The need for line transpositions seems to be a bit subtle. First, HV circuits generally differ from LV circuits in that the source and load impedances are much closer together—there is less of a gap between the two. Ideally, the perfect electrical source will not result in lower voltage during a short circuit, and the perfect load will not cause voltage drop when connected.
Increased source impedance means that, comparatively, less current will flow when the source is shorted out. Decreased load impedance will cause greater voltage drop when connected or ‘switched on’.
[Positive- and negative-sequence components in power systems correspond to phase-to-phase balanced and unbalanced conditions, where zero-sequence components relate to phase-to-ground characteristics.]
The idea is that 3-phase systems work most efficiently when everything is most closely balanced or symmetrical—especially voltages, currents and impedances. That applies to phase-to-phase characteristics as well and phase-to-ground or circuit-to-adjacent-circuit in the case of side-by-side [2-circuit] lines.
Another very critical situation is for protective relaying to be able to differentiate between faulted and unfaulted conditions on 100-mile circuits. The further a fault gets from the breaker and relaying point, the more difficult it is to differentiate it from simple load imbalance. The interconnected everything
in recent power systems does not help the issue at all—except make it all more interesting and get increasingly fun toys to fight the battle.
Schematically, line transpositions are illustrated as…
Scroll down to: “Transposition Tower” ”Each pylon carries lines in a multiple of three - usually 3 or 6 lines per pylon. In each set of three, one wire will be for each phase. Ideally, the capacitance of each line should be the same. However, the different heights at which each line is carried means there is a small difference in length between them. For short distances, the difference is tiny and can be discounted. However in areas where the lines run for hundreds of miles, such as across Africa or Continental Europe, these differences between the sets of wires can be enough to cause problems. So a transposition tower is used to swap the electricity from one line onto another, in order that over the entire line, the capacitance of each line will be more-or-less the same.”
[This message has been edited by Bjarney (edited 12-16-2004).]