A new close-loop based capacitor voltage control method for modular multilevel converter with the switching frequency of 150 Hz

This paper presents a DC voltage control method for modular multilevel converter (MMC) with a fixed switching frequency of 150 Hz. The proposed control method is developed based on the power balancing between reception and consumption. In order to evenly distribute power among the modules per phase leg, a certain relationship between DC and fundamental components in AC voltage of each module is considered as a mathematic constriction when applying selective harmonic elimination (SHE) method to calculate pulse pattern. Moreover, the different power loss of each modules may also cause the DC capacitor voltage deviation in a long-term run although the power absorbed are equal, so a close-loop based method of slightly changing the pulse width is introduced for canceling the power mismatching. The validity of the proposal is verified by computer simulation in both inverter and rectifier modes.