A Three-Layer Sequential Model Predictive Current Control for NNPC Four-Level Inverters with Low Common-Mode Voltage

The four-level nested neutral point clamped (4L-NNPC) inverter has recently become a promising solution for renewable energy generation, e.g., wind and photovoltaic power. The NNPC inverter can stabilize the flying capacitor (FC) voltages of each bridge through redundant switch states (RSSs). This paper presents an improved three-layer sequential model predictive control (3LS-MPC) method for 4L-NNPCs. This method eliminates weighting factors and removes the switch states that generate high common-mode voltage (CMV). Before selecting the optimal vector, we disable certain switch states which affect the FC voltages, continuing to deviate from the desired value. Then, adopting a two-stage optimal vector selection method, we select the optimal sector based on six specific vectors and choose the optimal vector from the seven vectors in the optimal sector. The feasibility of this method was verified in Matlab/Simulink and the prototype. The experimental results show that compared with classical FCS-MPC, the proposed 3LS-MPC method reduces the common-mode voltage and has better harmonic quality and more stable FCs voltages.