LLCL-Filtered Grid Converter With Improved Stability and Robustness

LLCL filter has been proven to introduce more filtering at the converter switching frequency, while using smaller passive components. However, like other higher order filters, it introduces resonance to the grid when used with a grid converter. Its stability and robustness are also affected by grid impedance variations, making its design more challenging. To address these concerns, a new parameter design method for the LLCL filter has been formulated in this paper, which when enforced, guarantees robust and stable grid current control regardless of how the grid conditions change. It is thus an enhanced method even with no damping added to the grid converter. The method has been applied to the development of an LLCL filter for testing in the laboratory with a 5-kW, 400-V, and 50-Hz grid converter. The method can also be applied to the lower order LCL filter with only a slight modification needed.