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Systems of Track Electrification: Composite System of Track Electrification
There is no system of track electrification which is good in all aspects, i.e. it suffers from some disadvantage.
The composite system (as the name implies) combines together any two of the systems of track electrification so as to incorporate good points of both the systems. The composite systems presently being used are as −
Single-Phase AC to Three-Phase AC System
Single-Phase AC to DC System
Single-Phase AC to Three-Phase AC System
The single-phase to three-phase system of track electrification is also known as Kando System. As the three phase AC traction system has some disadvantages such as tapping of power supply through two overhead conductors which is eliminated in the Kando System.
In this system, a single phase AC supply of 15 kV at 50 Hz is fed from the substation to the overhead distribution network. This single phase AC power at 15 kV is then tapped by the locomotive with the help of pantograph. The locomotive is equipped with phase converter which converts the single-phase AC to three-phase AC at the same frequency. This three-phase AC supply is then given to the three-phase induction motor.
Hence, by employing the kando system of track electrification, the advantage of producing power by three-phase induction motor are retained whereas the disadvantage of the tapping power from the two overhead conductors (in three phase AC system) get eliminated.
Single-Phase AC to DC System
In the single-phase AC to DC system of track electrification, the overhead line is fed from a substation at 25 kV, single-phase 50 Hz AC supply. This 25 kV single-phase AC supply is tapped by the locomotive through the pantograph and is then fed to the step-down transformer which is located in the locomotive which reduces the voltage to lower values.
The reduced voltage is further fed to the rectifier unit which is also installed in the locomotive. The rectifier unit converts the AC voltage to DC voltage and this DC voltage is given to the DC series motors which are fitted to drive the wheels.
Therefore, it can be noticed that the single-phase AC to DC system combines the advantages of the high voltage AC distribution at industrial frequency with the DC traction system.
Also, the single-phase AC to DC system of track electrification employing 25 kV at industrial frequency has been adopted for all future electrification in India. Some of the traction systems in India which are working on this supply system as follows −
Howrah to Tundla
Igatpuri to Bhusaval
Virar to Sabarmati
Madras to Tambaram
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