Traction Power Supply Systems
Traction power supply is the domain that tests the chain from the grid connection to the pantograph: incoming HV, traction transformer, rectifier or AC feed, switchgear, feeders, catenary and return. Questions focus on voltage classes, rectifier behavior, single-phase load unbalance, and what a feeder breaker actually protects.
Core rules
- AC traction classes are 25 kV 50/60 Hz and 15 kV 16.7 Hz; DC classes are 1.5 kV and 3 kV overhead, plus 750 V third rail. 25 kV is AC, never DC.
- A DC traction substation normally uses 12-pulse rectifiers; 24-pulse reduces ripple. For a six-pulse bridge the ideal no-load DC voltage is about 1.35 × the transformer secondary line-line RMS; rated voltage is lower under commutation and load.
- Single-phase AC traction produces negative-sequence current. Utilities may require Scott, Le Blanc, V/V, or a compensator such as an SVC or STATCOM.
- Neutral sections separate phases or feeding sections; the train coasts through them. DC feeders use high-speed breakers with current and di/dt settings.
Worked example
A 25 kV feeder supplies 8 MW at 0.95 power factor. Use the nominal 25 kV, not the 27.5 kV transformer secondary: I = P/(V·pf) = 8,000,000/(25,000×0.95) ≈ 337 A. If the transformer is 132/27.5 kV, 27.5 kV is the no-load secondary; the catenary nominal remains 25 kV. Using 27.5 kV would understate current by about 9%.
Traps
- Treating rectifier no-load voltage as the rated DC voltage.
- Mixing 25 kV AC with 1.5 kV DC nominal values.
- Ignoring power factor in feeder current.
- Assuming a neutral section is energized.