Railway electrification system

A railway electrification system is a way of supplying electric power to electric locomotives or multiple units.

One may distinguish such systems by:

• type of conductors (third rail or overhead wire)
• type of current:
  • direct current or alternating current
  • voltage
  • standard frequency or low frequency
  • single-phase or three-phase current

Direct current

Historically the first system was relatively low-voltage DC. Electric motors were fed directly from mains. Motors were controlled by switching resistors and by regrouping the motors in parallel and in series.

The common voltages are 600V for trams and metros, 750V for metros, 1500V and 3000V for common railways.

This system is quite simple, but low voltage requires thick wires and short distances between feeding stations, and some energy is lost in resistance.

The auxiliary machinery (fans and compressors) are driven by the same type DC motors fed directly from mains. However, the mains voltage is too high for them, so these auxiliary motors are very big and heavy.

The 1500V DC system is used in France, Netherlands and Japan. The 3000V DC system is used in Belgium, Italy, Poland, Czech Republic, Slovakia, former Yugoslavia and the former Soviet Union.

Low-frequency alternating current

Common commutating electric motors can also be fed alternating current, because reversing current in both stator and rotor does not change the direction of torque. However, this creates much stress on the motor. This stress can be reduced if the frequency is low, so many European countries have adopted a system of 16⅔Hz single-phase alternating current of 15kV (formerly also 6kV and 7.5kV).

Motors are fed through a switching transformer that allows to change voltage, so no resistors are required. The auxiliary machinery is driven by the same type commutating motors, but of lower voltage (from a separate winding of the main transformer), so these auxiliary motors are reasonably small.

The use of non-standard frequency means that one have to use motor-generators at the feeding stations, or have altogether separate electric power stations.

The low frequency alternating current system (15kV at 16⅔Hz) is used in Germany, Austria, Switzerland, Norway and Sweden.

Standard frequency alternating current

The first attempts to use standard-frequency (50 Hz) single-phase alternating current were made in Hungary in 1930s. However, only in 1950s this system began to be widely implemented.

Today the locomotives in this system use a transformer with rectifier that provide low-voltage DC current to motors (strictly speaking, they provide pulsating current). Motors are controlled by switching windings in the transformer.

This system is quite ecomomical, but it has its drawbacks: the phases of the external power system are loaded unequally, and there is some electromagnetic interference with aerial communication lines.

The 25kV 50Hz single-phase AC system is used in France, Great Britain, Finland, Denmark, former Soviet Union, former Yugoslavia and Japan.

Multisystem locomotives

Because of the variety of railway electrification systems (even within one country), trains often have to pass from one system to another. One way to do it is to change locomotives at the switching stations (these stations have overhead wires that can be switched from one voltage to another, and so the train arrives with one locomotive, and then departs with another). Often this is inconvenient. Another way is to use multisystem locomotives that can operate under several different voltages and current types. In Europe, it is common to use four-system locomotives (DC 1.5kV, DC 3kV, AC 15kV 16⅔Hz, AC 25kV 50Hz). Such locomotives do not have to stop when passing from one electrification system to another.