Booster Transformers

Booster transformers are used in electric railway AC catenary feeders to collect the return current from the rails and the earth to the return conductor.

Booster transformer is one which is often used towards the end of a power line to raise the voltage to the desired value. It is used for controlling the voltage of a feeder at a point far away from the main transformer.

In railways,

  • The electric current is taken from the catenary conductor to the locomotive, where the energy is used by electric motors, and fed to the earth connected rails, which are part of the return circuit.
  • From the rails, however, the return current may deviate around to unintended or harmful places like metallic pipe-lines, bridges, communication cables, etc. The stray currents bring about interference in communication systems and other electronic devices due to passing trains.

Booster transformers are used to eliminate the stray currents and the disturbances, obliging the return current to flow to the return conductor.

The secondary of the booster transformer is connected in series with the line, and its primary is supplied from the secondary of the regulating transformer. The output winding of the regulating transformer is so connected to the primary of the booster transformer that the voltage injected in the line (BB) is in phase with the supply (AA).

By changing taps on the regulating transformer, the magnitude of BB can be changed and thus feeder voltage AA can be regulated. The rating of regulating transformer is only the fraction of that the main 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.

The advantage of the above system is that the regulating equipment is independent of the main transformer so that a failure in the former will not throw the latter out of service.


  1. Booster transformer is used in railways for eliminating the flow of stray current. The stray current disturbs the communication system and also damage the electronic devices of the trains passing through them.
  2. They are used to maintain uniform voltage along the line that have distributed load, the voltage drops due to serial resistance is compensated periodically with a booster transformer, otherwise the voltage given to first users will go too high and last users too low.

Product information

  • Primary voltage: 15 or 25 kV
  • Current: usually 200 – 800 A
  • Frequencies: 16.67, 25, 50 or 60 Hz
  • Power ratings: 100-800 kVA
  • Hermetically sealed tank


Leave a Reply