The Frequency-Decrease Effect Under Harmonic Compensation of Capacitor Voltage Related to the Low Frequency Ratio in MMC-APF

The carrier phase-shift modulation strategy (CPS-PWM) with a low frequency ratio (the ratio of carrier frequency to fundamental frequency) can effectively reduce the switching loss of MMC-APF. However, the harmonic compensation in MMC-APF injects harmonics into the modulation wave, introducing new features in the analysis of the frequency-decrease effect under harmonic compensation. Taking 5th and 7th harmonic compensation as an example, this study quantitatively analyzes the frequency-decrease effect of MMC-APF with harmonic compensation, clarifying the particularly severe deteriorations - instability in the charging and discharging states of submodule capacitor and the voltage imbalance among submodules when the switching frequency equals the frequency of compensation current, and Low-frequency harmonics with a wider range of frequency variation impacting the effect of harmonic compensation. Based on analysis, an optimal design method is proposed to determine an appropriate frequency ratio to suppress the frequency-decrease effect in MMC-APF. Finally, the simulation verify the existence of the frequency-decrease effect in capacitor voltage when MMC-APF compensates harmonics, as well as the effectiveness of the optimal method.