SOLUTION

Interference of Transmission and communication line

In practice, it is observed that the power lines and the communication lines run along the same path. Sometimes it can also be seen that both these lines run on Sam supports along the same route. The transmission lines transmit bulk power with relatively high voltage. Electromagnetic and electrostatic fields are produced by these lines having sufficient magnitude. Because of these fields, voltages, and currents at induced in the neighboring communication lines. Thus it gives rise to interference c power line with communication circuit.

“if the power line is running along the same route as the communication line, there will be interference in
the communication line due to both electromagnetic and electrostatic effects.” The electromagnetic effect produces currents, which are superimposed on the communication signal, and thereby it is distorted. However, the electrostatic effect induces a voltage in the communication line, which may be dangerous in handling the telephone receiver.

Steps for Reducing Telephone interference

1. Transposing transmission as well as communication lines
2. There are various ways that can reduce telephone interference. Some of them are as listed below
3. The harmonics at the source can be reduced with the use of AC harmonic filters, DC harmonic filters, and smoothening reactors.
4. Use greater spacing between power and telephone lines.
5. The parallel run between the telephone line and the power line is avoided.
6. Instead of using overhead telephone wires, underground telephone cables may be used
7. If the telephone circuit is ground return then replace it with the metallic return.
8. Use microwave or carrier communication instead of telephone communication.

Transposition:- Transposition of overhead line conductors refers to exchanging the positions of power conductors at regular intervals along the line so that each conductor occupies the original position of every other conductor over an equal distance in order to reduce crosstalk and otherwise improve transmission.

Usually, telecommunication lines and power lines travel close to each other, then the current flowing through the power line produces magnetic flux linkage between telecommunication line so that voltage is induced in the telecommunication lines which causes disturbance in communication. This phenomenon is called radio interference which is suppressed by the transpose of the power line.

The transposition arrangement of the conductor can simply show in the following figure. The conductor in Position 1, Position 2 and Position 3 changes in a specific arrangement to reduce the effect of capacitance and the electrostatic unbalanced voltages.

Effect of Transposition:- The purpose of transposition is to balance the capacitances of the line so that the electrostatically induced voltages balance out in the length of a complete set of transposition, and this length is called a barrel.

Transposition also results in the decrease of electromagnetically induced e.m.f. on the wires, because of fluxes due to the positive and negative phase sequence, currents will add up to zero along the barrel.

This flux due to the zero-phase sequence or longitudinal current is unaffected by transpositions of power systems because it flows along the three conductors in parallel. To reduce the e.m.f. in the telephone line due to its zero phase sequence currents the telephone line is transposed by proper coordination of transpositions of the power and communication lines. The induced voltage can be reduced to very small proportions under normal working conditions.