At present

the travelling wave principle based DC line fast protection used in high voltage direct current(HVDC)/ultra-high voltage direct current(UHVDC)projects has the shortcoming of low ability of enduring transition resistance. And the differential principle based slow-speed protection for DC line grounding fault with high transition resistance usually trips slowly. In the AC line protection

a pre-acceleration used in near power side can trip fast without any selection. Then this non-selective trip can be rectified by reclosing breaker. This paper introduces the concept of pre-acceleration into DC line fast protection and aims to fast protect DC line under grounding fault with high transition resistance. The DC line pre-acceleration can trip fast without any selection

and then this non-selective trip can be rectified by several measures such as blocking converter. This paper analyzes several problems after introducing pre-acceleration into DC line protection

and estimates the value of transition resistance under the fault. The simulation verification shows that the DC line protection with pre-acceleration level can trip and have a margin to distinguish the fault in outside zone within 2 ms under line-end grounding fault with 500 Ω transition resistance.