Research On Control Strategy Of Interface Inverter In Microgrid

Microgrid is a new generation form based on the distributed generation system of renewable energy,which is an important way for distributed generation to make full use of its efficiency.The distributed power supplied in microgrid is mostly connected to the ac side by the inverters,and the control technology of the interface inverter in the microgrid is one of the key technologies of the research on microgrid.Microgrid has two operation modes: grid-connected mode and off-grid(island)mode.Microgrid operates in these two modes stably and can transfer smoothly between these two modes is the guarantee of its reliable operation.With the research background of low-voltage microgrid,This thesis based on the requirements of that low-voltage microgrid need to implement in island mode and grid-connected mode reliably and smooth transferring between two modes,research the control strategy of interface inverters in low-voltage microgrid.This thesis firstly established the mathematical model of three-phase voltage source inverter,completed the design of main circuit parameters and the parameters of the voltage-current double closed loop control system,and analyzed the characteristics of the output impedance of inverter;Secondly,the paper researched the control strategy of low-voltage microgrid which operated in island mode,introduced the conventional droop control strategy and virtual impedance technique,and put forward the voltage recovery strategy,improved the problem of voltage sag caused by virtual impedance technique;Low-voltage microgrid that operated in island mode with nonlinear load was studied,aiming at the problem of the harmonic current,this thesis proposed a new virtual resistance which associated with frequency,it changed the distribution of the harmonic current in low-voltage microgrid without affecting the distribution of fundamental current.The new virtual resistance improved the harmful effect on system side which caused by the harmonic current from the idea of filtering;And then expounded the control strategy of low-voltage microgrid that operated in grid-connected mode,smooth transferring between two modes of low-voltage microgrid was studied,adopted a pre-synchronization method and a smooth switchover control method based on controller status follow,achieved the smooth transferring between two modes in low-voltage microgrid;Finally build the Matlab/Simulink simulation model,the simulation analysis about the control strategy of island mode and the method of smooth transferring between two modes in low-voltage microgrid was made,and the simulation results verified the correctness and effectiveness of the control strategy in this thesis.