Transposition Of Conductors In Transmission Lines

Transposition of conductors: By transposition of conductors is meant the exchanging of position of the power conductors at regular intervals along the line, so that each conductor occupies the original position of every other conductor over an equal distance.

A complete transposition cycle is shown in Fig.below

If the spacing is unsymmetrical, even though the system operates under balanced condition, voltage drops of different magnitude will be there in the three conductors due to unequal inductance of the three phases.

Also, due to unsymmetrical spacing, the magnetic field external to the conductors is not zero, thereby causing induced voltages in adjacent electrical circuits, particularly telephone lines, that may result in telephone interference.

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To reduce this effect to a minimum the conductors are transposed as shown in Fig. above

It is enough to transpose either power line or the communication lines.

Under balanced operating condition, the magnetic field linking an adjacent telephone line is shifted 120° in time phase with each rotation of the conductor positions in the transposition cycle.

Over the length of the one complete transposition cycle of power line, the net voltage induced in the telephone line is zero as it is the sum of three induced voltages which are displaced by 120° in time phase.

Under unbalanced conditions, of course, where power currents flow in the earth or in overhead ground wires (zero sequence currents), voltages will be induced in communication lines and interference will take place.

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Transposition of conductors in transmission lines

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