Transposition of Line: The transmission line transmits a bulk amount of power at high voltages. e high power and voltage of these transmission lines give rise to electromagnetic and electrostatic fields. These fields are strong enough to induce voltages and currents in the neighboring communication line (such as a telephone line) and cause interference in them.
The electromagnetic and electrostatic fields produced by the power transmission line under extreme conditions (such as during faults in the power system network when high fault current flows through the line) make it impossible to transmit any useful signal or message over the communication line. Also, the interference may introduce extreme levels of induced voltages and currents in extremely dangerous to handle the telephone receiver.
Here we will be clarifying the interference caused by the power transmission line on the communication line and reducing the interference by transposition of the line.
Reduction of Interference by Transposition of Line
The power line near the communication line causes radio interference in the communication line. The radio interference on the communication line can be reduced by the transposition of the power and communication line at regular intervals.
The capacitance of the power line is balanced by transposing it, and the electrostatically generated voltages are balanced over the length of the entire set transposed line. The complete set of a transposed line is known as a barrel.
The electromagnetically induced voltages on the communication line due to the power line gets much reduced by transposition. This is due to the fluxes due to the positive sequence and negative sequence cancel out along the barrel. In order to counterbalance the effects of zero sequence current, communication lines are transposed at regular intervals along with the power lines.
Transposition of Power Line
Transposition of the three-phase power transmission line is done at regular intervals. Here, the lines are twisted in a plane at the right angles to the run of the line. The transposition of the power line is done at an interval of one-third of the length of the line.
Transposition of Communication Line
The communication line such as a telephone line consists of two wires. The two wires of the telephone line are transposed at regular intervals by a complete cross-over of the two wires.
Under extreme conditions such as a line to ground fault (L-G fault), a huge amount of current flows to the ground thereby resulting in a huge amount of zero-sequence current. Under such conditions, the transposition of the line may not be helpful.
During such faults, a considerable voltage may be induced between the conductor and ground so it becomes necessary that the communication lines insulation can withstand such voltages.
In more extreme cases, the telephone apparatus is isolated from the telephone line through a highly insulated transformer. For the safe discharge of the induced charge, the device is earthed.