Abstract: Most existing protection schemes for flexible DC transmission grid lines rely on boundary components for implementation, which makes them difficult to adapt to boundary-free grids with current-limiting reactor installed at the outlet of the converter station. To address this problem, this paper firstly analyzes the transmission and attenuation characteristics of the normal pole voltage traveling wave and the fault initial traveling wave, and the mathematical expressions for the normal pole voltage traveling wave integral value λ and the initial traveling wave domain frequency component ω_domi are constructed. Then, the fault characteristics of λ and ω_domi are analyzed in depth, and the study finds that their complementarity enables fast full-line protection for boundary-free DC grids. Based on the fault characteristics of the two parameters, a fast traveling wave protection method for flexible DC transmission grids independent of boundary components is proposed. In principle, this method does not rely on the filtering effect of current-limiting reactors, and the protection threshold setting has a clear theoretical basis. Finally, the reliability and applicability of the proposed method are verified based on the PSCAD simulation platform. Simulation results indicate that the proposed method has good selectivity in DC grids with different boundary structures, and can still operate accurately under 1 kΩ grounding faults, and has a strong ability to withstand transition resistances.