A conductor or phase is shifted to the next physical place in a regular sequence by rotating the conductors, according to the basic notion of transposition of line. In transmission lines for electrical power, the conductor arranges uneven space.
What is the Transposition of lines?
The transmission line transmits bulk power at relatively higher voltages. These lines give rise to electromagnetic and electrostatic fields of sufficient magnitudes which induce currents and voltages in the neighboring telephone lines. The currents so induced are superimposed on the true speech currents in the neighboring telephone wires and set up the distortion, while the voltage so induced raises the communication circuits potential. This field makes it difficult to transmit a message and raise the potential of the telephone receiver. To reduce the induced voltages due to electromagnetic and electrostatic induction, the conductors of the power and telephone lines are transposed. The transposition of lines helps to reduce the disturbances nearby communications circuits.
How do the regular transposition of power lines and communication lines reduces the radio interference?
By transposition of power lines the capacitance of the lines are balanced and also the electrostatically induced voltages balance out in the length of a complete set of transposition called barrel. The electromagnetically induced voltages on the conductors of the communication lines get diminished because the fluxes due to the positive and negative phase sequences cancel out along the barrel. This doesn’t apply to the Zero sequence currents. To reduce the effects of Zero sequence currents, communication lines are regularly transposed along the power lines’ transposition. Transposition of the lines is used to transpose the phase conductors in three sections so that each phase by rotation occupies each of the three-phase positions in the circuit.
Each transposition of a telephone line consists of a complete cross-over of the two wires.Each transposition of a 3 phase power line consist of twist of the line.To bring the phases back to their original positions three transpositions are necessary.Transposition of conductors are useful under the normal operating conditions but doesn’t provide the complete solution of the problem associated with abnormal conditions cause due to faults such as short circuit to earth.In such case high emf is induced due to electromagnetic induction and the faults results in large Zero sequence currents flowing along the wires in parallel and through the earth return.
Why is it necessary to arrange the communication line insulation so that insulators can withstand high voltages?
With the communication lines perfectly transposed and balanced no voltage between two conductors may exist. But a considerable voltage will be induced between each conductor and the earth. So, the communication line insulation must be arranged so that the insulators can withstand a voltage. sometimes in very extreme cases, electrostatic charging telephone apparatus is isolated from the telephone line utilizing highly insulated transformers and also earthed drainage coils and a lightning arrestor is used to dissipate induced charges.