Output impedance of grid-connected converter with active damping and feed-forward schemes

LCL-filter is commonly used as an output-side filter in a grid-connected converter due to its good attenuation capability at the switching harmonic frequencies. LCL-filter creates several resonances in the converter dynamics which shall be damped for ensuring robust performance of the converter. The resonant behavior introduced by the LCL-filter can be attenuated with active damping (AD) instead of passive damping to improve system efficiency. Moreover, the active damping affects especially the output impedance of the converter and the output impedance should be accurately predicted in order to perform impedance-based stability analysis. This paper presents the smallsignal model for inverter-current-feedback (ICF) converter as well as output impedance properties using only the inverter-side inductor current feedback for both control and active damping. Furthermore, traditional feed-forward (FF) from the grid-voltages combined with the active damping scheme is analyzed since their simultaneous effect of is not yet discussed in the literature. Experimental measurements with 3-kW prototype are presented to verify the analysis.