The Research Of Strategy For Microgrid Connected Grid,Off Grid And Smooth Switching

With the rapid development of the world economy in the 21 st century,non-renewable energy sources such as oil and coal are increasingly exhausted,energy shortage,environmental pollution and other issues are becoming increasingly prominent,which has aroused widespread concern of scientific researchers all over the world.With the rapid development of wind and solar power generation,microgrid emerges as the times require in order to effectively control the shock caused by large-scale distributed energy grid-connected.However,there are significant differences between the control strategies of the micro-grid in the two operation modes of grid-connected and off-grid,and the control strategies are different when the off-grid operation mode changes,which will lead to the jump of the controller output and seriously affect the stable operation of the system.Therefore,this thesis studies the micro-grid control strategy and smooth off-grid switching strategy.The specific contents are as follows:Distributed generation units and energy storage are the foundation of microgrid construction,and the microgrid with wind and solar energy storage is the typical representative of microgrid.Therefore,based on the analysis of the working principles of micro-sources such as photovoltaic power generation,hybrid energy storage and wind power generation,their mathematical models are established respectively,and their operation control strategies are given.The variable step size MPPT control is proposed to ensure the maximum power output of photovoltaic arrays.Hybrid energy storage system adopts voltage and current double-loop control strategy to maintain the voltage and frequency stability of the system;permanent magnet synchronous motor adopts stator Voltage-oriented double-loop control,so that the speed of the fan can be optimized,thus achieving the MPPT control of the fan.In the control strategy of micro-grid,especially the smooth switching strategy of parallel off-grid,it is pointed out that the essence of smooth switching of micro-grid is to design the control strategy of inverters,and the control performance of inverters directly affects the work efficiency of micro-grid system.On the basis of the topological structure and mathematical model of the inverters,taking the master-slave low-voltage micro-grid as the research object,and aiming at the problems of voltage amplitude difference and frequency difference when the grid-connected mode is running,a quasi-proportionalresonance PR control strategy for the current inner loop of the main control inverters is proposed to improve the control accuracy of the system.Based on the analysis of conventional islanding detection methods,an active frequency shift method with positive feedback is proposed to detect islands in order to improve the detection speed and reduce the detection blind area.Aiming at the sudden switching of load and sudden drop of output power in the process of smoothing switching control of conventional master-slave microgrid,an improved smoothing switching control structure is proposed.In order to effectively improve the fluctuation of voltage and frequency in the switching mode of microgrid,an improved droop control strategy suitable for low-voltage microgrid is proposed in the off-grid mode.In order to verify the smooth switching control strategy of the micro-grid in and out of the grid and between them,the simulation experiment model of master-slave structure micro-grid system is built by using MATLAB/Simulink software.Under the conditions of fast load switching and changing the output power of main inverters,the simulation experiments are carried out to verify and analyze the operation of micro-grid in grid-connected and off-grid,and the smooth switching between off-grid.The results show that the proposed control strategies of grid-connected,off-grid and smooth switching are reasonable and effective,which can keep the system running smoothly.