission application, even 6 phases is a burden.
What is the largest practical conductor sizing when considering multiple conductors braced together? E.g. what is the highest capacity 765kV three phase transmission line you could envision ever being built?