Merits and demerits of DC track electrification

In the dc system of track electrification, the contact line or third rail carries dc at 600–750 V (tramways, metros) or 1500–3000 V (suburban and main lines). Substations take ac from the grid and convert it to dc with rectifiers; dc series motors (or chopper-controlled motors) drive the vehicle.

Merits

1. Ideal motor characteristic: the dc series motor gives high starting torque, speed that falls with load, power $\propto \sqrt{T}$, and good load sharing.
2. Simple, light and cheap locomotive: no heavy transformer or rectifier on board.
3. Lower energy per train-km in frequent-stop urban service, due to good acceleration.
4. Small clearance and insulation at low voltage: suits tunnels, underground metros and the third-rail system.
5. No interference with telecommunication and signalling lines (no alternating magnetic field).
6. No single-phase unbalance: the substation rectifiers draw a balanced three-phase load from the grid.
7. Speed control is simple (series-parallel, chopper), and regenerative braking is possible with chopper control or by separately exciting the fields during braking.

Demerits

1. High line current at low voltage: heavy, costly overhead conductor or third rail and supports.
2. Large voltage drop and $I^2R$ loss; substations must be close together (3–5 km at 750 V, 10–20 km at 3 kV).
3. Costly substations: every substation needs rectifier equipment, and many substations are needed, so the capital cost is high.
4. Stray currents returning through earth cause electrolytic corrosion of underground pipes and cables.
5. Energy is wasted in starting resistances (with resistance control).
6. Regenerated energy can be used only if another train is drawing power nearby (rectifier substations cannot feed back to the grid without inverters).
7. A third rail at ground level is a safety hazard.