Power Supply
The electric power supply of Polish State Railways is an example of a power supply system used for a railway system operated at 3 kV. In this system all power is taken from the public grid and rectified in substations. Most substations are fed with voltages between 15 and 30 kV. Where lines have heavy traffic and higher speed, the substations are fed from the 110 kV-grid.
The switchyard of substations fed from voltages below 30 kV is indoors, outdoor switchgear is used at 110 kV. The distance between substations is between 15 and 28 kilometres. For reliability of supply, substations are usually fed by at least two powerlines. Each substation normally feeds two segments of the overhead wire, which are separated by a switch. As is common with DC systems, the negative pole is grounded. Half-way between two substations, there is a switch, which can be used to connect the overhead wires together. At some lines a three-phase AC line operated with 6 kV, 15 kV or 20 kV runs parallel to the railway line, either on the poles of the overhead wire or on separate poles. It is used for power supply of signals, level crossing equipment and other devices requiring electric power used for the rail. This line can be also used for emergency power supply of substations.
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