Enhanced Voltage Injection Control for Capacitor Voltage Balancing of MMC Under Low-Frequency Operation

초록

In this study, a control approach is presented that is designed for the Modular Multilevel Converter (MMC) to drive low frequency AC devices, such as motors that run at low speeds. The main objective is to improve the converter's dynamic performance, which is crucial for stable operation of motor drives. Theoretical analysis suggests that energy balance in cell capacitors used in motor drive applications may lead to instability during startup and low-frequency operation. Furthermore, the capacitors' energy pulsation is inversely proportional to the output frequency, resulting in significant pulsation at low frequency. Therefore, effective MMC operation requires advanced energy balancing techniques to reduce voltage fluctuations in individual cell capacitors. This paper introduces an enhanced strategy that injects leg offset voltage for low-frequency MMC operation, which is more efficient than existing methods. The proposed approach is evaluated through simulation with a low-frequency MMC output, and the results demonstrate its effectiveness by comparing and analyzing the size of the capacitor voltage ripple before and after the control is applied. © 2023 The Korean Institute of Power Electronics.

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