Transient protection method, system and device for extra-high voltage direct current transmission line

The invention provides a transient protection method, system and device for an extra-high voltage direct current transmission line. Aiming at the problems that the wavelet packet algorithm has frequency aliasing and the decomposition speed of characteristic signals is low, the invention provides an improved wavelet packet transformation algorithm, fault voltage and current are decomposed, and an abnormal frequency spectrum component is constructed as a criterion of unipolar and bipolar faults. In addition, the method comprehensively considers that the amplitude change of the voltage and current of the monitoring point after the fault is utilized to construct the criterion of fault starting, and then the criterion of internal and external faults is constructed by utilizing the voltage ratios delta in and delta ex under different frequencies. According to the method, the fault protection of the extra-high voltage direct current transmission line can be quickly and accurately realized, the phenomena of misoperat