LCL filter design and inductor ripple analysis for 3-level NPC grid interface converter

The harmonic filter for a 3-level neutral point clamped (NPC) grid interface converter is designed in this paper with good filtering performance and small component size. LCL topology is selected because of the attenuation and size tradeoff. The design for inverter side inductor L1 is emphasized due to its cost. A detailed inductor current ripple analysis is given based on the space vector modulation (SVM). The analysis derives the inductor volt-second and the maximum current ripple equation in line cycle. It also reveals the switching cycle current ripple distribution over line cycle, with the consideration of power factor and other factors. The total system loss is calculated with different ripple current. Inductor L1 is determined by the loss and size tradeoff. Also the capacitor and grid side inductor L2 is designed based on attenuation requirement. A damping circuit suppresses the LCL resonant peak. The filter design is verified by both simulation and a 200kVA 3-level NPC converter hardware.