Interference with Communication System in Electric Traction

This article explains the phenomenon of interference with the communication systems caused by the electric power lines employed for traction purposes. The phenomenon which modifies a signal in a disruptive manner, when the signal travels along a communication channel between its transmitter and receiver is known as interference in the communication system.

As the electric traction system uses high-voltage power lines, the electromagnetic and electrostatic fields produced by these traction power lines disturb the neighbouring communication lines, i.e., the traction power lines cause interference with the communication lines.

In electric traction systems, the track is used as the return path for the current. Some portion of this current flows through the track and the remaining portion the current flows through the earth as indicated in the following figure.

The part of the return current flowing through the earth causes interference with neighbouring communication lines. Therefore, to avoid the interference with the communication system, it is ensured that the return current flows through the restricted path, i.e. through the track. For this purpose, booster transformers are used in the AC traction system while negative booster is used in DC traction system. The booster transformer and the negative booster reduce the potential drop across the track.

Also, by reducing the resistance of the track, the return current can also be constrained to the restricted path. Usually, the resistance per km length of the continuous track rail is of the order of 0.05 Ω, but the resistance of the fish-plate-joints used for joining the commercial rail mechanically is greater than that of a trail. Therefore, to reduce the overall resistance of the track, the joints are supplemented with a good conductor bond. The modern bonds are of the flexible type and are designed to be as short as the conditions of locations allow. Generally, the bonds are fixed under the fish-plates to protect them from damage while carrying out the repairs to the paving.

In case of single-phase AC traction system, the interference to the communication systems is also caused by the harmonics present in the supply system.

The electromagnetic induction due to traction supply system also creates interference in the communication lines and signalling lines. In this case, the current flowing through the contact wire sets up a magnetic field around itself and this magnetic field links nearby communication circuit and causes interference by inducing EMF in the circuits.

Interference in the communication systems and signalling lines is also caused due to electrostatic induction in the circuits. There occurs a potential gradient between HV contact wire and the earth. Thus, the statically induced EMF in the communication and signalling lines is produced because of its positioning potential gradient.

Methods of Reducing Interference with Communication System

The effects of interference with the communication systems in electric traction can be reduced by taking following steps −

• All the communication systems are to be installed underground in cables instead of in overhead lines.

• The communication lines and signalling lines are provided with aluminium sheath instead of lead, and PVC or rubber covering is used to prevent sheath corrosion.

• Long communication networks are broken into small circuits by using isolation transformers.

• As much as possible, the communication circuits are to be installed at the distant places from the railway tracks.

• In order to avoid the magnetic linking of the communication lines with the traction power lines, booster transformer using return feeder is used.

Conclusion

Traction power lines produce electromagnetic and electrostatic fields that cause interference with the neighbouring communication lines and signalling lines. Also, in the traction systems, the track is used as the return path, due to high resistance of the track as compared to the earth, some part of the return current flows through the earth and causes interference with the communication lines.

As there must be some steps like increasing distance between communication lines and power lines, the use of booster transformer with a return feeder, splitting long communication line into smaller one, etc., to be taken to reduce the interference with the communication systems due to traction power lines.