This paper analyzes the influences of the attenuation of high frequency signals by the HVDC transmission lines and line boundaries on the current HVDC transmission line protections. The frequency characteristics of boundary and line are also studied. The traditional traveling wave protection approaches based on the derivative of electrical quantity generally have poor tolerance to transition resistance. To solve this, a single?ended protection method based on the second?order micro?component of electrical quantity is proposed, where the transient fault component information after the second?order differential transformation will be amplified, and the difference of second?order differential variables inside and outside the boundary are utilized to judge the fault. In the proposed DC protection scheme, full line protections can be realized only by the single?ended fault transient information. The simulation analysis is carried out in PSCAD/EMTDC, and the results show that this criterion can accurately distinguish internal and external faults, and select fault poles, which indicates its strong ability to withstand the influence of transition resistance and long transmission lines, thus can realize full?line quick action.