Abstract: Abstract: The topology of three-phase inverters can divide into a three-phase bridge structure and a single-phase combined structure. Three-phase combined inverters have their own advantages compared to normal three-phase bridge inverters, such as enhanced functionality, more choices for power resources, and simple maintenance, and so on. Therefore, it is worthwhile to study the technology of three-phase combined inverters, especially the study of the synchronous control method for three-phase combined inverters. In this paper, the synchronous control method for three-phase combined inverters was studied in the island operation and the process of pre-synchronization between island and grid-connected operations based on the Y/△ connection of three single-phase energy storage inverters. When in the island operation, a synchronous control method of master/slave mode through a time benchmark synchronization and a phase adjustment synchronization was proposed based on a CAN (controller area network) bus communication. With the help of a CAN bus, an automatic network was designed to determine the A, B, and C-phase for the three inverters. A synchronization frame was designed and sent every 20 ms by the A-phase inverter, which was taken as the reference phase and corrected the phase error of the sine signal, to make sure that the time benchmark synchronized and the phase adjustment synchronized .And on this basis, when in the process of pre-synchronization between island and grid-connected operations, a synchronous control method of improved PLL (phase-locked loop) through frequency adjustment synchronization and logic judgment controller was proposed based on a SOGI (second-order generalized integrator). The improved PLL added a virtual PLL compared to the traditional PLL for the calculation of the real gird voltage phase angle. And the logic judgment controller used the difference between the output voltage phase angle of the inverters and the real gird voltage phase angle as the input, which implemented the smooth handoff from the island mode to a grid-connection. The actual experiments of the three-phase combined inverters were finished to verify the proposed synchronous control methods. The controller of the inverters was developed on the popular digital processor, which is TMS320F28335 from Texas Instrument. The CAN bus baud rate was set to 125 kB/s. The reference voltage was set to 220 V, 50 Hz, and the capacity of each inverter was 8kVA. As the result, when in the island operation, the proposed method implemented the three-phase output voltages of three-phase combined inverters and always kept the phase difference for 120°, and the average of the unbalance of three phase voltage in 5 minutes was 0.4%, which is less than the specified value 1.3% and meets the standards. When in the process of pre-synchronization between island and grid-connected operations, the maximum of the unbalance of three phase voltage in 5 minutes was 2.1%, which is less than the specified value of 2.6% and meets the standards. Therefore, both in the island operation and the process of pre-synchronization between island and grid-connected operations, the results showed that the synchronous control method for three-phase combined inverters is reliable and useful.