A current differential protection using time-domain waveform comparison for HVDC transmission lines

High voltage direct current (HVDC) differential protection is affected by distributed capacitance current and unsynchronized two ends data. A novel current differential protection unaffected by unsynchronized data is proposed. Based on an analysis of fault characteristics with transmission line two ends unsynchronized data, the data synchronization status of external faults can be identified by comparing differential current under a distributed parameter model in time domain. For external faults, free from the influence of fault point current, differential current reaches its minimum value when data is synchronized and augmented significantly when data is unsynchronized. For internal faults, with the influence of fault point current, differential current always has great value regardless of synchronization status. The criterion of the proposed approach is composed under the distributed parameter model in time domain, and data unsynchronized time of external faults is identified by using the characteristic above, which is used to adjust operation current along with the threshold to prevent false tripping. Simulation results have verified that the new approach can identify high resistance faults reliably and sensibly. The proposed approach is not affected by distributed capacitance current nor does it require precise synchronization of two ends data.