Pilot protection for transmission lines of flexible DC grid based on voltage reflected traveling wave similarity

• The time domain expression of voltage reflected traveling wave in flexible DC grid when internal and external fault is derived by mathematical method. The time domain expressions of the voltage reflected traveling waves on both sides of the line are slightly different when internal fault, but are significantly different when external fault. • Constructing similarity quantification index using Bray-Curtis distance and Spectral angle. • The weight factor is set to reduce the influence of the rising edge of the fault traveling wave on the similarity. There are problems such as dead zones in the traveling wave protection range based on wave-form characteristics and limited ability to tolerate transition resistance. In this paper, pilot protection for transmission lines of flexible DC grid based on voltage reflected traveling wave (VRTW) similarity is proposed. Firstly, the time domain expression of VRTW on both sides of the line is derived during internal and external fault. From the mathematical analysis, it is proved that VRTW similarity is high during internal fault. And the similarity is low during external fault. In order to extract the differences in VRTW wave-form characteristics, the Levenberg-Marquardt algorithm is introduced for data fitting. Secondly, the wave-form similarity quantification index is constructed using Bray-Curtis distance and Spectral angle. By setting the weight factor, the impact of the rising edge of initial traveling wave on the index can be reduced. Finally, simulation results show that the proposed protection scheme can act quickly and accurately. The proposed protection has higher resistance to transition resistance and anti-noise ability. It has low requirements on sampling frequency and data synchronization.