Abstract
In a three-phase multilevel cascaded inverter (MLCI), when a failed module is bypassed, the number of modules in each phase becomes unbalanced. Under this condition, a method to obtain a balanced three-phase output is to move the neutral point of the MLCI. The neutral-point shift (NS) method has attracted a great deal of attention since the output of the inverter has a relatively low harmonic distortion when compared to other fault-tolerant operation methods. To move the neutral point, an offset voltage of the fundamental frequency component is injected. If the injected offset voltage exceeds the available range of the offset voltage, overmodulation occurs. Then, the output of the inverter is distorted. In this paper, the overmodulation regions of the NS method are analyzed for the fault-tolerant operation in the three-phase MLCI. In addition, a modified NS method is proposed to compensate for the overmodulation region. Both simulation and experimental results in a two-by-three MLCI demonstrate the performance of the proposed method.
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