An Optimized Fault-Tolerant Strategy Based on Uncoupled Zero-Sequence Voltage Injection for Cascaded Multilevel Converters

For cascaded multilevel converters, the extensive use of power cells increases the possibility of failure and reduces system stability. When some cells fail and are bypassed, the neutral point drifting strategy is often applied to solve the problem of unbalanced output line voltage. However, this traditional strategy focuses on the feasibility of constructing an equilateral triangle of output line voltage, which is deeply affected by the phase with the most faults and causes the waste of healthy cells in other phases. In this paper, an optimized fault-tolerant strategy is proposed to improve the output line voltage. By decoupling the faults of each phase and injecting zero-sequence voltages pertinently, it can reduce the mutual influence between the phases, thereby making more use of the healthy cells and increasing the DC bus voltage utilization under multiple fault conditions. The correctness and superiority of the proposed strategy have been verified by the simulation results.