Direction Protection Based on Time-domain Travelling Wave for Flexible MTDC Grid

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 $\Delta i$ and $\mathrm{d}i/\mathrm{d}t$ 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.