Research On DC Feeder Protection Of Flexible DC Traction Power Supply System

The traditional urban rail transit DC traction power supply system uses a 24-pulse diode rectifier unit,and its energy flows in a single direction,so the regenerative braking energy of the locomotive can’t be fully utilized;at the same time,the traction network voltage tends to fluctuate in the uncontrolled rectification mode,and renewable energy grid connection is difficult to achieve.In response to our country’s major strategic goal of "carbon peaking and carbon neutrality" and promoting sustainable social development,this dissertation proposes a flexible DC traction power supply system based on bidirectional inverters.Urban rail transit bears the heavy responsibility of relieving traffic pressure,so the safe operation of traction power supply system is particularly critical.Based on the flexible DC traction power supply system,this dissertation conducts indepth research on its operating characteristics,fault mechanism,fault characteristics,and DC feeder protection methods.First of all,this dissertation designs the device selection and control strategy of the bidirectional converter of the flexible DC traction power supply system in detail.Combined with the operating parameters of Beijing Metro Line 13,the model building work of flexible DC traction system,locomotive transmission system is completed in MATLAB/Simulink software.Through a large number of simulations and compared with the traditional DC traction power supply system,the performance of the built model is verified.Secondly,the fault types,fault mechanisms and fault characteristics of the DC side short-circuit fault of the flexible DC traction power supply system are analyzed.The DC feeder protection configuration used in subway engineering and the two main protection principles of high current trip and DDL(current rise rate and current increment)are introduced.Combined with the simulation data,the insufficiency of DDL protection is analyzed from the perspectives of locomotive starting current,over-segment charging current and changes in fault characteristics of the new system.In order to solve the problem that remote fault current or non-metallic fault current is indistinguishable from locomotive starting current and over-segment charging current,this dissertation proposes a single-ended quantity protection method based on timefrequency domain characteristics of transient energy increment.This method constructs the Teager energy protection criterion by decomposing the transient energy increment signal variational mode(VMD)and demodulating it with the Teager operator,which can achieve the purpose of quickly capturing the abrupt change characteristics at the fault moment.The simulation results show that the method has a significant effect on removing all kinds of middle and remote faults.Finally,this dissertation designs the subway 5G communication scheme.On the basis of realizing the data exchange of locomotive starting current and DC feeder current,a new principle of multi-terminal current differential protection based on time series similarity measurement is proposed.This method realizes data dimension reduction and symbolic processing by an improved Symbol Aggregation Approximation(SAX)algorithm.In order to overcome the influence of communication delay and jitter,the Dynamic Time Warping(DTW)algorithm is used to measure the similarity of symbol sequences.The simulation results show that the proposed protection method has the advantages of strong anti-noise capability,good quickness in identifying middle and remote faults,and high reliability.In conclusion,this dissertation builds a simulation model of a flexible DC traction power supply system,and verifies its effectiveness;the deficiencies of existing DDL protections are analyzed.In view of the problem that the remote fault current is difficult to distinguish from the starting current of the locomotive,two new DC feeder protection methods with improved performance are proposed.