Virtual Model Predictive Control for Inverters to Achieve Flexible Output Impedance Shaping in Harmonic Interactions with Weak Grids

The impedance-based method is an effective approach to investigate harmonic interaction problems in weak grids. Modeling and shaping of the inverter output impedance are two important aspects of this method. But for the inverter under finite-control-set model predictive control (FCS-MPC), the output impedance model is difficult to develop due to the nonlinear dynamic characteristics. In order to explore the FCS-MPC-based solution to harmonic interaction problems in weak grids, this paper firstly derives the mathematical output impedance model of the inverter under FCS-MPC based on the describing function method. The developed inverter output impedance model is highly robust to changes in operating conditions and parameters, and the proposed modeling method provides an effective way to describe the impedance characteristics of inverters under FCS-MPC. Then based on the virtual model predictive control, a flexible output impedance shaping method for the inverter is proposed. The ability to improve system stability and mitigate harmonics is evaluated through the impedance-based analysis, and a design method for the control parameters is presented. Finally, the theoretical model of inverter output impedance and the impedance shaping method developed in this paper are verified through simulations and experiments.