Research On Grounding Mode And Fault Line Selection Of Flexible Medium Voltage DC Distribution Network

In recent years,the rapid development and wide application of power electronic technology has made the trend of distributed power access,power supply and load direct current,etc.obvious.New energy such as wind energy and solar energy can be connected to the grid as distributed energy sources to improve energy utilization and reduce environmental impact,but it will affect the control methods and relay protection of the existing AC distribution network,while the DC distribution network is easier to distribute Because of this,it has received extensive attention in recent years.Firstly,analyze the topological structure and mathematical model of the DC distribution system,determine that the topological structure of the DC distribution network is radial,and explain its voltage level,power supply radius,converter structure,DC transformer structure,and grounding method.The control mode of the inverters on both sides of the DC power distribution system is described.A ±12kV flexible medium-voltage DC distribution system model was established in PSCAD.In the model,the VSC voltage source converter was used to achieve AC-DC conversion,and the DAB DC transformer was used to achieve DC voltage conversion and stabilize the DC distribution system.Run simulation analysis.Secondly,in order to make the flexible medium voltage DC distribution network have a certain fault ride-through capability,the grounding method of the neutral point of the converter transformer and the neutral point of the DC side capacitor of the converter is studied.When unipolar grounding occurs on the DC side of the distribution network When a fault occurs,a fault model is built,and the influence of different wiring methods on the transient characteristics of the fault is analyzed through the model.The theory and simulation are combined to determine the Yn/△ connection of the converter transformer and the split midpoint grounding of the DC side capacitor of the converter.Finally,the fault interval of the DC distribution network is divided,and the fault characteristics of the unipolar ground fault and the bipolar short circuit fault in the DC distribution network are further analyzed,and the identification criteria for the unipolar ground fault and the bipolar short circuit fault are proposed.For the magnitude of the fault current of each distribution line,a line selection method based on the transient current fault component is proposed.