Research On Transient Fault Characteristics And Protection Principle Of Flexible DC Grid Based On Marti Model

Flexible DC technology is increasingly applied to new energy grid connection,longdistance power transmission,isolated power supply and AC system asynchronous interconnection.Flexible DC grid will become an important part of smart grid.The fast developing speed of DC fault and fragility of power electronic device make high requirement for DC protection speed.The reliability of DC protection is facing severe challenges along with the protection speed increasing.Fast and reliable identification of DC fault is a key problem in the development of flexible DC grid.Fault analysis is an important basis for fault diagnosis,protection principle and protection threshold setting.Therefore,it is of great significance to study the fault characteristics and protection principle of flexible DC power grid for its development.This paper make flexible DC power grid based on modular multilevel converter(MMC)with half bridge sub model(HBSM)as the study object.The developing process of DC fault was researched with modal analysis method.The equivalent circuit of MMC was built based on physics equivalent principle.The Marti equivalent circuit,reflecting frequency dependent distribution parameter characteristics of the power line,was chosen as the best equivalent circuit of power line,according to the principle of equivalent circuit selection and the accuracy requirement of transient fault analysis of flexible dc power grid.As to symmetrical bipolar flexible DC system with different grounding model,fault boundary conditions were analyzed before MMC blocking under DC fault.The fault characteristics of flexible DC grid were analyzed by solving the equations of Marti equivalent circuit and boundary conditions.The accuracy of the analysis method was verified through simulation contrast verification,supplying accurate information for the protection principle study of flexible DC grid.Integrating the speed of traveling wave protection and reliability of longitudinal differential protection,a new principle of longitudinal traveling wave differential protection was proposed,based on the traveling wave theory,Marti model and fault boundary conditions.The proposed principle use reverse traveling wave differential as the protection criterion,which can effectively shorten the effect of communication delay on longitudinal protection.For symmetrical bipolar system with earth return and metallic return,the protection strategy use the decoupled sequence components to determine fault types and identify fault lines accurately.Through large amount of simulation on PSCAD/EMTDC,it shows that the proposed principle satisfies the fast protection requirements of flexible DC grid,with the advantages of quick and reliable identification of DC line faults,high fault resistance tolerance,and strong antiinterference ability.It can withstand noise interference of less than 5%,measurement error of less than 1% and all of the lightning interference.