Study On The Control Strategy And Stability For Photovolotaic System Of Microgrid

Because of the contradiction between society Energy demand and environmental deterioration,many country exploring the new-energy in recently years. Sufficient energy and without regional impact, so solar energy be researched as distributed power. Distributed power connect the traditional grid had a great impact on large grid,and the microgrid accessing can not only solve large-scale access problem, but also can make a variety of distributed power objects do their best. Therefore, the study of photovoltaic power generation system of microgrid control and the stability of the micro-grid has very important significance.The mathematical model based on the engineering of photovoltaic cells is analyzed in thesis, and a simulation mode is built.Photovoltaic array characteristic output and maximum power point tracking problem is simulated. Traditional control strategy is no longer meet the requirements of the micro-grid. According to the requirements of the micro-grid, droop thoughts on photovoltaic power generation to network is used and simulation analyzed. Finally, the stability of the photovoltaic system in the microgrid is simulated. A special operational status is the "island effect" in the operation of microgrid, the island run the system will bring certain hazards. So the control strategy of grid-connected inverter for avoiding island. A few kinds of detection methods to achieve the anti-islanding effect is given in this thesis. The detailed contents are as follows:(1) In this paper, the power generation principles and the electrical characteristics of photovoltaic array are introduced. Then a simulation of photovoltaic array based on the mathematical model is conducted. The influence of solar irradiation level and temperature on the output characteristics and volt-ampere characteristics of photovoltaic arrays are analyzed. The mathematical model shows the output of the photovoltaic cell have a great relationship with the external environment, so light and temperature changes in the simulation based on mathematical formulas. The simulation results show that when the light for a specific value, and the PV array short circuit current Isc and open circuit voltage Uoc are inversely proportional to temperature; when the temperature for a specific value, the PV array open circuit voltage Uoc and short circuit current Isc are proportional to light intensity.(2) According to the output characteristic curve in a state of the PV array output power for maximum PV array.In order to the PV array can always work at maximum power, PV array maximum power point tracking is adopted. This thesis introduces athe most commonly used in several tracking methods, and the simulation of incremental conductance method combining with the S-function simulation is established based on the Matlab.(3) The overall structure of the three-phase two-stage grid-connected photovoltaic (PV) system is presented based on output characteristics of photovoltaic cells. The DC-DC converter combined with the maximum power tracking of PV systems remains at the maximum power of the array, and keep DC voltage output at a desired value. After studying of the inverter control method, the conventional inverter control method is not applicable to micro-grid. According to operating characteristics of the micro-grid and using the droop thought to control the inverter, the inverter when the microgrid connected the grid is adopted PQ droop control; the inverter when microgrid islanded is adopted V/f control, and the control strategy is simulated and analyzed. The simulation results show that the microgrid can stably operated under the new control strategy; prove the new control strategy is feasible.(4) A microgrid containing two kinds of micro-power is built, one is photovoltaic power generation system with on loading, and the other DC source with on loading. The stability of the micro-grid simulated and analyzed based on recovery for different modes of transient failure and load changes in the microgrid. The simulation results show the micro-grid failure can be quickly restored to normal operation and stable operation of small interference under the above control strategy.(5) Because of photovoltaic systems in microgrid making solar converter and inverter accessing grid, microgrid should contain protection measures to deal with during the operation of a special kind of condition, the islanding. The three detection methods is given based on the international standards for PV systems as well as our standard. The impact of the microgrid instantaneous to enter the islanding can reduce when methods with control strategy.