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A neutral-point-clamped (NPC) three-level inverter is used more and more in the grid-connected power generation system. In order to achieve the lower current harmonic, an LCL filter is widely utilized due to its superior filtering performance. Passive and active damping control are studied to solve the resonant problem appearing in an LCL filter, active damping control overcomes the weakness of power loss existing in the passive damping method, thus it gradually becomes one of research focus. According to the feature of the NPC three-level grid-connected inverter and active damping method based on feedback control of the capacitor current, design method of the LCL filter is thoroughly discussed in this paper, simulation and experimental results show that the filter design method is right.