Zero DC voltage ride through of a hybrid modular multilevel converter — Part I: Principle and control implementation

In the second part of the paper, transient response of the hybrid Modular Multilevel Converter (MMC) under the zero dc voltage fault conditions is analysed and a suppression scheme is presented to reduce the transient voltage stress of sub-modules. An equivalent circuit of the hybrid MMC under the fault conditions is explored, and further quantitative analysis on the arm current and transient voltage stress of submodules are deduced. Based on the above analysis, a suppression strategy for reducing the transient voltage stress is proposed. It is found that transient voltage stress is relative to the arm fault current which is composed of ac feed-in current and capacitor discharging current in sub-modules, and capacitor discharging current´s influence is critical. The capacitor discharging current in sub-modules is associated with arm dc output voltage generated by the injected SMs. Thus, a corresponding scheme to regulate capacitor discharging current by common-mode voltage injection is proposed to reduce the transient voltage stress of sub-modules. Simulation results demonstrate the feasibility and validity of the analysis and proposed suppression scheme.