Researches in the area of distributed power generation system(DPGS) such as wind turbines, photovoltaic systems, fuel cells, etc. were been increased recently. DPGS are normally connected to the grid system through the inverter. Output current of the grid connected inverter includes higher order harmonics due to the PWM switching pattern of inverter. Typically, L filter was used between the inverter and the grid to reduce the switching harmonics. However, for applications above several hundred kilowatts, L filter becomes quite expensive to realize larger size of filter. Moreover, the system dynamics may be poor. Compared to the L filter, LCL filter is more effective on reducing harmonic distortion at the switching frequency, but it brings also the disadvantage of generating a resonace problem. Also, the design of LCL filter parameters is complex to consider many constraints, such as current ripple, filter size, reactive power absorbed by filter capacitor, etc. Therefore, it is important to choose the appropriate parameters for LCL filter parameters.
This thesis describes a design method of LCL filter for grid connected NPC three-level inverter. By analyzing the ripple current according to the switching of NPC inverter, the inverter side inductor can be designed. And filter capacitor is designed considering stand-alone mode. Also by considering the relation between filtering effect at the switching frequency and the ratio between grid and inverter inductors, the relation between the output current attenuation at the switching frequency and the ratio a between grid and inverter inductors and ripple current of inverter side inductor, optimized value of LCL filter can be designed. Finally, the described LCL filter design method is validated through the simulation and experimental results.