Modular multilevel converter composite submodule topology and control

The DC-side short-circuit fault suppression of modular multilevel converter (MMC) applied to DC transmission has been a technical problem. According to the different location of cutting off fault current, this study first analyses three possible suppression schemes: AC/DC circuit breaker, bridge arm damper, and MMC submodule (SM) topologies and their respective features. Then, characteristics of SM topologies with DC fault current self-blocking capability are summarised, and an improved composite SM topology is designed based on the full-bridge SM and multilevel capacitor storage structures. The suppression characteristics of the proposed topology under different operation modes, capacitor voltage balancing control during blocking, and self-starting process are also studied. Finally, a simulation model is built in the PSCAD/EMTDC to verify the DC-side short-circuit fault suppression characteristics and control strategies.