A Five-Level Six-Switch Hybrid FC-NPC-Based Unidirectional Power Converter Topology With Enhanced Reactive Power Capability

For unidirectional power flow applications at the medium-voltage (MV) level, multilevel converters with fewer active devices are cost-effective and more efficient than bidirectional ones. However, the existing unidirectional converters have limited reactive power support capability. This article proposes a novel three-phase five-level flying capacitor neutral-point clamped (FC-NPC) hybrid unidirectional power converter topology with only six active switches, two diodes, and two FC per phase which can give enhanced reactive power support. The average dc-link midpoint voltage is naturally balanced, which eliminates the need for a dc-link midpoint voltage controller. Moreover, unlike existing converters using similar number of devices, the proposed topology can operate over widely varying input supply frequencies. The proposed topology does not require any FC precharging circuitry either. In this article, the converter operating modes, switching-cycle average model, the modulation, and control strategies are presented. Loss calculation results for 5-MVA grid-connected rectifier application indicate that the proposed converter has the least loss compared with the existing five-level topologies having similar component count. Overall performance of the converter connected to the grid under maximum active and reactive power transfer conditions is experimentally verified on a 2-kW laboratory prototype.