Modular multilevel converter (MMC) is a preferred topology for higher power applications. The instantaneous voltage mismatch between the phase legs and dc bus in MMC leads to predominantly second-harmonic circulating current (CC). The sizes of submodule (SM) capacitor and arm inductor, power losses and current rating of semiconductor switches in MMC are influenced by the CC. This article proposes a circulating current optimization control (CCOC) scheme for MMC, which reduces the arm current peak value without increasing the SMs capacitors size for the same maximum voltage ripples. It enables the MMC to handle more power with the same rated switches. This controller works on the principle of controlling the amplitude and phase angle of CC to follow their respective optimized references. A detailed design and implementation method of CCOC is presented for all possible operating conditions of MMC in terms of modulation indices and load power factors. A comparative evaluation performed between CCOC and other major CC control schemes highlights its main features. The efficacy of the proposed scheme is validated using simulations and also using experimental studies on a seven-level three-phase grid-connected MMC prototype. The results demonstrate the effectiveness of the CCOC scheme and verify the proposed principles.

中文翻译：

---

MMC循环电流优化控制

模块化多电平转换器 (MMC) 是更高功率应用的首选拓扑。MMC 中相臂和直流母线之间的瞬时电压失配主要导致二次谐波循环电流 (CC)。子模块 (SM) 电容器和臂电感器的尺寸、MMC 中半导体开关的功率损耗和额定电流受 CC 影响。本文提出了一种针对MMC的循环电流优化控制（CCOC）方案，在相同最大电压纹波的情况下，在不增加SMs电容器尺寸的情况下，降低了臂电流峰值。它使 MMC 能够使用相同额定开关处理更多功率。该控制器的工作原理是控制 CC 的幅度和相位角以遵循它们各自的优化参考。针对MMC所有可能的工作条件，在调制指数和负载功率因数方面，提出了CCOC的详细设计和实现方法。CCOC 与其他主要 CC 控制方案之间进行的比较评估突出了其主要特点。所提出方案的有效性通过仿真以及对七级三相并网 MMC 原型的实验研究进行验证。结果证明了 CCOC 方案的有效性并验证了所提出的原则。所提出方案的有效性通过仿真以及对七级三相并网 MMC 原型的实验研究进行验证。结果证明了 CCOC 方案的有效性并验证了所提出的原则。所提出方案的有效性通过仿真以及对七级三相并网 MMC 原型的实验研究进行验证。结果证明了 CCOC 方案的有效性并验证了所提出的原则。