Major Traction Substation Equipments:
The Major Traction Substation Equipments are namely,
- traction transformer
- circuit breakers and
- interruptor.
1. Traction Transformer
Transformers are required to step down the grid supply voltage (220 or 132 kV) to 25 kV. These are specially designed so as to withstand severe operating conditions and have large short-time peak capacity. Its windings are designed with class A insulation and are specially reinforced to withstand the stresses of frequent short circuits on 25 kV overhead equipment. Transformers are designed to have large per cent impedance (not less than 8.5% in case of 10 MVA and 10% for 12.5 MVA transformers) so as to limit the effect of short circuit. Transformers need not to be provided with on-load tap changer as voltage at substations is maintained within permissible limits by the supply authorities. However, off-load tappings are provided on the hv side at – 10%, – 5%, 0% and +10% of rated voltage so as to provide voltage between 27,500 V and 23,750 V. This has the advantage of less copper in the taps and smaller switches, which also may be all immersed in the oil used for cooling the transformers. These transformers are provided with standard accessories such as conservator tank, Buchholz relay, breather, oil level indicator, drain valve, dial gauge thermometer, relief pipe and two thermostats to indicate the oil temperature rise. One is meant to give an alarm indication while the other is for tripping of circuit breaker in case of excessive temperature rise. The cooling arrangement is obviously oil cooling, which is provided by radiators. The radiators can be easily removed for maintenance and repair simply by closing valves in the inlet and outlet pipes connecting radiators and transformer.
2. Circuit Breakers
As already mentioned, for each single unit substation there are four line circuit breakers and one transformer circuit breaker. Line circuit breaker trips in case of fault in the overhead equipment. Transformer circuit breaker on 25 kV side usually remains closed and is intended to trip on the occurrence of internal fault on the transformer. The circuit breakers provided are of low oil content type and of rating 750 A at 25 kV and rupturing capacity of 500 MVA. Circuit breakers can be operated electrically by local push button or by remote control. These can also be operated manually.
3. Interruptor
Interruptor is a non-automatic type circuit breaker. It is not provided with protective relays to enable it to operate automatically in the event of occurrence of fault in the system. However, it can be operated for opening and closing of OHE by remote control from remote control centre. These are provided at feeding, subsectioning and paralleling posts and subsectioning posts. Circuit breakers and interruptors are provided with the following interlocks.
- Double pole switches cannot be operated unless the interruptor or circuit breaker is open so that sparking at the switches can he avoided.
- The door, if any, into the bay of circuit breaker or interrupter, cannot be opened unless the double pole switch is locked in open position.
- The circuit breaker/interruptor cannot be operated from remote control when its key is placed for operation locally.