INDUCTIVE INTERFERENCE
WITH
NEIGHBOURING
COMMUNICATION CIRCUITS
It is usual practice to run telephone lines along the same route
as the power lines. The transmission lines transmit bulk power at relatively
high voltages and, therefore, these lines give rise to electro-magnetic and
electrostatic fields of sufficient magnitude which induce are superposed 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
as a whole. In extreme cases the effect of these may make it impossible to
transmit any message faithfully and may raise the potential of the telephone
receiver above the ground to such an extent to render the handling of the
telephone receiver extremely dangerous and in such cases elaborate precautions
are required to be observed to avoid this danger.
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 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 distortion. 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 unbalance in power circuits and in telephone circuits.
4) Type of return telephone circuit i.e. either metallic or ground
return.
5) Screening effects.
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 A.C. harmonic filters, D.C. harmonic filters and
smoothing rectors.
ii) Use greater spacing between power and telephone lines.
iii) The parallel run 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 e.m.f.s are diminished. For zero sequence currents
the telephone lines are also transposed which is shown in the Fig.
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