A Hybrid Low Capacitance Modular Multilevel Converter for Medium Voltage PMSM Drive and Its Control Method

Abstract

To address the problem of large sub-module capacitor voltage ripple at low-speed conditions of modular multilevel permanent magnet synchronous motor drive system. First, a hybrid low capacity MMC (HLC-MMC) topology based on horizontal sub-module interconnection is proposed. Based on the threephase symmetry of the input fluctuation currents of the submodules of the HLC-MMC, a fluctuation current coupling channel is established between the submodules with the same horizontal position of the three phases. The fluctuation currents are coupled and cancelled in the interconnection channel, and the amplitude of the fluctuation currents flowing into the sub-module capacitors is reduced. A switch tube is connected in series with the DC bus, and its on/off control can reduce the average DC voltage component in the bridge arm voltage, so that the voltage ripple of the capacitor is reduced. Then, a new control method is proposed for the HLC-MMC topology, where the energy is equalized between the upper and lower bridge arms of each stage by adjusting the fundamental frequency current components. The switching frequency of the series switches on the DC bus is controlled by duty cycle, and the measured average voltage of the control sub-module capacitors is equal to the reference value voltage. The overall energy of the system is balanced and the submodule capacitor voltage ripple is further reduced. Finally, the correctness and effectiveness of the topology and the proposed control method are verified by simulation.INDEX TERMS Capacitor voltage ripple suppression, low-speed motor operation, modular multilevel converter, medium voltage PMSM drive.

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“…Higher voltages can be handled by this type of converters [1], [2] since the power switches are exposed to a lower voltage as a result; multilayer converters for medium and high voltage power applications have engrossed a lot of attention recently. Figure 1 depicts various applications of multilevel inverter [3], [4]. Modular multilevel converters (MMCs) are contemporary voltage source converters that can be used in various medium and higher voltage operations.…”