Optimized design of filter for transformerless three-level photovoltaic grid-connected inverter

Linking the capacitor neutral point of LCL filter to the DC midpoint for transformerless three-level photovoltaic inverter has an advantage of leakage current reduction. But it also brings large current virtual value and low-frequency resonance of bridge-side current, which leads to the reduction of the system efficiency and stability. Aiming at these problems, the common and differential mode model of three-level grid-connected inverter was constructed. Based on the equivalent model, the mechanism of the two problems was analyzed and a optimized design scheme was proposed. In the scheme, the capacitor of LCL filter was divided to two parts and the neutral point of the smaller one was linked to the DC midpoint. Then, the design principle of the improved LCL filter parameter was proposed based on bridge-side current ripple and common mode voltage source spectrum analysis. The problems of the system efficiency and stability reduction were solved when this proposed scheme was adopted. Feasibility and effectiveness of the optimized scheme are validated by experimental results.