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Protection of direct current (DC) transmission lines is one of the key difficulties to be urgently solved in the construction of the future voltage-sourced converter (VSC)-based DC grids. In this paper, a novel ultra-high-speed traveling-wave (TW) protection principle for DC transmission lines is proposed which is based on characteristics of modulus voltage TWs. First, the absolute value of the change in amplitude of the 1-mode voltage TW is used to construct the protection starting-up element. Then, the dyadic wavelet transform is utilized to extract the wavelet-transform modulus maxima (WTMM) of 1-mode and 0-mode initial reverse voltage TWs separately, which are used for fault section identification and selection of fault line successively. A four-terminal annular VSC-based DC grid electromagnetic transient model is established in PSCAD/EMTDC, and performance of the proposed novel ultra-high-speed TW protection principle is evaluated. Extensive simulation results under different fault conditions show that the proposed ultra-high-speed TW protection principle is excellent in rapidity, reliability and robustness. Therefore, the proposed novel ultra-high-speed TW protection principle in this paper can be adopted as an outstanding main protection for VSC-based DC grids.