Research On Control Strategy Of Power Distribution In Island Microgrid And The Smooth Switching Of Status

The microgrid system based on clean energy integrates scattered new energy sources to generate electricity,which not only solves the randomness,intermittentness and uncontrollability of new energy sources,but also realizes pollution-free power production process,optimizes the energy structure of the power system,and makes the power system more stable,provides a reliable technical solution to solve the electricity consumption of rural residents in remote areas.During the normal operation of the microgrid,including islanding,grid-connected,and the switching of island/grid-connected status,when the microgrid is operating in an island mode,the local load of the system is borne by the inverters according to the capacity ratio to prevent inverters from appearing over-current phenomenon,When the load changes,the bus voltage amplitude and frequency of the microgrid will not change suddenly.The microgrid does not have transient impacts during the switching of island/gridconnected status,and the transition is smooth.After the microgrid is connected to the grid,the load is completely transferred to the grid.In this paper,the following researches are conducted on the above issues:Firstly,this paper is based on the peer-to-peer mode droop control AC microgrid,the paper analyzes the principle of droop control,the adjustment characteristics of droop control in island and grid-connected status,designs the link of droop control in microgrid,establishs the Matlab/simulink simulation model.The simulation results show that the island microgrid based on droop control when the line impedance is resistive and inductive,there is power coupling,chaotic distribution of reactive power and circular currents between the two inverters.Secondly,the paper analyzes the reasons for the shortcomings in droop control,and the power distribution principle of droop control in island microgrid.Both the microgrid line impedance is inductive and matching line impedance with distributed power capacity are necessary conditions to realize power decoupling and reasonable distribution.Therefore,adding virtual impedance to droop control to solve the above problems,and using the Bode diagram to analyze the changes in the output characteristics of the system before and after the virtual impedance is added.Adding virtual impedance solves the shortcomings of droop control,but the virtual impedance makes the bus voltage drop.To solve this problem,the voltage and frequency secondary adjustment link is added to virtual impedance droop control,so that its bus voltage and frequency will not change suddenly when the load of the island microgrid changes.The power can be shared by the inverters according to the capacity ratio,and it can enhance the reliability of island microgrid.Establishing the Matlab/simulink simulation model,the simulation results show that the virtual impedance droop control of voltage and frequency secondary adjustment has a good optimization effect.Finally,when the island/grid-connected status is switched,virtual impedance droop control based on secondary adjustment of voltage and frequency and pre-synchronization control strategy can make the microgrid bus voltage amplitude,phase,and frequency the same as the grid before the grid is connected,in order to ensure that there is no transient impact during the switching of island/grid-connected status.Establishing the Matlab/simulink simulation model when microgrid operation status is switching.The simulation results show that the island microgrid can be smoothly switched to the grid-connected state,and show that the adopted control strategy has a very good control effect on the switching of operation status.