Series-parallel HBSM and two-port FBSM based hybrid MMC with local capacitor voltage self-balancing capability

Recently, hybrid Modular Multilevel Converter (MMC) composed of Half-Bridge Sub-Module (HBSM) and Full-Bridge Sub-Module (FBSM) is becoming a preferable topology in overhead line based HVDC transmission. Assuming that two switches are paralleled in each HBSM and FBSM to double the current ratings, this paper proposes an enhanced hybrid MMC topology based on the improved series-parallel HBSMs and two-port FBSMs. The two types of Sub-Module (SM) segments are obtained from direct mirror flipping of all the traditional SMs, but with more degrees of freedom for control. The proposed topology maintains the dc-fault ride through capability and can minimize the requirements of capacitor voltage sensors and control hardware. Then the nearest level capacitor voltage control strategy suitable for the new topology is proposed and implemented by controlling the specific switches to parallel adjacent capacitors for voltage equalization, and the spike currents during the parallel procedure can be suppressed by the distributed arm inductors, this is defined as capacitor voltage self-balancing in this paper. The effectiveness of this work has been validated in terms of steady and transient states on a two-terminal hybrid MMC based HVDC system.