Resonant Damping Analysis of Output Filter of Grid-Connected Inverters

The design performance of the grid-connected inverter directly determines the quality of the grid-connected output current as an interface between the distributed power generation system and the power grid. Grid-connected inverters usually use output filters to attenuate high-frequency harmonics, and output filters' filtering performance directly determines the system's performance. Based on the analysis of the existing output filter topology and filtering performance, an LLCLC-type grid-connected inverter output filter is proposed. To suppress filter resonance spikes for stable system operation, Rs and Rs-Cs methods are used to damp the LCL-type, LLCL-type and LLCLC-type filters. The damping effect on resonant spikes and high-frequency harmonic attenuation is thoroughly analyzed and compared for different damping methods applied to different filters. The analysis shows that LCL-type and LLCL-type filters have the same characteristic that the better the damping effect, the worse its trapping effect on the switching frequency harmonics. However, the trapping effect of LLCLC-type filters on switching frequency harmonics is not weakened by damping, so its overall filtering effect will be significantly better than that of LCL-type and LLCL-type filters under the same device volume cost, which is an outstanding advantage of LLCLC-type filters. Both simulation and experimental results verify the correctness and effectiveness of the proposed LLCLC-type output filter structure and resonant spike damping technique.