Abstract: L filter and LCL filter are often applied in wind-power system whose abilities to restrain the higher harmonic near inverter switch frequency are relatively limited. Therefore, LLCL filter was proposed, through inserting a small inductance at the capacitor branch of LCL filter, which could cause series resonance at the switch frequency, divert harmonic near the switching frequency, and then reduce the effects on the grid as many as possible. The structures and filter characteristics of L filters, LCL filters and LLCL filters were analyzed, and the design constraints of LLCL filters parameters were discussed. The simulation results show that LLCL filter can suppress higher harmonic near the switching frequency more effectively. Compared with the LCL filter, the output current from LLCL filter has fewer ripples, less harmonic near the switching frequency and less harmonic distortion under the same inductance value of filters.

Key words: wind-power system, converter, LLCL filter, switching frequency