Capacitor-less modular multilevel converter with sliding mode control for MV adjustable-speed motor drives

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.