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Ultra-high-voltage transmission raises high demand for ultra-high-speed operation of relay. In this study, the fault information contained in travelling wavefront is analysed on the basis of frequency-dependent characteristic of transmission line parameters and broadband characteristic of the fault waveform. It can be concluded that the shape change of the fault mode current wavefront is related to the fault distance and the magnitude of the fault mode current wavefront is related to the transition resistance. Hence, a principle of ultra-high-speed protection for DC transmission line based on waveform inversion transformation is proposed. The simulation verifies the speediness and reliability of the principle.