A comprehensive control system for multi-parallel grid-connected inverters with LCL filter in weak grid condition

Active damping methods are used for resonance damping in grid-connected inverters with LCL filter. In microgrids, parallel grid-connected inverters are coupled due to grid impedance introducing multiple resonances. In general, such coupling effect is not taken into account for modeling and controller design. For single grid-connected inverter, despite good performance, the system tends to become instable with parallel connection of other inverters. Moreover, the grid injected current can be distorted by the grid voltage harmonics. In traditional control system, grid voltage is used as a feedforward signal to achieve harmonic rejection capability by boosting the inverter output impedance. However, this method introduces negative phase angle which could lead to control system instability. In this paper, the control system design for multi-parallel grid-connected inverters using active damping is clarified. Inverters with different characteristics are also modeled in a weak grid as a multivariable system while coupling effect with a wide variation of grid impedance is taken into account. An improved grid voltage feedforward method is proposed to eliminate negative aspects of the traditional method. The simulation results in MATLAB/SIMULINK software demonstrate the effectiveness of the proposed control system.