MMC Capacitor Voltage Balancing Strategy Based on Carrier Rotation

Modular multilevel converter (MMC) is one of the most potential converter structures for medium/high power/voltage applications. Since the electrical energy is stored in the capacitances of its sub-modules during the operation of the MMC, the output power quality is directly related to the capacitor voltage. Therefore, capacitor voltage balance is of great importance. In this paper, a capacitor voltage balancing strategy for the carrier phase-shifted modulation strategy is presented for MMC by rotating the carriers, which not only solves the problem of capacitor voltage balance, but also avoids complex closed-loop modulation and voltage sequencing. Considering the potential additional switching process during carrier rotation, the algorithm is further improved to find the best rotation period. The experiment verifies the propositions.