| The storage capacity of primary energy is insufficient to support the sustainable development of our country because of the continued exploitation of primary energy. Due to the urgent need and use of energy, we urgently want to be able to find new energy sources to replace the primary energy. So, the renewable energy has become the preferred object. Currently it is one of the important topics explored to rationalize and maximize the use of these new energy.Grid-connected PV system is accessing large power grid in the form of micro-grid using solar energy, and it is a good way to supplement the insufficiency of energy supply. At present, there are three main difficulties in the study of solar photovoltaic connected grid, which are the MPPT technology, the grid inverter control strategy and the islanding detection technology. The following is the research on the above difficulty done in this paper.(1) Build the engineering mathematical model using Matlab/simulink simulation software, according to the working principle and the output characteristics of photovoltaic cells. Verify the correctness of the built model of photovoltaic cells through simulating the simulated photovoltaic cells and detailed analysing the output characteristics.(2) Dissertate three common MPPT methods, detailed analyse and compare each method in order to sum up the advantages and disadvantages of each method. On this basis, this paper presents a new method of improved perturbation and observation method. Build a simulation model in the Simulink of Matlab software applying this method, write the program in C language, and achieve MPPT simulation using S function in Boost Circuit. The simulation results show that the new method can track the maximum power point fast and efficiently, and can reflect the output characteristics of photovoltaic cells generates changes with the changes of outside temperature and light intensity. So, it proves the effectiveness of the built circuit model.(3) In this paper, in-depth study has been done on the control technology of later stage inverter of the two-stage three-phase structure grid-connected PV, refering on the common control strategy of PQ and V/f based on the droop characteristics. Then build simulation model in the Simulink of Matlab software, and do the simulation experiments of the microgrid with parallel operation of two photovoltaic micropowers DG1and DG2. Trough simulating three operating states of DG system analyse the performance of the control strategy. Simulation results verify the correctness and feasibility of the control strategy.(4) Analyse the formation process and causes of the island, and the operation mode and constitution after forming islands. Propose two modes of islanding, which are the static islanding mode and the dynamic islanding mode. Through the analysis of each mode, it is found out that the wide area information set control thinking proposed by the dynamic mode better meet the demands of reality.(5) Discuss the islanding detection method, and explain in detail every method’s working principle, and conclude the advantages and disadvantages of each method. On this basis, this paper presents the micro grid islanding detection method based on Clark transform. Build a simulation model of islanding detection in the Simulink of Matlab software, and do simulation analysis through the examples. The results show that the method has characteristics of rapid detection speed and high accuracy. At the same time, the detection method is simple, has no impact on the power quality, and has certain engineering application value. |
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