Major Equipment's of Electrical Railway Substation

Major Equipment's of Electrical Substation

Transformer, circuit breaker and interrupter are the major equipments in the traction substation. They are discussed below in detail.

Transformer :-

Transformer is the major component of traction substation. Normally, step down transformer is used to step the high voltage level from 220kV - 132kV to a working voltage level of 25kV. The capacity of the transformer is normally from 15MVA - 32.5MVA.
These transformers are oil immersed. Oil immersed transformer not only provides Traction cooling but also specially designed to endure severe working conditions as insulation. transformers are compared to ordinary transformer.
Since the transformer is oil immersed it is provided with buchholz relay along with standard accessories such as, breather, oil level indicator, drain valve filtering valve, conservator tank.

Traction transformer is specially designed to withstand severe short circuit conditions. They are required to have short time peak capacity.
In order to meet the requirement, traction transformer has 'A Class Type' insulation, so as to withstand the sort circuit at 25kV over head lines. They can tolerate the temperature as much as 110-120% instantaneous of their rated value.
Traction transformers are generally provided with off load tap changer instead of on load tap changer since the primary is connected to the grid network where voltage is maintained within permissible limits.
Off load tappings are provided on the primary side at - 10 %, - 5 %, 0 % and 10 % of rated voltage.
These transformers are provided with other standard accessories such as oil level indicator, drain valve, filtering valve, thermometer, relief tank and thermometers to indicate the rise in oil temperature.

Circuit Breaker :-

Circuit breaker is switch which automatically under faulty conditions such as overload or short circuit to protect an electrical circuit from damage. Its basic function is to identify faulty condition and break off the current flow.
Operates Circuit breakers used in traction substation are L of two types namely, transformer side circuit breaker and feeder side circuit breaker.
Transformer side circuit breakers will operate only under the faults within the transformer and are normally in close position. Whereas, feeder side circuit breakers operates under the presence of fault on the over head lines.
Most of the faults on the transmission line / OHE are temporary in nature. Under such circumstances faults automatically cleared within auto reclosing arrangement is made available to the circuit breaker. Reclosing time interval seconds. few are Therefore is adjusted to 30 seconds.
The rated capacity of the circuit breaker is 750 amps at 25kV and rupturing capacity of about 500MVA.
There are various types of circuit breakers available according to type of arc extinguishing media. But mostly Minimum oil Circuit Breaker is preferred to the substation application.
Overall tripping time of minimum oil circuit breaker is 0.14 seconds composed of 0.04 second for relay operation and 0.1 second for opening of breaker contacts.

Interrupters :-

Interrupter is a non- automatic type of circuit breaker. As per the requirement they are available in various types and ratings such as single pole interrupters, single pole interrupters, double pole SF6 interrupters etc.
The Interrupter is used at the following locations in the 25kV AC 50 Hz, Single Phase Traction Systems

As sectioning Interrupter at Sub Sectioning and Paralleling Posts (SSP)
As Paralleling Interrupter at Sectioning Post (SP) and Sub Sectioning and Paralleling Post (SSP)
As Bridging Interrupter at Sectioning Post (SP)
As feeding Interrupter at Feeding post (FP)
As Bus Coupler Interrupter at traction substation
For controlling Power Supply to Major Yards / Loco Sheds

Interlocking of interrupters can be of Single pole, double pole or bus coupler isolator type. Interlocking is provided between circuit breakers and interrupters for following safety operations

Double pole switches are unable to operate unless the interrupter or circuit breaker is in open position. This is to avoid sparking at switches.
Unless double pole switch is locked in open position, it should not be possible to open the door of bay circuit breaker or interrupter.