Research On Control Strategy Of Four Leg Grid Connected Inverter Under Asymmetric Fault In Microgrid

Complex source load uncertainty increases the risk of short-term fault in microgrid operation,so the fault ride through capability of micro source inverter is an important guarantee to realize fault tolerant operation of microgrid.Three phase four leg inverter is widely used because of its low capacitance demand,high utilization of DC voltage and excellent unbalanced load capability.As the core of this thesis,the control strategy of three-phase four bridge inverter under asymmetric fault is studied.Firstly,the concept of the microgrid has been introduced in this thesis as well as its development history.And the thesis focuses on the structural characteristics,application occasions and related modulation strategies of three-phase four leg inverter,and expounds the control strategy of micro source inverter which is mature at this stage.Secondly,the fault characteristics of microgrid by using the theory of symmetrical component method has been analyzed in this thesis.Considering that the single-phase load in the independent microgrid accounts for a large proportion and the probability of asymmetric fault is high,this thesis will focuse on it.Through the establishment of mathematical model for theoretical analysis,the characteristics of microgrid under various fault conditions are summarized.Through the establishment of mathematical model for theoretical analysis,the characteristics of micro grid under various fault conditions are summarized.Thirdly,the mathematical characteristics,modulation mode,control mode and special control strategy under asymmetric fault of three-phase four leg inverter are analyzed in turn: The mathematical models of three-phase four bridge inverter in static coordinate system and synchronous rotating coordinate system,as well as zero sequence component and zero sequence component sequence model are established.Through three models,the characteristics of three-phase four bridge inverter in different coordinate systems are introduced,and the final control strategy is determined.This thesis briefly analyzes several modulation ideas of three-phase four bridge inverter,and focuses on the three-dimensional space vector modulation calibration system based on abc coordinate system,and the open-loop operation of four leg inverter is realized by building a module in Simulink software.The concept of droop control is introduced,and the limitation of the control strategy under asymmetric fault is put forward.Combined with the concept of symmetrical component,the traditional droop control strategy is improved through the module for separating positive and negative sequence based on double synchronous rotating coordinate system,which has been verified by simulation;a variety of zero sequence control methods are compared and a new control strategy is proposed to improve the split sequence control,and the improved split sequence control is combined with the improved droop to achieve the best control effect,and the effectiveness of the control scheme is verified by simulation Sex.At the end of the thesis,the experimental platform of three-phase four leg inverter based on TMS320F28335 is designed and built,and the related principles and design steps of software design and hardware design are introduced in detail.The control performance experiment of three-phase four leg inverter under unbalanced fault condition is completed.Experimental results show that the proposed control method can improve the fault ride through capability of three-phase four leg inverter.