Fault direction identification is important for dc protection

which cannot only be used in directional pilot protection

but also plays an important role in non-unit protection (for excluding backward faults). Direction criteria based on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta i$ and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{d}i/\mathrm{d}t$ have been utilized in LCC-HVDC system extensively. However

the reliability and sensitivity of the protection are significantly influenced by fault transition resistance and line distribution capacitors. In a flexible MTDC grid

boundary characteristics of the DC inductor can be utilized to judge the fault direction. But when the DC inductor is only installed at converter outlet

this kind of direction protection cannot work. Therefore

this paper analyzes the frequency characteristic of the fault traveling wave (TW) first. On this basis

the direction protection based on the time-domain TW is proposed

which is immune from frequency-dependence line parameters. Compared with existing direction protection

it has higher operation reliability and sensitivity

and is independent of the line boundary. Finally

the feasibility and superiority of the designed direction protection are verified by simulation cases based on PSCAD/EMTDC.