Research On Three-Level Grid-Connected Converter Of Microgrid Based On LCL Filter

As the demand for energy in human society continues to grow,how to improve the utilization of new energy and its penetration in our energy structure is an important topic that scholars in various countries are studying and solving.As an important structure of distributed energy structure,microgrid is an important technical direction to improve the utilization rate of new energy,and has been concerned and researched by many scholars.However,the weak grid characteristics prevalent in the microgrid make it susceptible to three-phase unbalanced load and cause three-phase voltage imbalance.This problem will lead to the stable operation of the internal converter of the microgrid system and the normal use of the load.Come to some challenges.Therefore,it is of great significance to study the control strategy of the converter system under unbalanced grid conditions.In this paper,the two mainstream topologies of three-phase three-wire system and three-phase four-wire system are used to derive their mathematical models and their power constraint equations in three coordinate systems.Through comparative analysis,the three-phase four-wire system can achieve better control effects on system current and power control.In addition,the grid-connected filter is an important part of the converter system.This paper analyzes several common grid-connected filter characteristics.The comparative analysis shows that the LCL grid-connected inverter is comprehensively optimized in terms of filtering performance and control stability.of.A set of calculation steps for calculating LCL grid-connected filter parameters is proposed,and a damping control strategy based on capacitive current proportional feedback is studied.Three control objectives are defined for the problem to be solved in this paper.A three-phase four-wire three-level grid-connected converter integrated solution based on LCL grid-connected filter is determined.Different command current calculation strategies under different targets are derived,and the command current is tracked in the two-phase stationary coordinate system using the PR controller outer loop and the capacitor current proportional feedback inner loop.The simulation by MATLAB/Simulink shows that the system designed in this paper can achieve all the defined control objectives,and has important reference significance for improving the stability operation and power quality control of the system under the imbalance of microgrid.Finally,from the perspective of realizing system control performance and considering security and versatility,the top-level design of system hardware and software is given,and the main circuit structure diagram and program flow chart are given.