Various Systems of Railway Electrification

Several different types of Railway Traction Electric Power System configurations have been used all over the World. The choice of the system depends on the train service requirements such as 
I.    Commuter rail 
-Commuter rail typically includes one to two stops per city/town/suburb along a greater rail corridor
II.    Freight rail
-Rail freight transport is the usage of railroads and trains to transport cargo on land. It can be used for transporting various kinds of goods
III.    Light rail
-The LRT vehicles usually consist of 2–3 cars operating at an average speed of 55–60 km/h on the lines with more dense stops/stations and 65–70 km/h along the lines with less dense stations
IV.    Train loads
V.    Electric utility power supply.

Railway electrification loads and systems required for light rails, commuter trains, fast high-speed trains, and of course the freight trains are all different. The power demands for these different rail systems are very different. The selection of an appropriate electrification system is therefore very dependent on the Railway system objectives

Presently, the following four types of track electrification systems are available:

1. Direct current system—600 V, 750 V, 1500 V, 3000 V 
2. Single-phase ac system—15-25 kV, 16 23, 25 and 50 Hz 
3. Three-phase ac system—3000-3500 V at 16 2 3 Hz 
4. Composite system—involving conversion of single-phase ac into 3-phase ac or dc.
 

Direct Current Traction System

In this traction system, electrical motors are operating on DC supply to produce the necessary movement of the vehicle. Mostly DC series motors are used in this system. For tramways, DC compound motors are used where regenerative braking is required.
Regenerative braking 
In this type of braking the motor is not disconnected from the supply but remains connected to it and feeds back the braking energy or its kinetic energy to the supply system. The essential condition for this is that the induced emf should be slightly more than the supply voltage. 

The various operating voltages of the DC traction system include 600V, 750 V, 1500V, and 3000V.
•    DC supply at 600-750V is universally employed for tramways and light metros in urban areas and for many suburban areas. This supply is obtained from a third rail or conductor rail, which involves very large currents.
•    DC supply at 1500- 3000 is used for mainline services such as light and heavy metros. This supply is drawn mostly from an overhead line system that involves small currents.

Since in the majority of cases, track (or running) rails are used as the return conductor, only one conductor rail is required.

Both these supply voltages are fed from substations which are located 3-5 KM for suburban services and 40 to 50KMs for mainline services. These substations receive power (typically, 110/132 KV, 3 phase) from electric power grids.
This three-phase high voltage is stepped-down and converted into single-phase low voltage using Scott-connected three phase transformers.
This single-phase low voltage is then converted into DC voltage using suitable converters or rectifiers. The DC supply is then applied to the DC motor via a suitable contact system and additional circuitry.

 

Advantages
1. In the case of heavy trains that require frequent and rapid accelerations, DC traction motors are the better choice as compared to AC motors.
2. DC train consumes less energy compared to AC unit for operating same service conditions.
3. The equipment in the DC traction system is less costly, lighter, and more efficient than the AC traction system.
4. It causes no electrical interference with nearby communication lines.

Disadvantages
1. Expensive substations are required at frequent intervals.
2. The overhead wire or third rail must be relatively large and heavy.
3. Voltage goes on decreasing with an increase in length.

                Single-phase ac system

In this type of traction system, AC series motors are used to produce the necessary movement of the vehicle. This supply is taken from a single overhead conductor with the running rails. A pantograph collector is used for this purpose. The supply is transferred to the primary of the transformer through an oil circuit breaker. The secondary of the transformer is connected to the motor through switchgear connected to suitable tapping on the secondary winding of the transformer. The switching equipment may be mechanically operated tapping switch or remote-controlled contractor of group switches. The switching connections are arranged in two groups usually connected to the ends of a double choke coil which lies between the collections to adjacent tapping points on the transformer. Thus, the coil acts as a preventive coil to enable tapping change to be made without short-circuiting sections of the transformer winding and without the necessity of opening the main circuit. 
Out of various AC systems like 15-25 kV, 16 23, 25, and 50 Hz. Mostly the 25KV voltage is used in railways. The main reason for the 25kV voltage used in the railway is, that 25 kV AC is more economical than a 1.5kV DC voltage system. Since the 25kV voltage system has a higher voltage, the higher voltage reduces the current flow through the conductor; this reflects reducing the conductor size. The cost of the conductor gets less. 

However, there are other major advantages for using 25kV voltage system in railway are quick availability and generation of AC that can be easily stepped up or down, easy controlling of AC motors, a smaller number of substations requirement, and the presence of light overhead catenaries that transfer low currents at high voltages, and so on.

 

Disadvantages
1.  Significant cost of electrification.
2.  Increased maintenance cost of lines.
3.  Upgrading needs additional cost especially in case there are bridges and tunnels.

Composite System

As the name suggests this system is classified into two types 
I single phase to dc system
II single phases to 3 phase system

Single Phase to DC system

The first one single phase to dc system is used where the voltage level is high for transmission and the dc machine is used in the locomotive.
This system combines the advantages of high-voltage ac distribution at the industrial frequency with the dc series motors traction. It employs an overhead 25-kV, 50-Hz supply which is stepped down by the transformer installed in the locomotive itself. The low-voltage ac supply is then converted into dc supply by the rectifier which is also carried on the locomotive. This dc supply is finally fed to dc series traction motor fitted between the wheels.
                                                                                    Single-phase to 3 phase system

Single-phase to 3 phase system is used where 3 phase machine is used in the locomotive and Single-phase track available. In this system, the single-phase 16KV, 50 Hz supply from the sub-station is picked up by the locomotive through the single overhead contact wire. It is then converted into a 3-phase AC supply at the same frequency by means of phase converter equipment carried on the locomotives. This 3-phase supply is then fed to the 3-phase induction motor.

References: various EMC Europe IEEE papers, slideshare.net presentations, rail systems, etc.