A Grid-Connected Photovoltaic Power System With Solid State Transformer

With the shortage of disposable energy and the aggravation of environmental pollution, more and more attention has been paid to the development of renewable energy all over the world. Among them, the solar energy attracted much more attention as its rich, easy getting, clean and friendly to environment. Photovoltaic power generation is an important trend in the development of solar energy, and has become a hot research topic in the this field. Recently, with the development of research on this area, especially the solid state transformer, photovoltaic power generation system has broken the limit of traditional power generation mode, and gradually have the value to enter the large-scale promotion. Therefore, we introduce the topology structure of the solid state transformer in detail. By carring on the modeling and controlling strategy design, we connecte photovoltaic array to the 10 kV grid by the solid state transformer, and Simulink simulation verified its feasibility.First of all, we introduced the working principle and output characteristics of the solar photovoltaic cell. In order to track the maximum power point, we improved the disturbance observation method by comparing and analyzing several common methods.After that, we elaborated the working principle, equivalent model and topology structure of the solid state transformer.On this basis, we particularly analyzed the rectifier module, DC converter module, inverter module and mathematical model of solid state transformer respectively.Besides, we proposed control strategy for each module of the solid state transformer. In order to realize the flexible control of high voltage side with a stable DC voltage and the reactive power, we took closed-loop control strategy for the reactive power and voltage rectifier output voltage. In order to ensure a stable output DC voltage of the secondary side, we adopted a outer voltage loop and a inner current loop, i.e., a double closed loop control method on the middle part of the isolation. Inorder to output a frequency of the alternating current signal and achieve input/output stability and flexible active transmission power under the normal operation of the power system, we adopted a closed-loop output voltage control method on the low voltage side.Finally, according to the above analysis and research, in the MATLAB/Simulink environment we established photovoltaic maximum power tracking control model and solid state transformer model,and then put the photovoltaic cell model into the low voltage DC bus of the solid state transformer after series-parallel connection,ultimately realize control of the grid-connected photovoltaic system. The experimental results show that the improved perturbation and observation method can realize the maximum power point tracking quit well. After accessed to solid state transformer, the adopted control strategy can effectively achieve inverter output current connect to the grid with low harmonic and high power factor.