A current carrying conductor produces its own magnetic field around it. This magnetic field varying with time induces an electro – motive force (emf) in a conductor parallel to the current carrying conductor. This induced emf disturbs the communication and signalling line and while undertaking electrification works, precautions are required to be taken to prevent this side effect. The following steps are taken to limit the magnitude of the induced voltage
- All telecommunication circuits are taken in cables instead of aerial circuits thus limiting induced voltage. This has given another benefit of avoidance of pilferage and damage during storm etc.
- Long telecommunication circuits are broken by provision of insulating transformer.
- Telecommunication cables are provided with aluminium sheathing
- Screening effect is augmented by the use of additional conductors or steel tapes. Sheathing with steel tapes raises the permeability and is of much effect. The smaller the value of ground resistance of the sheath and the value of sheath
- The use of PVC or rubber covering to prevent sheath corrosion
- Use of booster transformer
For safety of equipment and personnel, the limits specified are not more than 430 V rms in tele-communication cable under conditions of short circuits in power circuit and not more than 60 V rms under normal condition of maximum current. AC Traction Manual para 20711 deals with Telecommunication facilities for AC Traction. Induced voltage can be derived by a simple formula
Induced Voltage = Mutual Impedance * Length in parallelism in Km * screening factor * Current
Considering a case study of Mutual impedance = 0.1250 ohms/km, length = 2 km, k=0.28 and I = 600 A
Then the voltage induced = 0.1250 * 2 * 0.28 * 600 = 42 V
Induced emf can be reduced by running a return current conductor in parallel to traction carrying conductor so that induced emf is nullified. Return current from rail leaks to earth and prefer to return via the earth to traction substation. Logical step to minimize the interference due to traction current is run return conductor parallel to overhead conductor and boosting the current through it. Provision of just the return conductor will help in diverting part of the return conductor into it and may reduce the induced emf partly.
Addition of booster transformer forces the earth current to move through a return conductor. Booster transformer is a 1:1 ratio transformer. Primary winding is connected in series with the contact wire. Any amount of current flowing through primary requires to be balanced by equal current in the secondary and therefore, the tendency of currents flowing through the stray path is reduced. Booster transformers are provided at around 5-7 Km distance.
Installing a boost transformer is having an effect on voltage regulation which drops to 10.39% from 2.88% with booster and without booster respectively. This also results in an additional energy consumption this is beside maintenance and reliability issue by adding overlap on booster transformer.
A typical mounting arrangement is shown below
Provision of Booster transformer along with return conductor was an acceptable feature for any electrification work undertaking in major cities. But over the period significant technological advancements are observed in communication system either optic fiber or wireless network and in built isolation, digital circuit or axle counter in signalling system.
Indian Railways having observed that the voltage induced is very low as compared to admissible level, has decided to remove booster transformer and now we don’t find any return conductor or booster transform any where on 25 KV network of Indian Railways. However Rapid Mass Transit system is still continuing with this arrangement. It is now high time this practice shall be reviewed and unnecessary provision having running cost be done away with. This will require taking clearance from Department of Telegraph and Signalling and Telecom department of Railway organisation. This will also require some measurements of induced voltage and shall be done to satisfy the laid down conditions. In fact, an extra design feature in cable and isolation work may be more economical then installing booster transformer as it involves running cost as well.
Southern Railway did actual measurement sometime in the year 2000 between Tambram and Chennai Beach and found isolated peak of 31 V well withing permissible limit.