Hybrid cascaded multilevel converter for medium-voltage large-capacity applications

Multilevel converters are particularly suitable to be used in medium to high voltage applications, such as in a Railway Power Compensator (RPC), since they can give better quality outputs and reach higher voltage rates compared with two-level converters. In this paper, a novel 6-stage hybrid cascaded multilevel converter for medium-voltage large-capacity applications is proposed. It can reach the voltage rate of 30 kV with minimum step of 2 kV so that the RPCs using it can be connected directly to the traction system without coupling transformers. It is a 31-level converter, which could reduce the harmonic contents in the output waveforms. However, the number of cascaded stages of it is much less compared with the ones using traditional cascading methods. The corresponding control method for the proposed converter is also designed. Simulation and hardware testing results are also provided to show the validity of the proposed hybrid multilevel converter and its control strategy.