The voltage source converter(VSC)-based high voltage direct current(HVDC)grid technology is an effective means for large-scale integration of clean energy. The fast DC line protection technology is one of the main challenges to HVDC grids. Based on the Zhangbei HVDC grid project

this paper proposes a fast protection scheme for overhead transmission lines of bipolar VSC-based HVDC grids equipped with DC circuit breakers. The protection is based on local measurements featuring fast response speed. The voltage gradient is used to detect faults rapidly

and the voltage derivative criterion is adopted to identify the protection zone in which the fault occurs thanks to the smoothing effect of the fault current limiter. And the faulty pole is identified according to the different propagation characteristics between the line mode and zero mode voltage

ensuring the selectivity of the protection as well as high reliability. The Zhangbei HVDC grid model is built in PSCAD/EMTDC and the performance of the proposed protection scheme is verified by simulation results of various fault conditions

as well as different operation modes of the HVDC grids. This work is supported by National Natural Science Foundation of China(No. 51577148)and National Key Research and Development Program of China(No. 2016YFB0900603).