Semi-Two-Stage Traction System based on the NPC Multisource Inverter for Tram Vehicles with Onboard Supercapacitors

This paper proposes an innovative semi-two-stage topology for the traction system of trams equipped with supercapacitors, with the aim to downsize the onboard dc/dc converters. The configuration is obtained through a non-conventional connection of a half-bridge chopper and 3L Neutral Point Clamped (NPC) inverter to the dc sources and ac traction motors. By means of a proper modulation strategy, the NPC converter works as a Multi-Source Inverter and can independently control the power absorbed or supplied by the catenary and supercapacitors. Due to limitations in the power-processing domain of the NPC circuit, a chopper must be also included in the traction circuit. However, it processes only a fraction of the total power of the supercapacitors. The proposed solution is compared against a traditional two-stage architecture, where a dc/dc converter processes the entire power of the capacitors and a 2L Voltage Source Inverter drives the traction motors. Extensive numerical analyses testify that the envisioned architecture shows encouraging performance and can yield a significant reduction of the dc/dc converter ratings, losses, volume, and weight.