A novel circulating current suppressing method of modular multilevel converter

In this paper, a novel circulating current suppressing method of modular multilevel converter (MMC) is proposed. The circulating current is caused by the difference between the dc bus voltage and the sum voltage of upper arm and lower arm. When using actual value of sub-module capacitor voltage to generate switching function, the sum voltage of upper arm and lower arm is constantly equal to the dc bus. So the circulating current is zero. But the voltages of sub-modules in one phase are not able to balance. So circulating current must to be injected to balance the sub-modules voltages. But the amplitude of injected circulating current is very smaller. Simulation results on a MMC based on three-phase inverter show validity of the proposed method.