A new line protection for HVDC systems using the ratio of currents of a DC filter and smoothing reactor is presented. Characteristic analysis on a DC filter shows that it behaves like an inductor at a high frequency band whose inductance is far less than the sum of smoothing reactor inductance plus converter equivalent inductance. Based on this characteristic

fault analysis shows that during high-frequency DC line faults (on either side)

the DC filter current amplitude significantly exceeds that of the smoothing reactor

with a current ratio much greater than 1. Whereas for external faults

near faulty end

DC filter current amplitude is less than the one of a smoothing reactor and current ratio at this end is smaller than 1

while the current ratio at the distance faulty end is still far greater than 1. The effectiveness of the protection is verified by a large number of simulation experiments. It does not need synchronization of signals at both ends. In addition

it has high sensitivity for high-resistance faults

and is insensitive to the length of DC transmission line and power flow change.