Evaluation of the hybrid four-level converter employing half-bridge modules for two different modulation schemes

A novel hybrid three-phase multilevel converter is proposed for medium-voltage applications. The converter employs a conventional three-phase voltage source inverter (VSI) linking series connected half-bridge modules at each phase. With the proposed connection, a large portion of energy can be processed by the VSI without the need for insulation or by employing a single transformer, while smaller power shares are processed within the half-bridge modules. Thus, the requirements for galvanically insulated dc sources are reduced. Modularity is naturally achieved. A modulation scheme for a four-level version is proposed and analyzed in detail. This scheme allows unidirectional power flow in all dc sources and, consequently enables diode bridges to be employed in the rectification input stage for unidirectional applications.