Suppose a single circuit AC transmission line. In Single circuit AC transmission line 3 conductors of all the 3 phases are unsymmetrical placed. Due to unsymmetrical spacing of the conductors Inductance and Capacitance of each conductors becomes different.
Suppose we assume that resistance is same for all the three conductors than due to difference in inductance and capacitance, all the three conductors willhave difference in Impedance. So, due to difference in impedance Voltage drop is different and unbalance of voltage is obtained. This is the reason why Transposition of Conductors requires in Electrical Transmission line.
Transposition of electrical transmission line is must when Long AC transmission line is to be erected. Because Voltage unbalance in Long transmission line becomes remarkable and cannot be tolerated. In case of short transmission line it does not require and not create much imbalance in voltage.
So, to make this voltage unbalance to minimum, the conductors are transposed such that the relative position of three phase conductors is interchanged after certain length of transmission line by putting Special Transposition towers.