NPC-based multi-source inverters for multimode DC rail traction systems

The need for decarbonisation has increased the interest in alternative propulsion systems for light rail vehicles, including fuel cells, batteries, and supercapacitors. These sources and storage devices can also be used in combination with an external supply to enable operation on both electrified and non-electrified tracks. In traditional architectures, the sources are connected via DC/DC converters to a common DC-link that feeds the motor drives. This paper addresses a new configuration where an NPC-based multi-source inverter is used for the integration of the DC sources to the traction motors in a single stage. Firstly, a thorough analysis of the converter’s operation is carried out with reference to its state-of-art control. The conditions for full control of the DC sources and traction motors over the entire vehicle speed profile are derived. Secondly, a novel modulation technique is proposed to complement the baseline control and enable the stationary charging of the energy storage from the electrified line through the converter. Thus, the multi-source inverter can be operated to match all the functionalities of a conventional multimode architecture, with the advantage of reduced number of power converters. Validations are carried out by means of simulations and extensive experiments on a laboratory test bench.