Parameter design and operating zone of hybrid cascaded multilevel converter under asymmetrical square-wave modulation method of the director switch

Hybrid cascaded multilevel converter (HCMC) is a newly developed modular multilevel converter (MMC) with DC fault isolation capability. It is consisted of wave-shaping circuit (WSC) and direct switch (DS). This study studies the parameter design and operating zone of HCMC employing asymmetrical square-wave modulation at the DS. Analytical method of determining the asymmetrical component is derived through the energy balancing between AC side and DC side. Method of determining the required additional full-bridge cells is then deduced. By analysing the energy fluctuation flowing through the WSC, method of dimensioning the cell capacitors is proposed. Size of cell capacitance and insulated gate bipolar transistor current ratings for HCMC and traditional half-bridge MMC are compared. Feasible operating zone of the HCMC is studied and the overall design process of HCMC is summarised. Semiconductor cost, cell capacitance and arm inductance of HCMC and other DC fault tolerant MMC topologies are compared. It is shown that HCMC is able to isolate DC fault current with significantly lower cell capacitance, lower number of power semiconductors and no arm inductance compared with other MMC topologies. Detailed simulations on PSCAD/EMTDC confirmed the theoretical studies.