A novel protection for HVDC transmission lines based on current ratio of DC filter and smoothing reactor

A new line protection for HVDC system using ratio of currents of DC filter and smoothing reactor is presented. Characteristic analysis on DC filter shows that it behaves like an inductor at 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 for DC line faults, for either side of DC line, at high frequency, DC filter current amplitude far exceeds that of smoothing reactor, and the corresponding current ratio is far greater than 1. Whereas for external faults, near faulty end, DC filter current amplitude is less than that one of 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 a high sensitivity for high-resistance fault, and is insensitive to the length of DC transmission line and power flow change.