Experimental Validation of a Quasi-Z-Source Modular Multilevel Converter With DC-Fault Blocking Capability

This article considers the design methodology and the modulation of the quasi-Z-source modular multilevel converter (qZS-MMC) with half-bridge submodules (HBSMs) and evaluates its performance in the voltage boosting mode for medium voltage applications. The qZS-MMC consists of two qZS networks inserted between the two terminals of the dc input source and the dc-link terminals of an MMC, which allows the generation of an output voltage larger than the input dc voltage. Two modulation schemes have been analyzed based on a mathematical derivation for the converter internal voltages, currents, and stored energy. The qZS circuit is proven to provide the qZS-MMC with HBSMs to deal with dc faults. The experimental results validate the performance of the proposed modulation schemes and the dc-fault blocking capability of the qZS MMC. Finally, the losses of the qZS-MMC are compared against a standard MMC using full-bridge submodules that can also provide dc-fault capability. The range in which the qZS-MMC is more efficient has been identified. Furthermore, the qZS-MMC can provide a significant reduction in a number of semiconductor power devices with the same performance.