Fault tolerance and overmodulation algorithm for three level neutral point clamped inverters

It has been a hot issue and also a difficulty to study the fault tolerance of multilevel converters. The topology changes to eight-switch three-phase inverter (ESTPI) if one leg of three-level NPC inverter is failed. An optimal modulation algorithm of ESTPI is proposed in this paper. Due to the loss of one leg, the DC-link voltage utilization of ESTPI is reduced by half. Hence, an overmodulation algorithm based on superposition principle is proposed to improve DC-link voltage utilization, without increasing DC-link voltage. The proposed method is easier to implement compared with conventional overmodulation strategies for less computation. Theoretical proof and experimental results show that the proposed overmodulation algorithm is effective, and the DC-link voltage utilization can be increased maximally by 10.3%.