The harmonic analysis and suppression methods for photovoltaic grid system based on improved LCL filter

For the resonance phenomenon in LCL filter interconnection interface of photovoltaic grid system, the mechanism of the interconnection node voltage resonance and the reason for resonance frequency changing are analyzed. In this paper, a control method based on virtual harmonic impedance is proposed. The virtual harmonic impedance is composed of virtual resistance and virtual inductance. It shows the characteristic of resistance and inductance in low frequency band and has good effect of harmonic suppression. It shows the resistance characteristics in high frequency band and suppresses the harmonic amplification. A improved LCL filter is designed based on virtual harmonic impedance control method. The improved method is LC resonance branch paralleled on damping branch and provides low resistance pathway for the harmonic current near carrier frequency. The filtering effect is strengthened and the method reduces power loss of the damping resistor. Finally the simulation experiments show the effectiveness of the proposed method.