(1. Key Laboratory of Smart Grid of Ministry of Education(Tianjin University), Tianjin 300072, China; 2. NR Electric Co. Ltd., Nanjing 211102, China)

Direct current(DC)line protection and fault handling technology are the key technical difficulties of the multi-terminal flexible DC gird due to the fault characteristics including high-speed rising of the fault current and the lack of natural zero crossing point. The particularity of the DC line protection in the multi-terminal flexible DC grid is analyzed. Considering the protection principle of the line commutated converter based DC system and the development of the protection for the point-to-point flexible DC system, this paper discusses the developing trend of the line protection for the multi-terminal flexible DC grid. Meanwhile, different kinds of the DC fault isolation are introduced, further developing direction is also discussed considering the fault isolation capability, investment and the coupling influence between the protection and control. Finally, a novel reclose strategy of the DC fault on overhead line for the point-to-point DC system is proposed, which does not lead to the second overcurrent damage during permanent faults. Afterwards, the performance requirements for the reclose strategy of the multi-terminal DC grid are discussed. This work is supported by National High Technology Research and Development Program of China(863 Program)(No. 2015AA050101).