Steady-state boundary assessment for modular multilevel variable-frequency drive systems using high-frequency injection method

Modular multilevel converters (MMC) have become an emerging research hotspot in the field of medium-voltage drives. However, excessive voltage ripple under low-speed and high-torque conditions represents the primary limitation of the full-speed range operation of the system. The high-frequency injection method is currently the most effective strategy for suppressing voltage ripple, but it increases the current stress in the bridge arm and carries the risk of overmodulation, introducing new constraints on the safe operation of the system. Therefore, assessing the steady-state operational boundaries of MMC variable-frequency drive systems using the high-frequency injection method can effectively reveal the application potential and parameter design of the system. This paper proposes a boundary assessment method with multiple constraints that considers the degree of freedom in high-frequency injection control. This method yields a frequency-maximum torque curve that satisfies constraints, and further analysis was conducted on the impact of the system parameters on the steady-state operating boundary. The accuracy of the proposed boundary assessment method was validated by measurements on a low-voltage MMC prototype that fed a field-oriented controlled permanent magnet machine.