This article presents the design and development of a 7-kV dc bus, 1-MVA, 0-1000 Hz modular multilevel converter (MMC) prototype for high speed medium-voltage variable frequency drives. The system is designed based on 1.7-kV silicon carbide (SiC) MOSFETs. First, the full in-depth design of the prototype is provided, including the submodules, arm inductors, and system integration. Then, the unique control scheme and the control hardware design are presented to achieve the full-frequency range operation. The power loss based on Si and SiC devices is analyzed. Finally, experimental results are presented to demonstrate the performance of both the SiC-based submodule and the full-scale MMC system. Compared with the traditional Si-based solution, the established SiC MMC can reduce the drive system volume by 75% and achieve 325% power density, and the peak energy efficiency reaches around 99.35%.

This article proposes a fixed switching frequency (FSF)-based improved model predictive control (MPC) for a five-level active neutral-point clamped (5L-ANPC) converter. Compared to the conventional MPC for 5L-ANPC converter, the proposed MPC significantly reduces the computational burden while ensuring the balance of dc-link and flying capacitor voltages. The proposed MPC optimally selects the control voltage vectors to reduce common mode voltage (CMV), leading to a small CMV of the converter. Using three voltage vectors action per control cycle in MPC, FSF is achieved. Steady-state and dynamic performances of the proposed MPC have been evaluated by experiments. The results verify the feasibility and effectiveness of the proposed MPC method.

An optimized model predictive control (MPC) method is proposed based on the selected switching states to control the four-level hybrid-clamped converter. The proposed method utilizes 64, instead of 512, switching states, compared with a conventional MPC, which greatly eases the calculation burden while achieving the high-quality regulation of output currents and the balance of capacitor voltages. The steady-state performance at both high and low frequencies is investigated, and results are compared with those by the traditional carrier phase-shifted pulsewidth modulation method. The dynamic response to the current or frequency step change is also evaluated. Both simulation and experimental results validate the feasibility and effectiveness of the optimized MPC method.

This work deals with the sizing of submodule capacitors for modular multilevel converters (MMCs) used for variable speed drives for medium voltage (MV) and high voltage (HV) electric machines. Variable speed drives are required to run over a wide frequency range from zero to a few hundred Hz or even over 1 kHz. Under low frequency operation (< 30 Hz), a large current ripple flowing through the MMC submodule capacitors will cause significant capacitor voltage fluctuation. This paper proposes an optimal submodule capacitor selection method based on the common-mode voltage injection. The proposed sizing model includes two functions: a) accurately estimating the capacitor voltage ripple based on the drive system parameters; and b) identifying the minimum capacitance of the submodule capacitors based on the allowed maximum capacitor voltage fluctuations. Simulation and experimental results show that the error between the actual and estimated values of the voltage fluctuation is within 3% using the proposed model.

When a 4-Level hybrid clamped converter operates at low frequencies, the high load current causes significant voltage fluctuations among the capacitors in the DC link. A mathematical model is developed to clarify the mechanism of excessive capacitor voltage fluctuations. An improved common mode voltage injection algorithm is proposed and integrated to the overall control method. With the algorithm applied, the voltage fluctuation minimization of the DC-link capacitors and the dynamic voltage balancing capability are investigated. Both simulation and experimental results show the effectiveness of the proposed algorithm, with a voltage fluctuation reduction over 50%.