Improved WAC Control Strategy for LCL Grid-Connected Inverters Considering Digital Control Delay

The traditional Weighted Average Current (WAC) control method loses its loop-order reduction characteristic due to control delay, resulting in significant instability risk for the system in weak grid conditions. This paper, based on the equivalent impedance model, analyzed that the main cause of system instability is the introduction of a positive feedback loop associated with grid impedance through the voltage feedforward channel, resulting in phase lag. To address this issue, the paper proposes the introduction of a Second-Order Generalized Integrator (SOGI) in the voltage feedforward channel to attenuate the impact of the positive feedback loop on the system, improve the phase margin of the equivalent output impedance in the medium to low frequency range, and enhance the system's adaptability to a wide range of grid impedance variations. Furthermore, in order to eliminate control errors in the indirect control of WAC and enhance the system's background harmonics suppression capability, the quasi-Proportional Resonant (QPR) controller is combined to improve the quality of grid-connected current. Finally, the effectiveness and feasibility of this improved WAC control strategy are verified through simulation in MATLAB/Simulink.