Comparison of H-NPC and parallel-H topologies for AC traction front-end converters

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. In this paper these two AC traction front-end converter topologies are compared theoretically based on 3 different aspects: hardware requirements, modulation technique necessities and current harmonic quality. The study 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.