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
  • AC frequency
  • single-phase or three-phase current

Direct current

Early electric systems used relatively low-voltage DC. Electric motors were fed directly from mains, and were controlled by using a combination of resistors and relays which connected the motors in parallel or series.

The common voltages are 600 V and 750 V for trams and metros, and 1500 V and 3000 V for railways. In the past, Rotary converters were used to convert utility (mains) ac power to the required dc voltage. Today, this is usually accompished via semiconductor rectifiers.

This system is quite simple, but requires thick wires and short distances between feeding stations; additionally, there are significant resistive losses.

Auxiliary machinery, such as fans and compressors are also powered by motors fed directly from mains. Consequently, these motors are often unusually bulky.

The 1500 V DC system is used in France, The Netherlands, Japan and Australia. The 3000 V DC system is used in Belgium, Italy, Poland, the Czech Republic, Slovakia, former Yugoslavia and in the former Soviet Union. 3000 V DC was also formerly used by the Delaware, Lackawanna & Western Railroad (now NJ Transit, now converted to 25,000 V AC).

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, including Germany, Austria, Switzerland, Norway, and Sweden have standardized on 15 kV 16-2/3 Hz (one-third the normal mains frequency) single-phase alternating current (earlier, 6 kV and 7.5 kV systems have been in use). In the United States (with its 60 Hz distribution system), 25 Hz (an older, now-obsolete standard mains frequency) was commonly used at either 11 kV or 12.5 kV.

Motors are fed through a switching transformer that allows voltage change, so no resistors are required. Auxiliary machinery is driven by low voltage commutating motors, powered from a separate winding of the main transformer, and are reasonably small.

The unusual frequency means that electricity has to be converted from utility power by motor-generators at the feeding stations, or generated at altogether separate electric power stations.

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 did this system become widespread.

Today, some locomotives in this system use a transformer and rectifier that provide low-voltage pulsating DC current to motors. Speed is controlled by switching windings in the transformer. More sophisticated locomotives use thyristor or IGBT transistor circuitry to generate chopped or even variable-frequency ac that is then directly consumed by ac traction motors.

This system is quite ecomomical, but it has its drawbacks: the phases of the external power system are loaded unequally, and there is significant electromagnetic interference generated.

The 25 kV 50 Hz single-phase AC system is used in France, Great Britain, Finland, Denmark, the former Soviet Union, the former Yugoslavia and Japan, while USA commonly uses 12.5 and 25 kV at 60 Hz.

Multisystem locomotives

Because of the variety of railway electrification systems, which can vary even within a country, trains often have to pass from one system to another. One way this is accomplished is by changing 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 1.5 kV, DC 3 kV, AC 15 kV 16-2/3 Hz, AC 25 kV 50 Hz). These locomotives do not have to stop when passing from one electrification system to another.