Model-Predictive-Control-Based Circulating Current Reduction and Energy Balance for Modular Multilevel Converter Under Asymmetric Arm Impedance Conditions

There are inevitably unbalanced states in modular multilevel converter (MMC), including switching states imbalance and parameters asymmetry in practical engineering, which will lead to energy imbalance, power loss, device damage and reliability decrease. To solve these issues, an improved model predictive control (MPC) based energy balance method is proposed in this paper. In the proposed method, the capacitor energy is analyzed and as the control objective directly, which aviods complicated asymmetric model analysis for MMC. Firstly, the relationship between the circulating current and arm energy is analyzed in detail. Then, the unbalanced components of the capacitor voltage of sub-modules (SMs) is injected into circulating current reference to control the energy between and within phases in balance. The proposed method achieves energy balance without influencing circulating current and increasing computational complexity. The feasibility and validity of the proposed control strategy are demonstrated by simulation results using Matlab/Simulink.