Dynamic Carrier Phase-Shift Control for Low-Frequency Capacitor Voltage Ripple Suppression in MMCs Under Motor Drive Electrical Operating Conditions

Modular multilevel converters (MMCs) are attractive for medium- and high-power motor drive applications; however, low-speed motor operation can cause significant submodule (SM) capacitor voltage ripple. In conventional carrier phase-shifted pulse-width modulation (CPS-PWM), the carrier phase displacement is usually fixed. This fixed setting does not account for the longer capacitor charging and discharging intervals that occur as a motor’s electrical frequency decreases. To address this issue, this paper develops a dynamic carrier phase-shift control strategy for MMCs under low-frequency motor drive electrical operating conditions. The proposed strategy relates the carrier phase-shift angle to the motor operating frequency through a piecewise function. A larger phase-shift angle is adopted in the low-frequency region, whereas the angle gradually returns to the conventional CPS-PWM value as the frequency increases. The strategy is implemented in the PWM generation stage through an equivalent carrier delay update, without modifying the MMC power circuit. Comparative MATLAB/Simulink simulations over the frequency range of 0.5–10 Hz are carried out to evaluate the capacitor voltage ripple, output current total harmonic distortion (THD), upper-arm current, and circulating current. The results indicate that the proposed scheme can effectively reduce the peak-to-peak capacitor voltage ripple at low frequencies while maintaining comparable AC-side output current quality and acceptable internal current behavior within the investigated converter-side electrical model.