Impedance Modeling and Stability Analysis of the IMAF-PLL-Based NPC Three-Level LCL Grid-Connected Inverter

The phase-locked loop of grid-connected inverters affects their output impedance characteristics and stability. Based on the NPC, three-level LCL grid-connected inverter, this article first analyzes the frequency characteristics of the improved moving average filter phase-locked loop (IMAF-PLL). Then, analyzing the influence of the neutral point control on the inverter impedance modeling, the harmonic linearization impedance modeling method combined with the symmetrical component method is used to establish a positive and negative sequence impedance model of the NPC three-level LCL grid-connected inverter. Furthermore, the frequency characteristics of the IMAF-PLL and the impedance model of the NPC three-level LCL grid-connected inverter are verified by simulations. Finally, the impedance-based analysis method is used to study the stability of the grid-connected system under different grid impedance conditions when using different phase-locked loops, and the theoretical analysis was validated experimentally.