Protection Schemes For Hvdc Tranmission Lines Based On Traveling Waveform Similarity And Time Difference Between Line Mode And Eatth Mode Of Polarity Wave

Compared with AC system,HVDC has the characteristics of large transmission power,long distance,narrow transmission corridor,low cost and flexible control mode,which is suitable for long-distance and large capacity transmission.HVDC transmission lines have a long distance and need to cross different terrain and climate.Poor working conditions may make protection failure.The traditional HVDC transmission line protection is affected by high resistance ground fault,information unsynchronization,abnormal sampling value and other factors.Therefore,the study of HVDC transmission line protection algorithm to improve the performance of line protection plays an important role in the safe and stable operation of the power system.In this paper,protection schemes for HVDC transmission lines based on traveling waveform similarity and time difference between line mode and earth mode of polarity wave are studied.In this paper,the propagation characteristics and traveling wave dispersion of fault traveling wave in HVDC transmission line are discussed,and the reflection and refraction of traveling wave at the point of wave impedance discontinuity are introduced.According to the fault characteristics of DC line,the existing traveling wave protection principles are expounded.A HVDC transmission line protection scheme based on improved DTW(dynamic time warping)traveling waveform similarity is studied.Based on the traveling wave propagation theory and the characteristics of traveling wave reflection and refraction,the similar characteristic of traveling waveform on both sides of the line at internal and external fault is studied.This paper introduces the DTW algorithm and analyzes its tolerance to out-of-sync and bad data.In order to improve the synchronicity and bad data tolerance of DTW algorithm,an improved DTW algorithm is proposed to calculate the improved DTW distance between traveling waves on both sides and establish the corresponding protection criterion for identifying internal and external fault.The voltage gradient operator is used as the starting criterion to detect the change of the traveling waveform.The tian-guang HVDC model is built in PSCAD/EMTDC,and a large number of simulation experiments are carried out to verify the effectiveness of the proposed protection scheme,which has strong synchronization resistance and bad data resistance,as well as a certain resistance to transition resistance.A single-end adaptive protection scheme for HVDC transmission line with polarity wave of time difference between line mode and earth mode is studied.Firstly,this paper introduces the boundary constitution of HVDC transmission line and the filtering effect of the boundary on the polarity wave,then this paper analyzes the characteristics of the wave velocity and frequency variation of the line mode and earth mode components and their influence on the fault polarity waveform and points out the reason for the decrease of the sensitivity of traditional traveling wave protection under high resistance fault.Using time difference between line mode and earth mode of polar wave,the slope of polarity wave is obtained by first order function fitting within the time difference.Through the introduction of a wave amplitude variation as the auxiliary criterion,a adaptive HVDC transmission line traveling wave protection scheme is proposed.Finally,this paper analyzes dead zone and influence factors of the proposed protection scheme.The HVDC simulation model is built in PSCAD/EMTDC,and the effectiveness of the proposed protection scheme is verified by simulation experiments.It has a strong resistance to transition resistance and a certain resistance to noise.