Active Damping Design of LLCL Grid-Connected Inverter Based on Biquad Filter

Because of the existence of LC series resonant branch, LLCL-type filters of grid-connected inverters are able to efficiently restrain the switching harmonics. However, the resonance issue of the system should be taken into account. Therefore, a biquad filter composed of resonator and notch filter (NF) is proposed to restrain resonance. The notch point and the resonance point of biquad filter are placed at the specified frequency in this method, and then the resonant peak is transferred to the stable region by phase transformation, so that the system does not cross -180° at the resonant peak. This method makes the system have high control bandwidth and strong robustness even in weak power grid. In this article, a proportional-integral multiresonant repetitive controller is applied to suppress low-frequency current harmonics, so as the steady-state and dynamic properties of the system are improved. Meanwhile, based on the active damping of biquad filter, the stability criterion of control system under weak power grid is given. Finally, the accuracy of the analysis and the validity of the method are demonstrated by simulations.