Inverter plays an important role for energy conversion and transmission in grid connected renewable power generation systems. The diode-clamped three-level inverter and its control under grid connected operation mode are studied. The math model of inverter is built in the two-phase synchronously rotating reference frame， and the vector relationship between voltage and current of the model under grid connected operation mode are analyzed. The vector control in d-q rotating reference frame and tracing control of grid current is adopted to achieve unit power factor for power transmission. The voltage unbalance of capacitor-leg in DC link of the inverter is analyzed in detail and a novel neutral point voltage balancing strategy based on fuzzy control is proposed. Simulation and experimental results verify the validity of the analysis and design.