Advantages of traction; traction fed from separate line

Advantages of electric traction

1. No pollution at the point of use; suits cities, tunnels and underground railways.
2. High starting torque and acceleration, giving higher schedule speed and line capacity.
3. Regenerative braking returns energy to supply and reduces brake wear.
4. Lower maintenance and running cost; electric locomotives have long life and high availability.
5. High overall efficiency when energy comes from large power stations or hydropower.
6. Better adhesion and smooth, jerk-free control; can haul heavier trains on gradients.
7. Ready for service at once; no fuel or water stops.
8. Uses domestic energy (hydropower in Nepal) instead of imported fuel.

Traction fed from a separate distribution line

In this system the vehicle carries no energy source; it collects energy continuously from a distribution network laid along the route (overhead contact line or third rail), fed from traction substations. Electric trains, tramways and trolley buses use it.

 Grid (132/66 kV) -> traction substation -> feeders
   -> overhead contact wire / third rail
   -> collector (pantograph, trolley, shoe) -> vehicle
   -> traction motors -> running rails (return) -> substation

Supply systems used:

• DC system: 600–750 V (trams, metros), 1500–3000 V (suburban/main line). Rectifier substations every few km; dc series motors.
• Single-phase AC system: 25 kV, 50 Hz (or 15 kV, 16⅔ Hz). Substations every 40–60 km fed directly from the grid; on-board transformer with rectifier/inverter.
• Three-phase AC system: 3.3–3.6 kV, using two overhead wires plus rails; induction motors; now obsolete because of complex overhead wiring.
• Composite system: single-phase ac line with on-board conversion to dc (rectifier) or to variable-frequency three-phase ac (VVVF inverter) for traction motors; the modern standard.

Merits: vehicle is lighter and cheaper, high power available, high efficiency, regeneration possible. Demerits: high capital cost of track electrification; vehicles tied to the electrified route; supply failure stops all traffic; interference with communication lines.