Research on low-order current harmonics rejections for grid-connected LCL-filtered inverters

The current quality is an important indicator to evaluate the performance of a grid-connected inverter. In practice, many factors including the grid voltage distortion, dead-time effect, non-ideal switches and dc-link disturbances would result in many low-order harmonics in the injected grid current. The inverter with either the typical grid voltage feedforward or the harmonic resonant (HR) control had some difficulties in rejecting the impact of multi-harmonic sources. In this study, the combination of the grid voltage feedforward and the multi-HR control is proposed to suppress low-order harmonics. It is found that all those harmonic sources are classified into two kinds, and a general way is provided to analyse the harmonic rejection of the inverter with the typical or the proposed strategy. The low-order current harmonics of the inverter with different strategies are clearly explained. The comparative analysis, simulations and experiments all indicate that the proposed strategy greatly improves the ability of the inverter to reject the current harmonics induced by multi-harmonic sources as long as the grid feedforward and the resonant control are complementary.