Modular Directed Series Multilevel Converter for HVDC Applications

Modular multilevel converter (MMC) and its hybrid topologies are highly preferred for high voltage direct current (HVDC) applications. A series MMC (SMMC) contains three single phase MMCs connected in series on the dc side which results in reduction in device count compared to MMC for a given dc link voltage. SMMC is highly suitable for low power HVDC applications such as transmission of remote renewable energy. A modular directed series multilevel converter (MDSMC) is proposed in this paper, which offers more compact structure and higher efficiency. A detailed analysis and operating principal of the proposed topology is presented. The converter sub-module capacitor size determination is also presented and compared with the existing topologies. The proposed topology requires smallest capacitor size. The working of MDSMC is verified by developing various component level models in PSCAD/EMTDC. The ability of the converter to provide wide range of modulation index (MI) variation is also verified. The results obtained from HVDC system developed proves the ability and suitability of the converter for HVDC application. Semiconductor losses in the converter are evaluated and are found to be lower than the exiting topologies.