What are the major equipment at a Traction Substation?

What is a Traction Substation?

An electric substation which converts electric power from the form provided by the electric utility company to an appropriate voltage, current and frequency to supply railways and other electric traction systems is known as a traction substation.

Major Equipment of Traction Substation

The main equipment which are provided at a traction substation are −

  • Traction Transformer

  • Circuit Breakers

  • Interruptor

Let's take a look at the functions of these three major equipment in detail −

Traction Transformer

The traction transformers are required to step down the high supply voltage (132 to 220 kV) of main grid to 25 kV. The traction transformers are specially designed so that they can withstand severe operating conditions and have large short time peak capacity.

The windings of the traction transformer are designed with class-A insulation and are specially reinforced to withstand the stresses of frequent short circuits on 25 kV overhead equipment. These transformers are designed to have large percentage impedance in order to limit the effect of short circuit. Typically, the percentage impedance is not less than 8.5% in case of 10 MVA and 10% for 12.5 MVA transformers.

The traction transformers need not to be provided with on-load tap changer as the supply voltage at substation is maintained within the permissible limits by the supply company. Although, the off-load tapings are provided on the high voltage side at −10%,−5%,0%,and +10% of the rated voltage so as to provide voltage between 27.5 kV and 23.75 kV. This has the advantage of less copper in the taps and smaller switches, which may be all immersed in the oil used for cooling of the transformer.

Usually, the traction transformers are provided with standard accessories like breather, oil level indicator, conservator tank, Buchholz relay, drain valve, dial gauge thermometer, vent pipe and thermostats to indicate the temperature of the oil.

In traction transformers, oil cooling arrangement is there, which is provided by radiators. These radiators can be easily removed for repair and maintenance by closing the valves in the inlet and outlet pipes connecting the radiators and transformer tank.

Circuit Breakers

In electric traction system, for each single unit substation, there are four line circuit breakers and one transformer circuit breaker.

Here, the line circuit breakers trip in case of fault in the overhead equipment and the transformer circuit breaker trips on the occurrence of internal fault in the traction transformer. The circuit breakers used in the traction substations are of minimum oil circuit breaker (MOCB) type and of rating 750 amperes at 25 kV and rupturing capacity of 500 MVA. These circuit breakers can be operated electrically by local push button or by remote control. These can also be operated manually.


An interruptor is nothing but a non-automatic circuit breaker. Interruptors are not provided with protective relays to enable them to operate automatically in case of occurrence of fault in the traction supply system.

Interruptor can be operated for opening and closing of overhead equipment (OHE) by remote control from the remote control center, however they are mainly provided at feeding, sub-sectioning posts and paralleling posts.

Circuit breakers and interruptors are provided with the following interlocks −

  • If the interruptor or the circuit breaker is not open, then double pole switches cannot be operated. This interlocking is provided to avoid the sparking at the switches.

  • If the key of the interruptor or the circuit breaker is placed for operation locally, then the circuit breaker or interruptor cannot be operated from remote control.

  • The door (if any) into the bay of the interruptor or the circuit breaker cannot be opened unless the double pole switch is locked in open position.