Grid Harmonics Suppression Scheme for LCL-Type Grid-Connected Inverters Based on Output Admittance Revision

In this paper, the influence of grid harmonics on the output current of grid-connected inverters with an LCL filter is investigated by means of the output admittance. With the complex transfer model of the output admittance, the full-feedforward scheme of grid voltage is derived, which, however, is difficult to be implemented due to the derivative terms in the feedforward link. A detailed theoretical analysis is also presented to explain the compensation error introduced by the active damping and delays when the grid voltage proportional feedforward is adopted. Then a feedforward scheme based on the band-pass filter (BPF) is proposed to compensate the grid harmonics at the selected frequencies, and the parameters of the BPF are derived to revise the output admittance. It has also been found that with the commonly used phase-locked loop (PLL), an additional admittance matrix is introduced. The compensation effect will be degraded when the PLL with a high bandwidth is used for tracking grid phase accurately. Therefore, a modified PLL is proposed to revise the output admittance again, for suppressing the output current distortion arising from the grid harmonics, which propagate to control system through the PLL. Finally, the experimental results verify the effectiveness of the proposed scheme, where the current harmonics are effectively suppressed.