Passivity-Based Design of External Passive Damper for LCL-Type Grid-Connected Inverter

To enhance the interactive stability between an
LCL
-type grid-connected inverter (GCI) and the grid, a passive damper (PD) is necessary for passivizing the output admittance of the GCI system since an active damper (AD) may not work in scenarios such as beyond the Nyquist frequency. This article explores the best installation locations of PD where higher efficiency can be obtained. To this end, a general admittance model of
LCL
-type GCI with an internal PD (IPD) or external PD (EPD) for different scenarios is derived. By leveraging the derived model, the passivity compensation burden is compared for IPD and EPD by analyzing the amplification and reduction characteristics of output admittance seen from the point of common coupling (PCC). Then, the EPD additivity conditions are figured out for different scenarios to guide the installation of the PD in the GCI system, which can fulfill full-frequency passive output admittance at the expense of relatively lower damping losses. Moreover, a design method for the EPD is also detailed in the article. Finally, GCI prototypes with high or low switching frequencies are tested in the laboratory and compared with state-of-the-art PD methods. The experimental results verify the effectiveness and superiority of the proposed EPD method.