Experimental Validation of a quasi Z-Source Modular Multilevel Converter with DC Fault Blocking Capability

This paper considers the design methodology and the modulation of the quasi Z-source modular multilevel converter (qZS-MMC) with half bridge sub-modules and evaluates its performance in voltage boosting mode for medium voltage applications. The qZS-MMC consists of two quasi Z-source networks inserted between the two terminals of the DC input source and the DC-link terminals of a modular multilevel converter (MMC), which allows the generation of an output voltage larger than the input DC voltage. Two modulation schemes have been analysed based on a mathematical derivation for the converter internal voltages, currents, and stored energy. The quasi Z-source circuit is proven to provide the qZS-MMC with half bridge sub-modules 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 is compared against a standard MMC using full bridge sub-modules 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 number of semiconductor power devices with the same performance.