To address the lack of backup protection scheme of current flexible DC transmission lines and the failure of protection scheme under the low signal-to-noise ratio. In this paper, a similarity measure based on the combination of wavelet threshold noise reduction and improved edit distance algorithm is proposed. According to the characteristic of large difference between noise signal and fault signal wavelet coefficients, the fault traveling wavelet signal is pre-processed with noise reduction by wavelet noise reduction method. When external fault occurs, the DC transmission line satisfies the invariance of traveling waveform characteristics, and the similarity of traveling waveform on both sides of the line is high. And the similarity is lower for internal faults. According to this characteristic, an improved edit distance algorithm is proposed to realize the calculation of the similarity of the traveling waveforms of faults on both sides of the line and establish the criterion for identifying faults internal and external of the flexible DC transmission line. Through simulation, it is verified that the algorithm can accurately identify internal and external faults, and has stronger resistance to noise interference and transition resistance compared with EDR algorithm.
 

wavelet noise reduction,flexible DC transmission,traveling wave longitudinal protection,edit distance