For Metro DC traction power systems, the commonly adopted nominal voltages are 750V DC and 1500V DC. The DC substations are normally fed from a High Voltage (HV) 11 kV or 33 kV 3-phase AC network in the UK. To convert the 3-phase AC voltage to DC voltage, a modern DC substation employs a 3-winding transformer (delta-delta-star) to feed a 12-pulse rectifier (consisting of 2 x 6-pulse diode bridges, the outputs of which are connected in either parallel or series to form a 12-pulse rectifier). Note that the secondary side delta and star windings are phase shifted by 30deg for cancelling harmonics and to minimize ripples on DC output.
The input line current consists of 12-steps and contain 50Hz (or 60Hz in some countries) fundamental frequency and characteristic harmonics for a 12-pulse rectifier, i.e. 11th and 13th (600Hz±50Hz), 23rd and 25th (1200Hz±50Hz), 35th and 37th (1800Hz±50Hz) and so on. These harmonics are a function of the fundamental 50Hz current. The magnitudes of these harmonics are in turn dependent on the service level and power rating of each train in service (i.e. Pac=Pdc; Pdc=Vdc*Idc, Pac=Vac*Iac*power factor, with Idc being the total DC substation current drawn by all the trains in section).
Besides causing additional transformer core loss and copper loss, these harmonics (if excessively large) can mess up other electricity users’ systems, e.g. causing signaling failure in railway control system, lifts and escalators control failures, protection system failure for shopping centers etc. In these situations the installation of harmonic filters will be required.
For Commuter AC traction power systems, the commonly adopted nominal voltage is 25kV AC. The AC substations are normally fed from a High Voltage (HV) 6.6kV, 132kV, AC network, although for high speed lines 275kV and 400kV AC supplies are adopted in the UK. A commonly used 132kV/25kV single phase transformer takes supply from two phases of a 132kV AC network. One pole of the 25kV secondary side is tied to ground to create the 25kV single supply required to drive the trains.
A modern AC train employs a 4-quadrant PWM AC/DC front end converter and it produces a range of characteristic harmonics which are too complex to discuss here. These harmonics can potentially cause resonance on the traction systems and are reflected in the HV 132kV AC network including the power transformers on the power systems. When resonances occur on the AC traction power systems, harmonic filters are normally required to filter and damp the harmonics to ensure that the over-voltages are not excessively high on the traction systems and the HV supply systems.