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.