Sub-module capacitor voltage balancing and DC fault ride-through are essential to ensure the safe and reliable operation of modular multilevel converters (MMC) in voltage source converter based high voltage direct current(VSC-HVDC). Hence, this paper proposes the improved sub-module(ISM) topology that can meet the demand of DC fault ride-through and capacitor voltage self-balancing at the same time. Under the normal operating state, ISM can get the output level of 2u_c, u_c and 0 by connecting the two capacitors in sub-module under series, parallel and bypass modes, respectively. Considering the characteristics of parallel-connected sub-module capacitors, capacitor voltage balancing can be realized only with the hardware without additional control schemes. Moreover, in the case of the system DC short-circuit fault, ISM can solve the problem with the counter electromotive force generated by capacitor voltage. The simulation results of MATLAB/Simulink verify the correctness and effectiveness of the proposed ISM topology

modular multilevel converter (MMC);sub-module topology;capacitor voltage self-balancing;DC fault ride-through