Analysis and Suppression of the Frequency-Decrease Effect in the Capacitor Voltage Related to the Low Modulation Frequency Ratio in an MMC System

Abstract

The carrier phase-shift pulsewidth modulation (PWM) strategy is widely used in modular multilevel converters (MMCs), where the modulation frequency ratio of switching and fundamental frequency is a major parameter. However, a low modulation frequency ratio, which is often implemented to decrease the power loss in a high-power MMC system, can greatly decrease the harmonic frequencies of the submodules' capacitor voltages. Such effect is defined as the frequency-decrease effect in the capacitor voltage, which can cause several severe deteriorations, such as the low-frequency fluctuations, the unstable charge-discharge state of submodules' capacitors, and unbalance phenomenon of the capacitor voltages, thus significantly deteriorating the system working performance. This study first proposes the mechanism of the frequency-decrease effect. A quantitative analysis of the relations between the frequency-decrease effect, the modulation frequency ratio, and corresponding deteriorations is then provided, indicating the root cause of the frequency-decrease effect. Based on the analysis, an appropriate enhanced balance strategy is introduced into the system. An optimal design method is used to determine an appropriate modulation frequency ratio and to mitigate the frequency-decrease effect; and an improved balancing method, which combines the direct-bias injection and condition selection to determine the modulation wave, is used to further improve the system performance. The frequency-decrease effect, the corresponding deteriorations, and the efficiency of the enhanced balance strategy to suppress the frequency-decrease effect are finally demonstrated via simulations and experiments.