Medium-voltage (MV) motor drives have become an appealing application for modular multilevel converters (MMCs). Starting and operation at low speeds result in wide fluctuations of the low-frequency ripple components in the sub-module (SM) capacitors DC link voltages, which can adversely affect system performance and system lifetime. A solution for this problem is to replace the low-frequency (LF) SM capacitor with a power decoupling circuit (PDC) that is independent from the converter line frequency. In this paper, a power decoupling approach based on the flux cancelation method is proposed. A three-winding high-frequency transformer (HFT) is employed to magnetically couple and cancel the three-phase symmetrical ripple power. However, this approach has two main challenges. (1) The ripple powers through the HFT are a function of the value of the leakage inductances. (2) Different leakage inductances and ripple power unbalance between phases cause unequal ripple voltages. As a result, phase-shift ripple rejection control is needed. Conventional liner controllers have several problems, such as bandwidth limitations, stability margins, and slow dynamics near-zero-speed operation. In addition, linear controllers are designed for a specific ripple frequency. In this paper, a frequency-independent ripple rejection sliding mode controller (SMC) is proposed to overcome the limitations of linear controllers. The SMC is applied to pass the SM capacitor voltage ripple into the HFT. Thus, the ripple is canceled out in the HFT magnetic core regardless of the converter line frequency. The proposed control is suitable for adjustable-speed applications. The performance of the proposed scheme is verified via simulation and experimental tests.

中压 (MV) 电机驱动器已成为模块化多电平转换器 (MMC) 的一个有吸引力的应用。低速启动和运行会导致子模块 (SM) 电容器直流链路电压中的低频纹波分量大幅波动，这会对系统性能和系统寿命产生不利影响。此问题的解决方案是用独立于转换器线路频率的电源去耦电路 (PDC) 替换低频 (LF) SM 电容器。在本文中，提出了一种基于磁通抵消方法的功率去耦方法。三绕组高频变压器（HFT）用于磁耦合和消除三相对称纹波功率。然而，这种方法有两个主要挑战。(1) 通过 HFT 的纹波功率是漏感值的函数。(2) 不同的漏感和相间纹波功率不平衡导致纹波电压不等。因此，需要相移纹波抑制控制。传统的班轮控制器有几个问题，例如带宽限制、稳定性裕度和接近零速运行的缓慢动态。此外，线性控制器专为特定的纹波频率而设计。在本文中，提出了一种与频率无关的纹波抑制滑模控制器 (SMC)，以克服线性控制器的局限性。SMC 用于将 SM 电容器电压纹波传递到 HFT。因此，无论转换器线路频率如何，HFT 磁芯中的纹波都会被抵消。所提出的控制适用于可调节速度的应用。通过仿真和实验测试验证了所提出方案的性能。