Analysis of Robustness Enhanced LCL Filter Design Based on Stability Region

In LCL-type grid-connected inverter system, the resonance frequency will shift down and cross 1/6 of the sampling frequency due to the variation of grid impedance in weak grid, which will lead to instability. To enhance the robustness, an improved stability region based LCL filter design which adds a restriction on the lowest resonance frequency can be introduced. In this paper the influence of this restriction on the filter parameters have been studied and the results shows that the influence is greater under higher switching frequency. However, it is proved that the restriction will only lead to a limited total inductance increment. Therefore, the robustness enhanced filter design is proved to be of great practical value. At last, the analytical results are verified through experiments and simulations.