In ac–dc power conversion system, modular multilevel converter (MMC) has been a promising technology, especially for high-voltage application. In such kind of application, the common-mode voltage (CMV) generated by power electronic switching can cause damage to electrical components and increase the electromagnetic interference (EMI) to the system. Hence, this article proposes a modulation scheme to reduce the CMV of MMC applied in high-voltage system, such as high-voltage dc. Usually, MMC consisting of numerous submodules (SMs) adopts the nearest level modulation (NLM) with the low switching frequency, so the CMV of the NLM scheme is analyzed, which indicates that the peak value of CMV is one-third of SM voltage. If one more or one less SM is turned on compared with NLM, the CMV caused by modulation can be eliminated theoretically. Subsequently, a modulation scheme based on the nearest zero CMV vector (NZCMVV) to reduce CMV is proposed for MMC. The NZCMVV scheme utilizes the two adjacent levels as candidate levels, and the theoretical ZCMVVs are picked out. Finally, the ZCMVV closest to the reference vector is selected to approximately replace the reference. The proposed method is simple and easy to implement. Simulated and experimental results validate that the proposed scheme can reduce CMV and CM current compared with the traditional NLM method.

中文翻译：

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交直流电源转换系统中低开关频率MMC共模降压方案

在交流-直流电源转换系统中，模块化多电平转换器（MMC）一直是一种很有前途的技术，特别是对于高压应用。在此类应用中，电力电子开关产生的共模电压 (CMV) 会损坏电子元件并增加对系统的电磁干扰 (EMI)。因此，本文提出了一种调制方案，以降低应用于高压系统（如高压直流）的 MMC 的 CMV。通常，由众多子模块（SM）组成的MMC采用开关频率低的最近电平调制（NLM），因此分析了NLM方案的CMV，表明CMV的峰值是SM电压的三分之一。如果多转或少转SM与NLM相比，理论上可以消除调制引起的CMV。随后，针对MMC提出了一种基于最近零CMV向量（NZCMVV）的调制方案来降低CMV。NZCMVV 方案利用相邻的两个级别作为候选级别，并挑选出理论上的 ZCMVV。最后，选择最接近参考向量的 ZCMVV 来近似替换参考。所提出的方法简单且易于实现。仿真和实验结果表明，与传统的 NLM 方法相比，所提出的方案可以降低 CMV 和 CM 电流。