High-power modular multilevel converter optimal design for DC/DC converter applications

High power DC/DC converters and DC hubs are required to interconnect the high-voltage direct current (HVDC) systems with different voltage levels. Low frequency 50/60 Hz modular multilevel converters (MMCs) are known to provide better power quality, higher reliability, and lower switching losses compared to conventional two-level voltage source converters (VSCs). This study presents the optimal design of an MMC VSC for medium frequency applications suitable for DC/DC converters and DC hubs. The design aims at minimising the MMC total power loss, size, and weight as these are great importance for HVDC converters. The optimal values for the main MMC parameters including cell capacitance, arm inductance, number of cells, and operating frequency are determined using an in depth study carried out in Matlab and PSCAD. The effect of different voltage balancing methods on converter performance is also investigated. It is shown that a number of cells within the range of 20-56 with an operating frequency around 300 Hz/500 Hz for onshore/offshore applications give best trade-off between converter total loss and overall size and weight.