A pilot protection scheme for UHVDC lines based on backward traveling-wave difference

As a backup protection for HVDC lines, legacy current differential protection needs long time delay to avoid the influence of distributed capacitor current under external faults. A new protection principle is proposed based on the fact that under internal faults, the backward traveling wave at the side away from the fault point and the forward traveling wave at the side near the fault point will not satisfy the transmission function of the DC line, whereas the backward traveling wave at the side away from the fault point and the forward traveling wave at the side near the fault point satisfy the transmission function under external faults. The proposed principle is verified in PSCAD/EMTDC, and simulation results show that, it can identify the fault reliably and rapidly, while having better tolerance to high fault resistance.