Research On Fault Location Of Flexible DC Distributiongrid

With the extensive development of power electronics technology and the improvement of new energy consumption demand,flexible DC distribution network has gradually become a research hotspot.Compared with the traditional AC distribution network,the flexible DC distribution network based on modular multilevel converter(MMC)has the advantages of low loss,high power quality and flexible control.At present,the completed flexible DC distribution projects mostly use cable lines with lower operation and maintenance cost and higher reliability to transmit electric energy.Although this can improve the power supply reliability of distribution network,most of the faults of cable lines are permanent faults.And the cable lines in the city are mostly laid underground,which makes troubleshooting difficult.Therefore,the research on the accurate fault location technology of flexible DC distribution network is of great significance to ensure the rapid recovery of power supply after fault,improve the reliability of system power supply and reduce the maintenance cost.Firstly,this dissertation expounds the development process and research background of flexible DC distribution network,and points out the main technical challenges faced at present.Among them,fast and accurate positioning after fault is one of the bottlenecks restricting the rapid development of flexible DC distribution network.According to the different fault location principles,this paper introduces the research status of fault location technology of flexible DC distribution network at home and abroad,summarizes the defects and shortcomings of existing location technology,and explains the research value and significance of this subject.Then,the basic working principle of flexible DC distribution network is studied,and the topology and control principle of MMC are introduced.On this basis,the main topology and grounding mode of flexible DC distribution network are analyzed.Taking AC grounded and DC ungrounded systems as the research object,the transient characteristics of fault current and voltage under single pole grounding fault and double pole short-circuit fault are analyzed.The correctness of the theoretical analysis is verified by building a simulation model,which provides a theoretical basis for the research of fault location.Next,a fault location method based on active current control is proposed for two terminal asymmetric,"hand-in-hand" flexible DC distribution network.Using the high controllability of hybrid MMC,the post fault current is designed into two stages: stage I and stage II.Phase I mainly realizes the rapid suppression of fault current and provides conditions for the smooth transition of phase II.Phase II realizes accurate fault location by injecting high-frequency reverse sinusoidal current with specific frequency.The design principles of each stage and the positioning principle of stage II are studied respectively,and the corresponding simulation model is built in PSCAD / EMTDC to verify the positioning performance of the proposed method.The simulation results show that the method can determine the fault location within10 ms and has high positioning accuracy.Finally,for the two terminal symmetrical flexible DC distribution network based on hybrid MMC,a two terminal fault location method based on current adaptive control is proposed.Based on the detailed derivation of the fault location expression based on the two terminal method,the current derivative calculation error and numerical instability of the traditional two terminal method are analyzed.The adaptive slope attenuation of fault current is proposed to correct the current derivative,and the weighted linear regression analysis of the numerator and denominator of the traditional two terminal positioning expression is carried out to improve the numerical stability.The simulation results show that this method can quickly determine the fault location without DC line isolation,and has high positioning accuracy.