| This thesis was supported by Natural Science Foundation of Chongqing(CSTC 2009BB6190) and independent research project of State Key Laboratory of Power Transmission Equipment & System Security and New Technology(2007DA1051270 9208).Distributed generation(DG) develops rapidly due to many advantages such as less pollution, high energy efficiency and flexible installation locations. But a large number of DGs access grid also produced a number of adverse effects. As a kind of new energy access solution, scholars proposed microgrid concept in this century. Generator, load, energy storage and control equipment are combined in microgrid from the system point of view, forming a single control unit, supplying the user with electricity and heat energy. Microgrid powers are mostly micro-powers, including micro-turbines, fuel cells, photovoltaic cells, wind generators etc. Photovoltaic generation is renewable, pollution-free, easy installation, and the technology is mature, so it will become an important micro-power in microgrid. The grid-connected and independent photovoltaic generation systems are both affected by the volatility of photovoltaic generation. Therefore research on the access mode and dynamic features of photovoltaic generation in microgrid is important for the applications and research on microgrid. Based on the study of the photovoltaic generation, inverter and control model, the impact of the photovoltaic generation on the grid and microgrid is studied in this article. The method of using the average power output of photovoltaic generation compatible with energy storage equipment to optimize the capacity of energy storage equipment is proposed. The main contents are as follows:Photovoltaic generation and photovoltaic inverter principle and model are analyzed to study the characteristics of photovoltaic generation. According to the working principle of photovoltaic inverter, current-mode control mode being used, the output current of the inverter is considered as the controlled variables. The output current is controlled in time so that the output current of inverter is in the same phase and the same frequency with the grid voltage, reaching the purpose of grid-connected power generation. The maximum power point tracking control mode is summarized. The advantages and disadvantages of the constant voltage tracking method, current scanning method, perturbation and observation method, open circuit voltage proportional coefficient method, the proportion of short-circuit current method, conductance increment method are analyzed.The effect of the volatility of on the distribution network and microgrid is researched in this paper. DG is equaled to current source. The actual measurement output current of the DG is decomposed into a limited number of sine wave. The frequency domain method for computing the fluctuation of the whole grid voltage is used. Studies show that when the capacity of photovoltaic generation is smaller than the grid system, the influence of the output power fluctuations on the voltage and frequency of distribution network can be ignored. Since photovoltaic generation is not regulatory, micro-turbine and other micro-adjustable power supply are needed to add. By studying micro-turbine speed/load control model and the fuel control model, there exists coordination problem between the rapid change of the power of photovoltaic generation and slow adjustment power as micro-turbine. Micro-turbines will appear oscillation regulation and cause the frequency fluctuation of microgrid.The harmonic of PV inverter is analyzed in the paper. Aimed at the volatility of photovoltaic generation, the photovoltaic energy management mode of using the average output power to select the capacity of storage equipment is proposed, photovoltaic power is smoothed. Electrical energy emitted by photovoltaic generation a day can be calculated by means of integral method, the average power of photovoltaic generation can be obtained. When photovoltaic generation and energy storage equipment output the average power in the photovoltaic generation, the capacity of the energy storage equipment is the smallest at this time. This is the as the basis of the selection of energy storage equipment. Photovoltaic cells and energy storage equipment are AC coupled. The output power of photovoltaic cells are stabilized by the energy storage equipment and can go together with adjustable power supply as micro-turbine during transient process, providing short-term power support, therefore frequency fluctuation can be effectively reduced of microgrid in the transition process from the grid-connected model to the islanding model.The photovoltaic generation and microgrid model are established in PSCAD/EMTDC, showing that the proposed theory and method in the paper are right. The microgrid running in grid-connected model and in island mode is simulated. The output power of energy storage equipment is increased in the islanding model. The common power of the energy storage equipment and photovoltaic generation is unchanged and is down in different slope and at different time. These cases are analyzed. |
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