Research On Mode Switching Method Of Grid Connected And Islanding For Microgrid Inverters

The efficient and convenient operation mode of the microgrid and the good user experience are closely related to the stable and reliable control system.When there is an accident in the main grid,the microgrid should quickly disconnect with the main grid and switch to the off-network operation mode.When the accident is eliminated,microgrid needs to be incorporated into the main grid and continue to work.However,in the process of switching operation mode,it will inevitably bring impact on the microgrid and the main grid.Therefore,this paper aims at researching the smooth switching between grid connected and off grid operation mode,realizes seamless handover of operation mode in microgrid,and ensure the microgrid running smoothly.First,analyzing the mathematical model and control principle of microgrid inverter.On this basis,to keep the consistency of the control strategy when the inverter is running,analyze the droop control strategy based on P-f and Q-V,and design the modules of the droop control part.At the same time,analyze the islanding detection method when switching the mode.The purpose is to determine the time of island occurrence and change the droop control equation,so as to improve the accuracy of reactive power allocation.Emphatically analyze the active frequency disturbance method and the frequency disturbance method with positive feedback,which lays the foundation for the later connection to the off-network switching.Secondly,aiming at the problem of droop control strategy needs different droop coefficient,and from the network and operation mode switching to different droop coefficient during mode switching,the paper proposes a microgrid control strategy based on dynamic coefficient which is adjusted in real time in the connecting grid,off grid and operation mode switching.Aiming at the defects that the output voltage amplitude is not the same in off-grid mode,add voltage compensation into the droop control equation,improve the accuracy of reactive load distribution,ensure that the system has good dynamic response and prepare for the realization of smooth switching operation mode.Then,the conversion process and control strategy between the two operating modes of the grid connected and off network are analyzed.When moving from the grid connected mode to the off-grid mode,aiming at the long detection time of the traditional islanding detection algorithm,a new islanding detection method based on enhanced frequency positive feedback is proposed,which shortens the islanding detection time and improves the dynamic characteristics of the system.When the microgrid inverter is switched from the off-grid mode to the grid connected mode,the pre-synchronous control strategy is adopted to synchronize the output voltage amplitude and the phase of the inverter quickly when connected to the grid,to achieve smooth switching.Finally,comparing the traditional fixed coefficient method and method proposed in this paper by simulation and experiments.The result shows that the effect of this method in the microgrid operation mode switching of the inverter is obviously superior to the traditional method,to show that the proposed method is advanced,correctness and feasibility.