Analysis and application of passive damping LLCL filter in active power filter

The output current of shunt active power filter (SAPF) contains a lot of switching frequency harmonics components. Therefore a low-pass passive power filter is often inserted between a SAPF and the grid to limit the excessive current harmnics. The paper presents a novel passive damping LLCL filter for SAPF. The LLCL filter inserts a small inductor in the capacitor branch to compose a series LC circuit that resonants at the switching frequency. The LC branch has very low impedance at the switch frequency, which can strongly eliminate the harmonic current around switching frequency. At the same time the LLCL filter can drastically reduce the total inductance of the system. The comparative analysis and discussion of the LCL and LLCL filter is presented and evaluated. Compared with the traditional LCL filter, the LLCL filter has a more excellent performance. Simulation in MATLAB/SIMULINK is presented to verify the theoretical analysis.