For a mesh DC power grid with current-limiting reactor installed at the outlet of a converter station, there is no boundary between the lines, and protection based on the boundary effect is no longer achievable. In addition, traditional differential protection has problems with long delays and strict communication synchronization requirements. Therefore, a pilot protection scheme based on the sign of the resistance component in measured impedance is proposed. Through analysis of the Peterson equivalent circuit of a mesh DC power grid, it is found that when an internal fault occurs, the signs of resistance component in measured impedance at both ends of the line are the same, while when an external fault occurs, the two signs are opposite. According to this characteristic, the real part of the measured impedance is extracted by continuous wavelet transform (CWT), and a protection criterion is constructed. Theoretical analysis and simulation results show that the proposed protection scheme can identify various types of HVDC line faults reliably and quickly, and can withstand high resistance and strong noise, without the need for strict synchronization of double-end data.