Stability Analysis and Active Damping for <i>LLCL</i>-Filter-Based Grid-Connected Inverters

A higher-order passive power filter (LLCL filter) for a grid-connected inverter is becoming attractive for the industrial applications owing to the possibility of reducing the cost of copper and the magnetic material. To avoid the well-known resonance problems of the LLCL filter, it is necessary to use either passive or active damping methods. In this study, the stability of the LLCL-filter-based grid-connected inverter is analyzed and a critical resonant frequency for the LLCL filter is identified when sampling and transport delays are considered. In the high-resonant-frequency region, active damping is not required; however, active damping is necessary in the low-resonant-frequency region. The basic LLCL resonance damping properties of different feedback states based on a notch filter concept are also studied. Then, an active damping method using capacitor current feedback for the LLCL filter is introduced. On the basis of this active damping method, a design procedure for the controller is given. Last, both simulation and experimental results are provided to validate the theoretical analysis of this study.