An improved resonant frequency based LCL filter design method for grid-connected inverters

Resonant frequency of an LCL filter in grid-connected inverters decreases with increase in inductive grid impedance. It is also affected by drop in magnetic permeability of filter inductor with increase in current. This resonant frequency variation affects grid-voltage harmonic rejection and compromises stability. Hence, an improved resonant frequency based LCL filter design method is presented in this paper. A resonant frequency band is identified by considering four factors: grid-voltage harmonics, grid strength, filter inductance non-linearity and control stability boundary. The LCL parameters are then derived so that the resonant frequency remains within this identified band for a wide operating range. Division-Summation digital control method is used to elaborate the design method. Experimental results have demonstrated stability and much improved harmonic rejection under wide parameter variations.