Interference of Power Lines Interview Questions and Answers:

1. How do power lines cause interference with neighbouring communication lines ?

Ans. The transmission lines transmit bulk power at relatively higher voltages and, therefore, these lines give rise to electromagnetic and electrostatic fields of sufficient magnitude which induce currents and voltages in the neighbouring communication lines. In extreme cases, the effects of elec­trostatic and electromagnetic fields produced by power lines may make it impossible to transmit any message faithfully and may raise the potential of telephone receiver above the ground to such an extent to render the handling of the telephone receiver extremely dangerous.

2. How the radio interference caused by power lines in the neighbouring communication lines be reduced ?

Ans. Radio interference caused by power lines in the neighbouring, communication lines can be reduced by regular transposition of both the power lines and communication lines. By transposition of both power lines and communication lines voltages induced due to electrostatic and electromagnetic effects are reduced.

3. How do regular transposition of power lines and communi­cation lines reduce the radio interference ?

Ans. By transposition of power lines the capacitances of the lines are balanced and, therefore, electrostatically induced voltages balance out in the length of a complete set of transpositions known as barrel. Also electromagnetically induced voltages on the communication line conductors get diminished as the fluxes due to positive and negative phase sequence cancel out along the barrel. This, however, does not apply to zero-sequence currents and communication line is also transposed regularly along with the transposition of power line so as reduce the effects of zero-sequence currents.

4. Why is it necessary to arrange the communication line insulation so that insulators can withstand high voltages.

Ans. With communication lines perfectly transposed and bal­anced no voltage between two conductors may exist and yet a considerable voltage will be induced between each conductor and earth. So it is necessary that the communi­cation line insulation is so arranged that insulators can withstand such a voltage.

5. How can the radio interference be reduced in extreme cases of electrostatic charging ?

Ans. In extreme cases of electrostatic charging it may become necessary to isolate the telephone apparatus from the telephone line completely by means of highly insulated trans­formers, and also, to ensure the dissipation of the induced charges by means of such devices as earthed ‘drainage’ coils and lightning arrestors.