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 ωdomi are constructed. Then

the fault characteristics of λ and ωdomi 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.