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The modular multilevel converter (MMC) based on half-bridge modules is a power converter topology suitable for high-power medium-voltage variable-speed drives. However, the voltage of its flying capacitors is negatively affected when low frequencies appear at the AC port. This paper analyzes the influence of using a variable DC port voltage in a machine-side MMC by implementing a closed-loop approach, ensuring a constant voltage fluctuation in the capacitors of the MMC during the whole operating range. The effectiveness of the proposed control scheme is demonstrated through simulation studies and experimental validation tests conducted using a 7.5 kW experimental prototype composed of an induction machine fed by an MMC with 18 half-bridge cells.

The modular multilevel converter (MMC) has emerged as a suitable topology for high power drive applications. However, the voltage fluctuations of its floating capacitors increase the control complexity of the converter. In this study, the MMC dc-port voltage is manipulated to regulate the amplitude of these fluctuations to a constant value during the whole frequency range. The proposed approach has several advantages when compared with the conventional ones since it minimises the voltage fluctuation in the capacitor cells. Additionally, it decreases the common-mode voltage at low frequencies and the capacitor rms current, increasing their expected lifespan and reducing the winding insulation damages and the leakage currents in the bearing of the machine. The effectiveness of the proposed control strategy is validated with a laboratory-based prototype composed of 18 power cells, feeding a vector-controlled induction machine.

Modern variable-speed drives must be able to deal with ride-through faults with dips of 70–90% to ensure the continuity of the process without the energy store station discharge and carry out significant effects to its performance. So, the existent contingencies to such case are tripping-method from grid, robust DC-Link with higher capacitors and additional power electronic hardware. Nevertheless, they comprehend a common drawback for being costly in the industry. Therefore, the Power Loss Ride Through behaviour on variable-speed drives is an important feature which is a worst-case where the system can suffer dips near to 99% on grid. Recently, Modular Multilevel Cascade Converters have been applied in Variable-Speed Drives applications due to their capability to operate at high-power rates and features such as enhanced fault-ride through capability during critical dip voltage (e.g., 99% of amplitude) in grid. Therefore, this paper presents a control strategy based on a transition mechanism between outer control loops on the nested control of a Modular Multilevel Matrix Converter drive with the aim to improve the Power Loss Ride-Through performance through keeping the energy store station uniform and stable. Simulation results obtained using PLECS software are presented to validate the effectiveness of the proposed control strategy.

The Modular Multilevel Converter (MMC) is a topology suitable for high-power medium-voltage variable-speed drives. However, the behaviour of this converter is highly affected by the power fluctuations of its modules. This paper analyses the influence of the dc-port voltage on these power fluctuations. It is demonstrated that the manipulation of this voltage offers several advantages for the operation of the drive. For example, it is possible to reduce: the voltage fluctuations in the capacitors, the capacitance of each cell or the application of mitigating variables during low machine speeds. Additionally, a reduction in the required common-mode voltage during low-speed operation is also feasible. The effectiveness of the proposed control schemes is demonstrated through simulation studies.