Research On Control Technology Of Three-phase Four-leg Microgrid Inverter Based On Virtual Synchronous Generator

Micro-grid is an important carrier of energy,it can provide real-time and fast power to different regions.However,with the rapid development of science and technology,the number of power electronic devices in the micro-grid is increasing,which leads to the poor ability of micro-grid to resist external interference and dynamic response.The virtual synchronous generator technology can simulate the external characteristics of synchronous generators,provide enough damping support and inertial support for the micro-grid,so as to effectively solve the problem of stable operation of the micro-grid.In this thesis,the operation control method of micro-grid inverter based on VSG under unbalanced load is studied.Firstly,with the help of the mathematical model of traditional synchronous generator and its primary frequency and primary voltage regulation characteristics,the active frequency regulation control loop and reactive voltage regulation control loop of virtual synchronous generator are derived.Secondly,aiming at the poor capacity of traditional inverters with asymmetric and non-linear loads,an improved sequential control method based on three-phase four-leg inverters is proposed,which uses dual-rotating coordinate system technology to decouple the positive and negative sequence voltage of the output of the inverters,and achieves the independent control of the positive and negative sequence voltage.At the same time,aiming at the independent zero sequence voltage,a virtual construction technology of alpha-beta coordinate system is designed,the coordinate system technology realizes more effective control of zero sequence voltage.Then,in order to enable the inverter to switch smoothly off-grid with unbalanced load,a dual-mode off-grid switching control method of VSG based on three-phase four-leg inverter is proposed,which can realize off-grid mode switching without changing the control algorithm.In order to improve the performance of VSG under asymmetric conditions,a VSG control algorithm based on sequential network model is proposed,by combining with pre-synchronization control,the impulse current of the inverters with unbalanced loads is reduced when connected to the grid,on this basis,voltage and current double closed-loop control is added to improve the dynamic performance of the system.Finally,the proposed control algorithm is validated by MATLAB/simulink,and the experimental control platform of the inverter based on VSG control is built,and the preliminary experimental verification of the VSG algorithm is carried out.