A traction system (i.e., for railways, trams, trolleys, etc.) involving the use of electricity is called an Electric Traction System. The system of track electrification refers to supplying electric power to the locomotive by means of overhead distribution lines running along the track.
Based on the availability of supply, area of track electrification, and type of electric traction service (mainline, urban, or suburban), the following track electrification or electric traction systems are employed,
- DC Traction System,
- Single-phase AC Traction System,
- Three-phase AC Traction System,
- Composite Traction System.
DC Traction System :
In the dc traction system, dc series motors are employed due to their high starting torque. The operating voltage range of the motor lies between 1500-3000V. However, dc compound motors with operating voltage at 600V are also used for tramways and trolleybuses where regenerative braking is required.
Usually, an ac supply at a voltage range between 33kV to 100kV is fed to the substation. The substation is equipped with a transformer to step down the voltage level and rotary converters to convert ac power into dc power. Then after the dc power supply is fed to the locomotive through an overhead conductor.
The operating voltage of the dc traction system depends upon spacing between substations and the type of motor employed. This type of system is preferred for suburban services and road transport where there are frequent stops.
Advantages of DC Traction System :
- The characteristics of dc motor are better than ac motor and also they are more suitable for traction system.
- The maintenance cost of a dc motor is low compared to ac motor.
- The weight per kW output of dc motor is less as compared to ac motor.
- Better speed control.
- In a dc traction system, rail act as a return conductor, thus only one conductor is used.
Disadvantages of DC Traction System :
- Since it is difficult to obtain high voltage levels in a dc system, a conductor of a large cross-section for distribution is required.
- The overall cost of the substation is high due to the additional converting equipment required for converting ac to dc.
- The number of substations to be erected will be more since the voltage level cannot be increased beyond the prescribed value.
Single-Phase AC Traction System :
In this traction system, ac power is supplied to the locomotive. Generally, ac series motors with operating voltage in the range of 300V-400V at 16 2/3 or 25Hz frequency are used. A voltage level in the range of 15kV to 25kV at normal frequency is fed to the overhead distribution wire from the substation. Later in the locomotive, the high voltage is stepped down to 300V-400V at a frequency of 16 2/3 or 25Hz using a step-down transformer and frequency changer.
If the power supply is from generating stations that are exclusively meant for the traction system, then it is easy to obtain the required voltage level at 16 2/3 or 25Hz frequencies. But in practice, this is not the case, because the power supply is taken from an industrial frequency network.
Thus substation in addition to the transformer has necessary to have frequency-changing equipment that converts the incoming three-phase supply to a single-phase low-frequency supply. The single-phase ac traction system is preferred for mainline and suburban services.
Advantages of AC Traction System :
- The spacing between two substations is usually taken as 50-80 km because of low current requirement at high voltage.
- Rapid acceleration and retardation have less effect on the system.
- Less cost.
- Low-frequency operation improves commutation, efficiency, and power factor of ac series motor.
- Low-frequency operation reduces the line reactance and hence voltage drop.
Three-Phase AC Traction System :
In a three-phase traction system, three-phase induction motors operating at 3000 to 3600V at a frequency of 16 2/3 Hz are used. Two overhead conductors are used and the track acts as the third conductor. Two overhead wires per track are necessary, so the line voltage of distribution systems has to be limited to values below the highest voltage used for single-phase systems.
Three-phase induction motors have to be built to operate at moderately high voltages. The line voltage may be chosen such that the motors may be supplied directly from the overhead wires. The sub-station receives power at a high voltage where it is stepped down and converted to the desired frequency.
Advantages of Three-Phase Traction System :
- Three-phase induction motors are simple, robust, and efficient with low-cost regeneration.
Disadvantages of Three-Phase Traction System :
- Difficult to collect current from overhead conductors.
- Induction motors have low starting torque, high starting current, and difficult speed control. However, these drawbacks can be overcome by using power electronics based circuits.
- The induction motors have almost constant speed characteristics which are not suitable for a traction system. However, speed control can be achieved by cascade connection and pole-changing methods.
Composite Traction System :
This system combines the advantages of dc and ac (three-phase/single-phase) systems. Single-phase system is preferred due to a better distribution system and easy current collection. Hence, in a composite system, power distributed to the locomotive is single-phase ac wherein it is converted to three-phase or dc at the required level. There are two types of composite systems as given below.
Single-Phase to Three-Phase System (Kando System) :
In this system, single-phase power at 15kV, 50Hz is transmitted to the locomotive. The single-phase supply will be converted to a three-phase supply by a phase converter and fed to a three-phase induction motor on the locomotive. Power electronics based inverter circuits are used to generate ac at low frequency i.e., 10Hz.
At low frequency, the three-phase induction motor develops high starting torque without drawing excess current from the supply. Speed control can also be obtained by varying the supply frequency. This system combines the advantages of a single-phase system and a three-phase induction motor.
The advantages of single-phase ac to three-phase ac traction system are,- The difficulty of collecting current in a three-phase system can be overcome.
- Less expensive than a three-phase distribution network.
- The design of the substation is simple and flexible in substation location.
Single-Phase AC to DC System :
Single-phase power supply at 15kV to 25kV, 50Hz is distributed and fed to the locomotive. The locomotive is equipped with a transformer that will step down the voltage. This stepped-down voltage is fed to a converter which converts ac to dc.
The dc thus generated will be fed to a dc traction motor. This system combines the advantages of using efficient high voltage single-phase ac distribution and dc series motor characteristics which are suitable for traction.