Modular Multilevel Converter Switching Frequency Harmonics Analysis and Suppression Through Cell Voltage Control

This paper investigates the impact of reference's bias on the high frequency voltage spectrum for the modular multilevel converters (MMC). Analytical solution of the voltage spectrum under carrier Phase Shifted Pulse Width Modulation (PSPWM) is derived using double Fourier integral analysis. The distribution of the switching frequency voltage harmonics is found to be directly related to the ratio between the dc-link voltage and the cell voltage. A control method utilizing this phenomenon is thereby proposed to suppress switching frequency harmonics in the Differential Mode Voltage (DMV) and Common Mode Voltage (CMV). Cell voltage control trade-offs considering different cell numbers, different ac voltages, 3 ^rd harmonics injection, and cell voltage stress are also investigated. The analysis and control method are validated through experiments on a 1.25 megawatt MMC testbed.