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.

中文翻译：

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用于三相 MLCI 容错操作的改进中性线移位方法

在三相多电平级联逆变器 (MLCI) 中，当绕过故障模块时，每相中的模块数量变得不平衡。在这种情况下，获得平衡的三相输出的一种方法是移动 MLCI 的中性点。与其他容错操作方法相比，逆变器的输出具有相对较低的谐波失真，因此中性点偏移（NS）方法引起了极大的关注。为了移动中性点，注入基频分量的偏移电压。如果注入的失调电压超出失调电压的可用范围，则会发生过调制。然后，逆变器的输出失真。在本文中，针对三相 MLCI 中的容错操作，分析了 NS 方法的过调制区域。此外，提出了一种改进的 NS 方法来补偿过调制区域。2×3 MLCI 中的模拟和实验结果都证明了所提出方法的性能。