Transposition of the conductors in a transmission system means exchange in the position of the transmission line conductors at regular intervals in a transmission system along the line in a manner that each transmission line conductor occupies the position of every other transmission conductors over an same distance. In a power transmission system if the power lines are not equally spaced ( asymmetrically spaced ) even though the system operates under balanced condition, voltage drops of different magnitudes exist in the three conductors. This difference in voltage drops is because the unequal inductance of the three phases (mutual inductance of conductors are different in unsymmetrical spaced lines). Also due to asymmetrical spacing the magnetic fields external to the conductors is not zero. This causes the induced voltages in the adjacent electrical circuits particularly telephone lines that may result in Interference in the telephone circuits.
To reduce this effect to minimum the conductors are transposed. Transposition can be made either to the power conductors or telephone (communication ) lines.In a transposed line each of the three conductors occupies all the three positions relative to the other conductors for one third of the total length of the transmission line. The purpose of transposition is due to balance the capacitance of the line so that electro-statically induced voltages balances out in the transposed lines.
Under balanced condition the conductors magnetic field linking an telephone line is shifted by 120o out of phase. As the power lines are transposed, over the length of one complete transposition cycle ( three different positions of the power conductor) of power line, the net voltage induced in the telephone line is zero. This is because the sum of three induced voltages which are displaced by 120o in phase combines zero.