Active damping of LLCL-filter resonance based on LC-trap voltage and capacitor current feedback

The LLCL-filter is emerging into grid-connected converters due to a smaller size compared to the conventional the LCL-filter. Similar to LCL-filter, the high order resonance introduces potential instability to the overall system which should be properly damped either passively or actively. In this paper, different feedback coefficients like the proportional, derivative, integral, high pass and low pass feedback coefficients of the filter capacitor current and the LC-trap circuit voltage are investigated for damping the filter resonance. Active damping methods are analyzed by using the concept of the equivalent impedance with and without the delay effect of an LLCL-filter based grid-connected inverter. Their corresponding equivalent circuits for the purpose of resonance damping are given to identify whether the feedback coefficient should be negative or positive for the different state-feedback methods. Then the stability of the different methods and the design of the feedback coefficient are analyzed and compared using root loci. Finally, experimental results are presented to validate the theoretical analysis.