Study On The Protection Scheme Of High Voltage Direct Current Transmission Line Based On Current Propagation Characteristics

High Voltage Direct Current(HVDC)technology has the advantages of large transmission power,long transmission distance and good economic benefits,and it plays an increasingly important role in long-distance large-capacity power transmission.There are a large number of DC transmission lines,and it has complex working environment and high failure rate.Therefore,reliable protection of DC transmission lines is of great significance to ensure the safe and stable operation of the power system.However,the HVDC system line protection in the actual project has certain problems:(1)The traveling wave protection used in the existing projects has a weak ability to withstand the transition resistance,which leads to insufficient reliability.(2)The principle of longitudinal differential protection cannot reflect the transient characteristics of the fault,resulting in insufficient speed.Therefore,in response to the above two types of problems,this thesis focused on the impact of transmission system on the fault current,with the goal of improving the reliability and speed of the existing protection schemes,and carried out research on HVDC line protection scheme based on transmission characteristics.The main tasks of the thesis are as follows:(1)The configuration schemes of Siemens traveling-wave protection and longitudinal differential protection carried in existing projects are introduced,and the simulation model is built according to the actual protection configuration scheme.The operation of traveling wave protection and longitudinal differential protection under various types of faults in existing projects are analyzed by simulations.The problems of insufficient adaptability of travelingwave protection and longitudinal differential protection are analyzed theoretically.(2)Aiming at the problems of low accuracy of fault identification and poor performance under transition resistance of traveling wave protection for HVDC transmission lines,a nonunit protection method based on fault current propagation characteristic is proposed.Based on the equivalent circuit of HVDC transmission system,the propagation characteristics of fault current from the commutation side to the line,from the line to the commutation side and on the line are analyzed respectively,and then the differences of fault current characteristics on the two sides of the line boundary of the rectifier station are analyzed when fault occurs at different locations.Based on this,the identification criterions of internal and external faults are constructed by characteristic frequency band current,and the non-unit protection scheme of DC line is designed.The simulation results based on PSCAD/EMTDC indicate that the proposed protection method can accurately identify internal and external faults,and has good performance under high transition resistance and noise interference.(3)Aiming at the problem that the existing HVDC transmission line pilot protection is greatly affected by distributed capacitance,which leads to slow operation speed and system locking,a HVDC transmission line directional pilot protection scheme which is not affected by distributed capacitance is proposed.By analyzing the transmission characteristics of the mode currents,it is obtained that the distributed capacitance has a small influence on the aerial mode current.Based on the mode impedance of the transmission line,the characteristic of the current amplitudes at both sides of the line under internal and external fault are derived respectively.Taking the difference in the characteristic of the amplitude ratio of the aerial mode current fault component at the rectifier and inverter measurement points as the protection criterion,a fault identification scheme was constructed.And the theoretical analysis provides a basis for the selection of the protection threshold.Simulation results show that the proposed protection scheme can reliably identify internal and external faults,and the protection result is not affected by distributed capacitance.It has low requirements on sampling and communication,and has strong resistance to transient resistance and noise.