Capacitor voltage balancing based on fundamental frequency sorting algorithm for modular multilevel converter

In recent years, Modular Multilevel Converter (MMC) is an emerging and highly attractive multilevel converter topology for High Voltage Direct Current (HVDC) transmission and Flexible AC Transmission System (FACTS). The technology on how to balance the floating DC capacitors plays a key role in the new converters. A Fundamental Frequency Sorting Algorithm (FFSA) based on Carrier Phase Shift PWM (CPS-PWM) is proposed to balance the floating DC capacitors. The main idea is to change the corresponding relationship between the CPS-PWM carriers and the submodules according to the capacitor voltage increments during the previous fundamental period. It utilizes no current detection, avoids the excessive frequent sorting and saves calculating resources for the controller so that more submodules can be dealt with. Moreover, the proposed FFSA guarantees that all the switching frequencies of the submodules are strictly identical, which equals to the PWM carrier frequency. It is beneficial for the thermal design of the sub-modules and the lifetime of the power switches. Finally, simulation and experimental results verify the validity of the proposed balancing method.