Design of LLCL-filter for grid-connected converter to improve stability and robustness

The LLCL-filter has recently emerged into grid-connected converters due to the improved filtering capability which ensuring a smaller physical size. An LLCL-based grid-connected converter has almost the same frequency-response characteristic as that with the traditional LCL-filter within half of the switching frequency range. The resonance frequencies of the LLCL-filters based grid-connected converters are sensitive to the grid impedance as well as cable capacitance, which may influence the stability of the overall system. This paper proposes a new parameter design method for LLCL-filter from the point of stability and robustness of the overall system, when the grid-side current control is used. Based on this design method, the system can be stable without damping and also robust to the grid parameters variation. The influence of delay and parameter variations is analyzed in details. Last, a design example for LLCL-filter is given. Both simulations and experimental results are provided through a 5 kW, 380V/50 Hz grid-connected inverter model to validate the theoretical analysis in this paper.