An Improved Circulating Current Injection Method for Modular Multilevel Converters in PMSM Drives

Modular multilevel converter (MMC) is an emerging and potential technology in medium-voltage motor drive applications. One of the main challenges of applying this topology in motor drives is the ripple and imbalance of submodule (SM) capacitor voltages, especially at low speeds with constant load torque. A promising method to solve this problem is high-frequency circulating current injection. The existing injection methods only focus on the low-frequency range and can not ensure the SM capacitor voltage balance in the high-frequency range. In this paper, the capacitor voltage balance of MMC in PMSM drives is analyzed, and an improved circulating current injection technique is proposed, which guarantees the balance of capacitor voltages in full-frequency range. The proposed technique is successfully verified by simulation analysis.