19-12-2012, 06:05 PM
Power Supply Installation in Electrical Traction
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GENERAL DESCRIPTION OF FIXED INSTALLATIONS
1.0 POWER SUPPLY ARRANGEMENTS AT SUB-STATIONS 1.0.1. Power Supply
25 kV, AC, 50 Hz single phase power supply for electric traction is derived from the grid of State Electricity Boards through traction sub-stations located along the route of the electrified sections at distance of 35 to 50 km apart. The distance between adjacent substations may however be even less depending on intensity of traffic and load of trains.
At present there are broadly four different arrangement in existence as under
1. The Supply Authorities supply power at 220/132/110/66 kV Extra High Voltage (EHV) at each traction substation which is owned, installed, operated and maintained by the Railways.
2. The Railway receives 3-phase power supply from the supply Authority at a single point near the grid substation from where the Railway runs its own transmission lines providing its own traction sub-stations.
3. All EHV and 25 kV equipment is owned, installed, operated and maintained by the Supply Authority except 25 kV feeder circuit breakers which are owned, installed, operated and maintained by the Railway.
4. All EHV and 25 kV equipment is owned, installed, operated and maintained by the Supply Authority but 25 kV feeder circuit breakers alone are operated on remote control by the Traction Power Controller (TPC).
Duplicate Supply
1. Fig 2.01 shows schematically the arrangement at a typical traction sub-station.
2. To ensure continuity of supply under all conditions, the high voltage feed to the traction substations is invariably arranged wither from two sources of power or by a double transmission line, so that if one source fails the other remains in service. Suitable protective equipment is installed at the substations to ensure rapid isolation of any fault in transmission lines and substation equipment, so that the power supply for electric traction is maintained under all conditions.
3. At each traction substation, normally two single phase transformers are installed, one which is in service and the other is 100% stand by. The present standard capacity is 21.6 MVA (ONAN)/30.2 MVA (ONAF).
However transformers of capacity 13.5 MVA (ONAN)/10.8 MVA (ONAN) have also been used at many of the substations. These transformers step down the grid voltage to 25 kV for feeding the traction overhead equipment (OHE). 25 kV feeders carry the power from the substations to feeding posts located near the tracks. Each feeder is controlled by a single pole circuit breaker equipped with protective devices.
Voltage Regulation
The permissible variation of the bus bar voltage on the busbars at the grid substations is +10% and -5% i.e. between 27,500 V and 23750 V. The tappings on the transformers are on the secondary winding and set to ensure the voltage is maintained as high as possible but not exceeding 27.5 kV at the feeding post at any time.
1.0.4 25 kV Supply at Traction Substations
1. On the secondary side one transformer circuit breaker and one feeder circuit breaker are installed with associated double pole isolator the busbar connections being such that full flexibility of operation is assured.
2. The traction substation is designed for remote operation.
3. The facilities exist to change over from one feeder to the other by means of isolator/bus coupler.
4. One end of the secondary winding of the transformer is solidly earthed at the substation and is connected to track/return feeder through buried rail.
Feeding and Sectioning Arrangements
1. The generation and transmission systems of Supply Authorities are 3 phase systems. The single phase traction load causes unbalance in the supply system. The unbalance has undesirable effects on the generators of the supply Authorities and equipment of other consumers. If its value becomes excessive.