Modular multilevel converters have become the preferred topology for industrial motor drives operating at medium and high power levels due to their modular design, scalability, and low harmonic distortion. However, a critical challenge emerges during low-speed motor operation: submodule capacitor voltage ripple increases significantly, threatening converter reliability and lifetime.
The root cause lies in conventional carrier phase-shifted pulse-width modulation control strategies. These methods use fixed carrier phase-shift angles that work well across the full operating frequency range but fail to account for the extended charging and discharging intervals that occur when motors run at reduced electrical frequencies. As frequency drops, capacitors experience longer charging cycles, leading to excessive voltage ripple on the DC-link capacitors.
This research introduces a frequency-responsive phase-shift control method that dynamically adjusts the carrier phase displacement based on the motor's instantaneous operating frequency. The strategy employs a piecewise function: larger phase-shift angles are deployed in the low-frequency region to better manage the extended charge transfer intervals, while the angle gradually returns to standard values as frequency increases toward nominal operation. Crucially, the control logic operates entirely within the PWM generation stage, requiring no modifications to the MMC power circuit hardware.
Simulation testing across 0.5–10 Hz motor operation demonstrates substantial improvements. Peak-to-peak capacitor voltage ripple decreases noticeably at low frequencies while maintaining acceptable AC-side current quality and internal circulating currents. Output current total harmonic distortion remains comparable to conventional methods, and upper-arm currents stay within acceptable limits.
This approach addresses a persistent limitation in MMC motor drive systems without additional hardware investment, making it particularly valuable for industrial applications requiring variable-speed operation, such as compressors, pumps, and conveyor systems. The technique enhances converter longevity and performance in low-frequency regimes where conventional control struggles.
