Research On Three-phase Cascaded Photovoltaic Grid-connected Inverter

Solar energy is abundant,and it belongs to renewable energy.Therefore,photovoltaic power generation using solar energy is favored by domestic and foreign companies and scholars.In PV power systems,there are topologies with a transformer and topologies without a transformer.Because isolated photovoltaic power generation systems have isolation transformers,they have the disadvantages of large size,large weight,high cost,low efficiency,and low power density.In order to solve this problem,experts and scholars have proposed non-isolated photovoltaic power generation systems in recent years.However,the photovoltaic power generation system that removes transformers will create new problems,in which leakage current is one of the major problems.How to suppress leakage current has become a common concern in the industry and academia.Firstly,the non-isolated inverter topologies of single-phase and three-phase at home and abroad,and the methods of suppressing leakage currents are introduced.It also determines the main objects of this research and the work that needs to be done.Then the common mode model is analyzed and its modulation schemes are introduced for the conventional and improved three-phase cascaded H4 photovoltaic inverters.The inherent reason that the conventional and the improved cascaded H4 grid-connected inverter topologies fail to reduce the leakage current is clarified.Based on this,a novel three-phase cascaded H5 photovoltaic grid-connected inverter is proposed.The common-mode models of high-frequency and low-frequency of the system are established,and the factors influencing the system leakage current are analyzed.A new carrier modulation scheme is proposed to ensure that the common-mode voltage is constant.The reason why the leakage current can be effectively suppressed is analyzed,and a mathematical model of a novel three-phase cascaded H5 inverter is established.According to the model,the current loop controlled by the grid and its parameters are designed.The scheme is verified through simulation.Finally,a three-phase cascaded H5 photovoltaic grid-connected inverter experimental platform is designed.The main circuit,drive circuit and its control circuit are designed.Combining the TI chip model TMS320F28335 and the Xilinx XC3S400-PQ208 FPGA chip digitally controls the main circuit system.By comparing the improved three-phase cascaded H4 inverter with the novel three-phase cascaded H5 inverter,the effectiveness of the proposed scheme is verified.Aiming at the novel three-phase cascaded H5 and its new carrier modulation scheme,the experiments of startup transient and three-phase input voltage unequal are performed,which further validate the effectiveness of the proposed scheme.