Active damping of grid-connected LCL filter resonance by means of an observer loop to estimate the capacitor current is a well-known method. However, with variation of the grid side inductance due to grid conditions, the system robustness has to be checked. This work discusses the effect of the observer design on the system robustness against grid side inductance variation. Different values of resonant frequency, lower and higher than a critical value of one-sixth of the sampling frequency, are considered to clarify the system robustness at different resonant frequencies. Simulation and experimental results are presented to confirm the system operation at specific values of grid side inductance and resonant frequency.