A fault-tolerant approach for hybrid modular multilevel converter using negative voltage levels

This paper focuses on the operation of hybrid modular multilevel converter and proposes an effective approach that guarantees the full performance of the converter even in post-fault condition. The main novelty of this paper is utilizing negative arm voltage levels in a way to compensate the missed voltage. After the fault occurrence, the DC component of the arm voltages as well as the DC-link voltage are adjusted based on the fault states to restore the nominal balanced line-to-line voltage in the post-fault condition. In comparison with the previous methods, the proposed method does not add any hardware to the circuit to restore the nominal output voltage. The capacitor voltage of submodules, also, does not increase after fault occurrence. Therefore, the converter can maintain its nominal line-to-line voltage in post-fault condition without extra implementation costs or overdesign requirements. To validate the effectiveness of the proposed method, detailed simulation and experimental results are provided.