AC Traction Front End Converter THD Analysis

Rectifier locomotives, which used AC power transmission and DC motors, were common, though DC commutators had problems both in starting and at low velocities. Todays advanced electric locomotives use brushless three-phase AC induction motors. These polyphase machines are powered from GTO or IGBT-based inverters. The cost of electronic devices in a modern locomotive can be up to 50% of the cost of the vehicle. Also they introduce harmonics due to use of diode rectifier for converting AC to DC, which further get injected inside the line of locos. To reduce these harmonics in a context of limited switching frequency, as it is the case in railway traction applications, due to the relatively high transferred power and the necessity to limit power losses, two are the main solutions used for increasing input catenary current harmonic quality (and power): parallel association of several converters or using multilevel NPC topologies. Here two AC traction front-end converter topologies are compared theoretically based on 3 different aspects: hardware requirements, modulation technique necessities and current harmonic quality. This research has been mainly focused in the third aspect, catenary current quality, which is considered as the most important. Consequently current quality has been analyzed in detail and validated in simulation and experimentally by means of various physical converter prototypes. The complete model is simulated in MATLAB/SIMULINK and the results are analyzed to see that the system works as required.