Research On Control Strategy Between Island Mode And Grid-Connected Mode Of Micro-Grid Inverter Based On Virtual Synchronous Generator

As the carrier of clean energy,microgrid not only improves the utilization ratio of clean energy,but also improves regional power supply.However,microgrid with a large number of power electronic devices has a fast response speed,lacks the characteristic similar to damping characteristics and rotor inertia of synchronous generator(SG),which makes the anti-interference and the dynamic response of microgrid poor.Therefore,the virtual synchronous generator(VSG)technology is used,which means that the electromechanical transient and external characteristics of the SG are applied to the control algorithm of microgrid inverter.Then,the microgrid inverter has the dynamic and static characteristics similar to the SG,and it can operate stably in the grid-connected mode and island mode.Therefore,the voltage-controlled VSG is used as the object,and the problems of grid-connected and island operation and switching control for microgrid inverter are mainly studied in this paper.This paper has solved the difficult problem of the smooth switching between the two operation modes,which has important theoretical value and practical significance.The main researches are as follows:First of all,the P-f and Q-V control units of the VSG is designed based on the excitation control and electromechanical transient characteristics of SG.Simultaneously,in order to increase the inductive output impedance of the SG,the synchronous impedance based on the second-order generalized integrator(SOCI)is introduced,and the quality of the output voltage and current is improved by the inner control loops.Secondly,the flexible switching control strategy based on quasi-synchronization algorithm is designed with the help of the quasi-synchronization principle and the characteristics of "secondary frequency regulation" and "secondary voltage regulation" in the power system.And the phase locked loop(PLL)technology,the key technology of flexible switching,is improved.Then,a PLL that can achieve accurate phase locking under non-ideal voltage conditions is proposed,which avoids the influence of the non-ideal voltage on the phase locking accuracy and realizes the flexible switching of the VSG.In addition,for the voltage source inverter of the microgrid,an islanding detection algorithm based on the frequency and voltage positive feedback and power disturbance is designed,which realizes the effective and fast detection of the unplanned island.The switching algorithm suitable for VSG control is designed to realize the seamless switching between grid-connected mode and island mode,which ensures the uninterrupted power supply for the important loads.Finally,the simulation model of VSG is built in Matlab/simulink,and the above algorithm is fully simulated.At the same time,the low-power experimental platform of the VSG system is built,and the control strategies are verified by experiments.