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To address the leakage current problem of transformerless three-phase inverters for photovoltaic (PV) grid-tied systems, H8 and improved H8 inverters were proposed to alleviate the leakage current of the PV grid-tied systems to a certain extent. However, these inverters cannot avoid the high-frequency switching of the generated common mode voltages (CMVs) among multiple levels, resulting in the unsatisfactory effect of the leakage current suppression of these inverter topologies. In order to solve these problems, an auxiliary power supply based nine-switch (AP-H9) inverter with reduced leakage current is proposed in this paper. A DC auxiliary circuit containing a switch is added to the inverter based on the H8 inverter, and one or more diodes are connected in reverse parallel with other corresponding switches to form the H9 inverter topology. By properly controlling the nine switches and using the reverse diodes to divide the DC voltage , the CMV of the system is limited to 2/3 times of the DC voltage, which can effectively suppress the leakage current of the PV grid-tied system. In addition, a model-free predictive current control (MFPCC) method based on ultra-local model is presented to eliminate the dependence of inverter control performance on system parameters. This method uses the voltage and current information of the previous two moments to realize the online update of the gain and dynamic part of the ultra-local model, and then obtain the predicted voltage vector for the next moment. Moreover, the switching loss optimization constraint is considered in the cost function to further improve the efficiency of the inverter. Finally, an experimental prototype is developed. The simulation and experimental results show that the proposed inverter and its control method can effectively suppress the leakage current of PV grid-tied system, and are insensitive to parameter changes.