The inductance and capacitance of each phase will be different in case of conductors of three phase line being spaced irregularly. The apparent resistance of the conductors is also affected on account of transfer of power between the phases, which occur due to mutual inductance. Thus all the three line constants are affected by irregular spacing of the conductors in a 3-phase line.
Also, due to unsymmetrical spacing, the magnetic field external to the conductors is not zero, thereby causing induced voltages in adjacent electrical circuits, particularly the telephone lines that may cause disturbances in the telephone lines.
The unbalancing effect on account of irregular spacing of line conductors can be avoided by transposition of line conductors. Transposition of line conductors means changing the positions of the three phases on the line supports twice over the total length of the line. In practice the line conductors should be so transposed that each of the three possible arrangements of conductors exists for one-third of the total length of line. This is illustrated in 4.41.
An unbalanced system, may be of conductors symmetrically placed, cannot be represented by three simple inductances, without making transposition. The effect of unbalanced currents is neutralised in case of conductors being transposed regularly at intervals.
ADVERTISEMENTS:
The transposition of conductors also reduces the disturbances to the nearby communication circuits. In composite line, the line carrying telephone line conductors below the power line conductors, it is also necessary to transpose the telephone line conductors in order to keep down the disturbances.