Analysis and Improvement of Harmonic Quasi Resonant Control for LCL-filtered Grid-Connected Inverters in Weak Grid

Harmonic Quasi Resonant (HQR) control is widely adopted to suppress the harmonic currents for LCL-filtered inverters in stiff grid. However, inverters with HQR control may observe instability when the point of the common coupling (PCC) behaves uncertain impedance or the HQR control frequency is out of the bandwidth of the system control loop. Besides, it could not effectively eliminate the harmonics which is beyond the control frequency. Therefore, to improve the stability performance and enhance the harmonic suppression for inverters, this paper first discusses the merits and limitations of the conventional HQR control based on the impedance model and then introduces a phase compensation (PC) method for conventional HQR control. The proposed method can improve the stability and suppress a wide frequency range of harmonics for inverter when the weak grid connected. The correctness of the mathematical model for the inverter output impedance and the adverse effect of the conventional HQR control due to the existence of the grid impedance is verified by simulations and experimental results respectively. Applicability of inverters with the proposed method to the wide grid impedance variation is also validated on a single-phase PV inverter.