Inductive Interference with Neighbouring Circuits

Inductive Interference with Neighbouring Circuits

AU : Dec.-04, 05, 11, 13

In practice it is observed that the power lines and the communication lines rim along the same path. Sometimes it can also be seen that both these lines run on same 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 are induced in the neighbouring communication lines. Thus it gives rise to interference of power line with communication circuit.

Due to electromagnetic effect, currents are induced which is superimposed on speech current of the neighbouring communication line which results into distrorion. The potential of the communication circuit as a whole is raised because of electrostatic effect and the communication apparatus and the equipments may get damaged due to extraneous voltages. In the worst situation, the faithful transmission of message becomes impossible due to effect of these fields. Also the potential of the apparatus is raised above the ground to such an extent that the handling of telephone receiver becomes extremely dangerous.

The electromagnetic and the electrostatic effects mainly depend on what is the distance between power and communication circuits and the length of the route over which they are parallel. Thus it can be noted that if the distortion effect and potential rise effect are within permissible limits then the communication will be proper. The unacceptable disturbance which is produced in the telephone communication because of power lines is called Telephone Interference. 

There are various factors influencing the telephone interference. These factors are as follows

1) Because of harmonics in power circuit, their frequency range and magnitudes.

2) Electromagnetic coupling between power and telephone conductor.

The electric coupling is in the form of capacitive coupling between power and telephone conductor whereas the magnetic coupling is through space and is generally expressed in terms of mutual inductance at harmonic frequencies.

3) Due to imbalance in power circuits and in telephone circuits.

4) Type of return telephone circuit i.e. either metallic or ground return.

5) Screening effects.

1. Electromagnetic Effect

Consider a line transmitting power telephone line with two conductors E below the power line D and E below conductors. The two lines are running on the same supports. This is shown in the Fig. 1.35.1.

Consider the loop formed by the conductors A and D. Let the radius of each power line conductor be r. Let the distance between conductors A and D be D1 whereas the distance between conductors A and E be D₂. Assuming that in loop AD, A is contributing to the emf induced in D. If we neglect the internal flux linkages then the inductance of this loop is given by,

The inductance of the loop AE is given by,

The mutual inductance between conductor A and the loop DE is given by,

Similarly the mutual inductance between conductors B and C and loop DE can be obtained. Let these mutual inductances be MB andMC respectively. These mutual inductances are due to fluxes which have a phase displacement of 120°. Hence the net effect of the magnetic field will be

M = M_A + M_B + M_C

Here M is the net mutual inductance which is the phasor sum of the three inductances.

If the current flowing through the power line conductors is I and the supply frequency is f then the voltage induced in the communication conductors D and E is given by,

V = 2π  f IM volts/m

From the above expression, it can be seen that with increase in distance between power and communication line, the values of MA , MB and Me nearly becomes equal in magnitude and with the result that the net inductance M becomes very very small. As a consequence the voltage induced in telephone lines also diminishes.

The voltage induced in the neighbouring telephone line is directly proportional to frequency. If third harmonic is present then voltage equal to 3 times the voltage due to fundamental frequency will be induced in telephone lines. Also as the higher frequencies may come within audible range they produce a distortion effect.

The presence of harmonics and multiples of third harmonics will not cancel as they are in phase in all power line conductors. In balanced condition the total induced voltage due to harmonic currents in power line is additive. In unbalanced condition i.e. during fault the flux linkage and corresponding voltage induced is very high which may prove to be dangerous for telephone circuits.

If the distance between power line and the telephone line is increased then the induced voltage in telephone line can be reduced. It can also be reduced by transposing the line.

2. Electrostatic Effect

Consider the line conductor A running parallel to infinite plane (i.e. earth). Let D be the conductor from neighbouring telephone line. Conductor A' is the image of conductor A below ground as shown in the Fig. 1.35.2. The potential distribution between the conductor and earth is exactly same as that of its image and the plane.

The potential of point A with respect to the earth is given by,

In the similar fashion, the voltages VDB andVDC can be calculated. Finally the resultant potential of point D with respect to earth is given by,

V_D = V_DA + V_DB + V_DC

The above addition is the phasor addition.

Similarly the resultant potential of point E, V_E can be calculated using the same procedure.

3. Steps for Reducing Telephone Interference

There are various ways that can reduce the telephone interference. Some of them are as listed below

i) The harmonics at the source can be reduced with the use of AC harmonic filters, DC harmonic filters and smoothening reactors.

ii) Use greater spacing between power and telephone lines.

iii) The parallel rim between telephone line and power line is avoided.

iv) Instead of using overhead telephone wires, underground telephone cables may be used.

v) If the telephone circuit is ground return then replace it with metallic return.

vi) Use microwave or carrier communication instead of telephone communication.

vii) The balance of AC power line is improved by using transposition. Transposition of lines reduces the induced voltages to a considerable extent. The capacitance of the lines is balanced by transposition leading to balance in electrostatically induced voltages. Using transposition the fluxes due to positive and negative phase sequence currents cancel out so the electromagnetically induced emfs are diminished. For zero sequence currents the telephone lines are also transposed which is shown in the Fig. 1.35.3.

Transposition of conductors proves to be effective under normal working conditions which may be inadequate under faulty conditions. 

Example 1.35.1 A 3 phase, 60 Hz, 160 km long 132 kV transmission line delivers a total load of 20 MW at 0.8 pF lagging. The conductor arrangement is as shown in the Fig. 1.35.4. The lowest conductor being 25 m above the ground. The radius of each power conductor is 0.6 cm. Calculate the induced voltage at fundamental frequency in the telephone circuit due to electromagnetic effect. Also determine the potential of telephone conductors S and T above the earth due to electrostatic effect only.

Solution :

The potential of points above earth is 3332.35 volts.

The potential of point T above earth is 2897.19 volts.

Review Questions

1. Write a short note on inductive interference betweeen power and communication lines.

AU : Dec.-04, 05, Marks 8

2. Derive the expression for the voltage induced in communication lines due to the current in power lines.

3. Explain about interference between power and communication circuits.