Minimization of DC-Bus Current Ripple in Modular Multilevel Converter Under Unbalanced Conditions

In a modular multilevel converter (MMC), the zero-sequence currents at twice the fundamental frequency and its multiples flow through the dc bus and cause dc-bus power fluctuations during unbalanced conditions. These fluctuations appear in dc-bus current, considering that the dc-bus voltage is stiff. In this paper, a model-predictive control is proposed to minimize the ripple in dc-bus current. The proposed approach takes the advantage of internally generated zero-sequence voltage to mitigate the zero-sequence current during unbalanced conditions. To implement the proposed approach, a generalized mathematical model of the MMC, which includes the zero-sequence voltage, is presented. The performance of the proposed approach is verified through MATLAB/Simulink simulations on a 4.16-kV/5-MW system and dSPACE/DS1103-based experiments on a 208-V/3-kW laboratory prototype.