As can be seen from Table 5, when high-resistance faults occur on DC lines, the values of D1 at the fault points f1, f2, f3 and f4 are all less than smaller than the setting value of Dset2.

Therefore, according to the protection criterion, the faults are judged to be internal faults.

Combined with Table 4 and 5, the proposed protection criteria can accurately identify internal and external faults. In the case of a high transition resistance at the end of the line, when the fault occurs in the DC line, the protection can also operate reliably and remove the high resistance fault quickly. When the fault occurs outside the DC line, the protection will operate correctly.

In order to explain more clearly the question, combining Tables 4 and 5 above, the results of the internal and external fault identification can be obtained, as shown in Fig.9. In Fig.9, f1 (10%) represents the simulation result of a fault f1 occurring 10% distance of the total line from rectifier side, similar for other fault types.