Railway substations
A railway traction substation is the fixed installation that connects the public electricity grid to the traction power distribution network, converting grid voltage and frequency to the supply characteristics required by the overhead contact system or third rail.
Each traction substation receives high-voltage power from the national transmission or distribution grid — at voltages from 33 kV to 400 kV depending on the country and the power demand of the line — and delivers it to the catenary at the appropriate traction voltage.
Function and architecture
For 25 kV AC systems, the substation contains a step-down transformer connected across two phases of the three-phase grid supply, producing a single-phase 25 kV output fed to the overhead line equipment via circuit breakers, disconnectors, and protective relays.
For DC systems, the substation adds rectifier groups that convert the AC secondary output to the DC catenary voltage; EN 50328 covers electronic power converters for DC traction substations.
Substations are typically arranged in a radial busbar configuration, the simplest architecture for a linear traction network.
Sectionalizing switches between substations allow power to be rerouted around a failed unit, maintaining supply continuity along the line. Major stations and junctions often host substations to manage the high and variable loads generated by accelerating trains.
Spacing and load design
Substation spacing depends on the traction voltage and expected traffic density. Conventional 25 kV AC systems are designed with substations every 20–50 km, constrained by the maximum permissible voltage drop at the pantograph under peak load conditions.
The 2×25 kV autotransformer system, which distributes at 50 kV between a dedicated feeder wire and the running rail, uses autotransformer posts at approximately 15 km intervals between substations spaced 60–80 km apart. DC systems at 1.5 kV require substations every 2–5 km on busy urban lines because the higher currents in the lower-voltage distribution produce steeper voltage gradients.
Regulatory and protection framework
EN 50122-1 addresses protective provisions for electrical safety and earthing in fixed railway installations. EN 50119 covers the overhead contact line system fed by substations.
Protection systems at substations must detect and isolate faults on the catenary within milliseconds to limit damage and prevent hazards to personnel. Remote control and monitoring of substations — supervision of switching states, protection relay status, and power metering — is standard practice on modern networks, with substations typically unmanned.