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.This work is supported by National Natural Science Foundation of China (No. 51177128

No. 51477131) and Ph.D. Programs Foundation of Ministry of Education of China (No. 20110201110056).