The sub-modules of modular multilevel converters(MMCs)have different operation time and switching frequency when the carrier level-shifted pulse width modulation(CLS-PWM)is adopted. As a result

the capacitor voltages of the sub-modules are seriously unbalanced and the loss and heating of the power switches is uneven as well

moreover

the system is not able to work normally with the CLS-PWM scheme. The principle of the traditional CLS-PWM is investigated and the inherent problems are identified

based on which

an improved CLS-PWM strategy is proposed for MMC based on the carrier rotation. Simulation results show the sub-module capacitor voltages can be balanced well even without particular control of each sub-module capacitor voltage by using the proposed method and thus the overall control algorithm of the system is simplified.