On Master-slave Structure-based Smooth Switching Of Micro-grid Operation Mode

The application of micro-grid is of great significance to integrating solar energy,wind energy and other new energies.Stable operations of micro-grid under grid-connected mode and island model are closely related to its control strategies.When modes are switched,if states of the controller under the two modes are mismatched,transient shocks may easily appear,which is not conducive to stable operations of the micro-grid.Therefore,the control strategy of smooth switching of operation state of microgrid is researched in the thesis.Firstly,operation control strategies of AC micro-grid with master-slave control structures are studied in the thesis and LC filter parameters are designed according to topology structures of micro-power inverter.When micro-grid operates in the grid-connected mode,the master control source inverter adopts PQ control mode to output constant power of the micro source.When micro-grid operates in the isolated island mode,the master control source inverter adopts V/F droop control to ensure the stable environment needed by micro-grid under the isolated island mode.When the low-voltage micro-grid isolated island mode is running,the circuit is resistive.The frequency and voltage of the system have large fluctuations if V/F drooping characteristics are used to regulate the system,so the resistive ratio is introduced in the thesis on the basis of the V/F droop control so as to stabilize voltage and frequency of the system.Besides,the effectiveness of the improved control strategies is verified through simulation.Secondly,the principle of micro-grid island detection is analyzed.According to the advantages and disadvantages of the current disturbance method and the frequency migration method,this paper adopts the island detection method combining the frequency migration method and the current disturbance method,which can effectively solve the power quality problem caused by the current fluctuation and the capacitive blind spot problem existing in the frequency migration method.When micro-grid grid-connected mode is switched to the isolated island mode,the state compensation is adopted to track the grid voltage before the switch,which can avoid voltage fluctuation and current shock problems and ensure the smooth switch from the grid-connected mode to the isolated island mode.When the isolated island mode is switched to the grid-connected mode,the voltage synchronization operation is realized through the pre-synchronous control,which can ensure the smooth switch of grid-connected process.Finally,a master-slave structure microgrid model is built in Matlab/Simulink software to simulate the microgrid under the conditions of switching load and mode switch,and analyze its operation characteristics in grid-connected,isolated island operation and two modes switch.The simulation results verified the effectiveness of the control strategy adopted in the thesis.