what is Transposition of lines?
Transmission line transmits bulk power at relatively higher voltages.These lines gives rises to electromagnetic and electrostatic fields of sufficient magnitudes which induces 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 distortion,while the voltage so induced raise the potential of the communication circuit.These field makes it difficult to transmit message and raise the potential of the telephone receiver.In order to reduces the induced voltages due to electromagnetic and electrostatic induction the conductors of the both the power line and the telephone line are transposed.Transposition of lines helps to reduces 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 reduces the effects of Zero sequence currents communication line are transposed regularly along the transposition of the power lines.Transposition of the lines are used to transpose the phase conductors in three sections in such a way that each phases by rotation occupies each of the three phase positions in 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 earth.so it is necessary that the communication line insulation must be arranged that the insulators can withstand a voltage. sometimes in very extreme cases of electrostatic charging telephone apparatus are isolated from the telephone line by the means of highly insulated transformers and also earthed drainage coils,lightning arrestor are used to dissipate induced charges.
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