Transposition

Hello sapan

You don't say how short the transmission line is, nor the frequency.

Radio Frequencies:

If it is at Radio Frequencies, and a few hundred times the wavelength of the transmitted/received signal to/from the Antenna, than is is advantageous to transpose the parallel conductor pair, of parallel transmission lines, at carefully calculated intervals.

Power Frequencies:

If you are talking about a single phase low voltage power line, 110 - 1100 Volts, operating at 50 - 60Hz, there is no real electrical advantage in transposition.

If you are talking about a Medium Voltage 3 phase power line, 3.3 - 11kV, operating at 50 - 60 Hz, there is no real electrical advantage in transposition.

If you are talking about a High or EHV 3 phase power line, 22kv - 66kV or 110kV - 1.1MV, operating at 50 - 60 Hz, there is no real electrical advantage in transposition.

DC EHV Transmission Lines:

No real electrical advantage.

However, in New Zealand, when the 500kV DC line, (with no conductor transpositions), was first switched on, and placed under full load, it gave airplane instruments of the day (some 40+ years ago, and non-digital, non-GPS) false readings, as the navigation instruments of the day sensed the large magnetic effect, over hundreds of miles of countryside, which appeared to shift the position of that island on which I presently live.

The anomaly was noted, and aircraft instruments were later calibrated with the phenomenon in mind, for those visiting here.

However: Mechanical Advantages

There is often mechanical advantages to transposing power lines, the main one being the need for equal mass/weight either side of a central pole, in long straight distances of pole line.

If the weights (mass of copper) are different on each side, the poles may, after some time, develop a lean in the heavier direction, or because of the way the wind loading (blowing wind torque) is different on each side of that long straight pole line.

Thus in 3 phase power lines, with a central pole and horizontal crossarms, there is normally one phase conductor on an insulator towards the outer end of each crossarm, while the inner phase conductor zig-zags across and back each pole being on the opposite side of the pole, than the pole immediately to either side in the pole run.

Thus the wind loading and mass/weights become relatively equal on either side of the centrally placed poles, along the entire pole run length.

Analogue telephone lines:

In earlier days, to minimise "crosstalk" between circuits, telephone wires were rotated a quarter turn in groups of two pairs (The quad system), on horozontal crossarms, the four wires (two circuits) of each quad rotated as a unit, one insulator per pole, until at the 5th pole they were back in the original position, the process being repeated.

Railway circuits - Signalling and Telephone:

Refer above for the same reason as Analogue Telephone Lines.

Trust you are now more informed, and enjoy each visit to the CR4 Forum.

Kind Regards....

In which cases transposition has real electrical advantage?

Hello Guest,

Read my Post immediately above, and it tells you exactly what you want to know.

If you don't understand, then you need more basic education.

Kind Regards....