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 electromagnetic and electrostatic fields of sufficient magnitude which induce currents and voltages in the neighbouring telephone lines.
The currents so induced are superimposed on the true speech currents in the neighbouring telephone wires and set up distortion while the voltages so induced raise the potential of the communication circuit as a whole. In extreme cases the effect of these fields 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.
Electromagnetic Effect on Telephone Line:
Single Phase Single Circuit Line and Telephone Line:
Consider two power conductors A and B of a single phase single circuit line and two telephone conductors C and D of a telephone line running on the same pole as the power conductors as shown in Fig. 8.1.
Let the distances between the power conductors and telephone conductors be dAC, dAD, dBC and dBD respectively.
First of all consider the loop formed by the conductors A and C. The self-induction of this loop is given by one-half of the expression of the self-induction of a pair of parallel conductors, because for this loop only conductor A is contributing to the emf induced in conductor C.
Denoting this self-induction by LAC, we have,
LAC = 0.2 loge dAC/r mH/km
ADVERTISEMENTS:
Similarly the self-induction of the loop formed by conductors A and D,
LAD = 0.2 loge dAD/r mH/km
Mutual inductance between conductor A and the loop CD,
MA = LAD – LAC
= 0.2 loge dAD/dAC mH/km
Similarly the mutual inductance between conductor B and the loop CD, MB is given as,
MB = 0.2 loge dBD/dBC mH/km
The net mutual inductance, which is phasor sum of two inductances, is given as,
ADVERTISEMENTS:
M = MA + MB = 0.2 loge dAD/dAC -2 loge dBD/dBC
∵ MA and MB are in phase opposition
= 0.2 loge dADdBC/dACdBD mH/km
The voltage induced in the loop CD is given by,
ADVERTISEMENTS:
Vm = 2f MI volts per km
where I is the current flowing through the power conductor and f is the supply frequency.
Three Phase Single Circuit Line and Telephone Line:
Consider three power conductors A, B, and C of a 3-ɸ single circuit line and two telephone conductors P and Q of a telephone line running on the same transmission tower as the power conductors, as shown in Fig. 8.2.
Let the distances between the power conductors and telephone conductors be dAP, dAQ, dBP, dBQ, dCP and dCQ respectively.
First of all consider the loop formed by the conductors A and P. The self-induction of this loop is given by one-half of the expression for the self-induction of a pair of parallel conductors, because for this loop only conductor A is contributing to the emf induced in conductor P.
Denoting this self-induction by LAP we have,
LAP = 0.2 loge dAP/r mH/km
Similarly the self-induction of the loop formed by the conductors A and Q.
LAQ = 0.2 loge dAQ/r m H/km
∴ Mutual inductances between conductor A and the loop PQ,
Similarly the mutual inductance between conductor B and the loop PQ, MB and the mutual inductance between conductor C and loop PQ, MC are given as,
MB = 0.2 loge dBQ/dBP m H/km
and MC = 0.2 loge dCQ/dCP m H/km
These three mutual inductances are the result of fluxes which have a phase displacement of 120°.
Therefore, the net effect of the magnetic field will be,
M = MA + MB + MC
where M is the net mutual inductance which is the phasor sum of the three inductances. The voltage induced in the loop PQ is given by:
Vm = 2πfMI volts per km
where I is the current flowing through the power conductor and f is the supply frequency.
It is to be noted that since the net mutual inductance M is the phasor sum of mutual inductances MA, MB, and MC, therefore, there is a partial cancellation of the induced voltages due to fundamental of the power current and as the distance between the power conductors and the telephone conductors is increased, the mutual inductances, MA, MB and MC progressively become equal in magnitude and thus M diminishes.
But the presence of third harmonics and multiples of third harmonics will not cancel as they are in phase in all the power conductors and, therefore, are dangerous for communication circuits. These harmonics are troublesome in two ways- firstly because the induced voltage is proportional to the frequency, and secondly, because the higher frequencies come within the audible range.
Example:
A single-phase, 50 Hz power line has horizontal configuration with 1.2 m spacing. A telephone line is run horizontally on the same support and is located symmetrically 0.85 m below the power line. Determine the voltage induced per km in the telephone line if the current in the power line is 120 A. Take the spacing between telephone conductors as 0.4 m.
Solution:
Refer to Fig. 8.1
dBD = dAC = 0.9394 m
dBC = dAD = 1.1673 m
The voltage induced per km in the telephone line,
Vm = 2fMI = 2õ × 50 × 0.086883 × 10-3 × 120 = 3.275 V/km Ans.
Effects of Earth Fault Currents:
There may be a large voltage drop in the earth itself, due to high soil resistivity when an earth fault occurs from a line to earth and there is a large return fault current. In case the telephone-circuit earth is within the resistance area of the power system earth, the potential of the telephone-circuit earth may be raised well above true earth potential and so a serious hazard may be present.
Electrostatic Effect on Telephone Line:
We know that when a conductor, such as A (Fig. 8.3), runs parallel to an infinite plane (earth for example), the potential distribution between this conductor and the plane is exactly similar to that between its image and the plane. Hence if q is the charge per unit length of the power conductor A and hA is the height of the conductor A from the ground, the potential of conductor A with respect to earth is given by,
Similarly potential of conductor P due to the conductor A,
Similarly, we can determine the potential of telephone conductor P due to power conductors B and C and hence the potential of telephone conductor P due to power conductors A, B and C will be,
VP = VPA + VPB + VPC
Similarly, the potential of telephone conductor Q due to power conductors A, B and C can be determined.
Transposition of Lines:
In extreme cases the, effects of the electromagnetic and electrostatic fields set up by power lines become so strong to make it impossible to transmit any message faithfully and render the handling of the telephone receiver extremely dangerous. In order to reduce the induced voltages due to electromagnetic and electrostatic induction, the conductors of both the power line and the telephone line are regularly transposed, as shown in Fig. 8.5.
By transposition of 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 conductors get diminished as the fluxes due to the positive and negative phase sequences cancel out along the barrel.
This, however, does not apply to zero sequence currents and that is why the telephone line is also transposed along with the transposition of power line, as shown in Fig. 8.5. Each transposition of a telephone line consists of a complete cross-over of the two wires, while each transposition of a 3-ɸ line consists of a twist, in a plane at right angles to the run of the line, of one-third of a revolution. Thus three transpositions are then necessary to bring the phases back to their original positions.
Electrostatic charging of the telephone line also results in the flow of currents which tends to interfere with the clarity of the speech. This effect cannot be eliminated entirely by transposition of lines.
Although, transposition of conductors is helpful under normal operating conditions but does not provide complete solution of the problem under abnormal conditions caused due to such faults as a short-circuit to earth. In such a case high emf is induced due to electromagnetic induction since the fault results in large zero-sequence currents flowing along the wires in parallel and through the earth return.
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.
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.