Design Of Modular Parallel Three-phase Photovoltaic Grid-connected Inverter

With the exacerbation of the world’s energy crisis, much attention has been paid to the photovoltaic (PV) power generation systems. In PV power generation systems, grid-connected inverter is one of the most important parts. As the power of the PV generation system increased, the power of the grid-connected is also needed to increase. In high-power grid-connected inverter applications, single grid-connected inverter exposed some shortcomings. With the power increased, the current stresses of power devices and the volume of the inverter will also increase. Moreover, if the single grid-connected inverter has some fault, the loss will be immeasurable. Grid-connected inverter parallel technique is an effective method in solving high-power grid-connected inverter applications. However, parallel inverters will cause the problem of circulating current, which will lead to the imbalance of the output current of the inverter. Also the quality of the output current will decrease, and even cause damage to the inverter. In this paper, the parallel control and the circulating current of the grid-connected inverter is researched.The grid-connected parallel inverters based on three-phase grid-connected inverter. So this paper analysis the topology of the three-phase grid-connected inverter and the small-signal model of different coordinates is built. The control theory of SVPWM is also analyzed. Moreover, this paper introduces the software phase locked loop (PLL) and the maximum power point tracking (MPPT) methods. The design of the current and the voltage loop of the three-phase grid-connected inverter are analyzed as well.Then the grid-connected inverter parallel system is modeled. Besides, the circulating current path and the circulating power are analyzed. The reason that causes the circulating current is explained in theory. And the effects of different parameters on circulating current are analyzed, such as the filter parameter, switching frequency, dead time and so on. Based on the analysis above, this paper uses master-slave method to control the grid-connected inverter and use zero-sequence current suppression method to suppress the circulating current. Due to the characteristic of the SVPWM control, the zero vector distribution will not affect the control of the fundamental control, but it will affect the zero-sequence current. Based on this theory, a zero-sequence current controller is designed to suppress the zero-sequence circulating current.Finally, the simulation is made based on Matlab/Simulink to verify the analysis. Experiment prototypes of three-phase grid-connected parallel inverters are built as well. And the performance of the zero-sequence current controller is tested in different parameters.