Analysis and reduction of capacitor ripple current in modular multilevel converter for variable speed drives

The rms value of capacitor ripple current is one of the important parameters for capacitor selection in Modular Multilevel Converter (MMC) based medium voltage adjustable speed drives. In this paper, an expression for the rms value of ripple current of each submodule capacitor in MMC is derived analytically. The analysis reveals that like capacitor ripple voltage, the ripple current also increases at lower operating speed. It is shown that this current can be reduced by reducing the dc-bus voltage with the operating speed. Furthermore, in this method, the capacitor ripple voltage is also reduced without the need of injecting any circulating current at lower operating speed. To verify the claims, the simulation study is carried out in Matlab/Simulink and the results are compared for different values of dc-bus voltage at various operating speeds.