Analysis and Suppression of the Frequency-Aliasing Phenomenon Related to the CPS-PWM Strategy in an MMC System

The sorting-algorithm-based voltage-balance strategy has been widely used in modular multilevel converters (MMC) due to its various advantages. However, because this strategy is operated in a digital-discrete system with limited operating speed, the frequency -aliasing phenomenon inevitably occurs, which mixes the harmonics in the number of inserted submodules, distorts arm voltages and alternating currents to reduce the power quality, and causes the phase delay to decrease the system stability. Therefore, this study first proposes the mechanism of frequency-aliasing phenomenon in the MMC system. A qualitative analysis of the relations between the frequency-aliasing phenomenon, the voltage-balance strategy and corresponding deteriorations is then provided, indicating the root cause of frequency-aliasing phenomenon and magnitudes of these deteriorations. Based on the analysis, an optimal design method is then proposed to determine the appropriate balance frequency; a code restructuration method is presented to rearrange the sorting algorithm and increase the balance frequency, thus effectively mitigating these deteriorations. The deteriorations caused by the frequency-aliasing phenomenon and effectiveness of proposed methods are finally demonstrated via simulations.