Fig.3 Additional network diagram when an internal fault occurs
From Fig.3 (a) can be seen, when a positive ground fault (f 1) occurs in the DC line, the line currents at both ends flow from the converter station to the fault point. Protection installation at 4 and 5 positive pole waveform polarity taken is positive, the waveform is positively correlated, the fault waveform distance is small, less than the setting value.
From Fig.3(b) can be seen, when a negative ground fault (f 2) occurs in the DC line, the line currents at both ends flow from the fault point to the converter station. Protection installation 4 and 5 negative pole waveform polarity taken is negative, waveform is positively correlated, the fault waveform distance is small, less than the setting value.
From Fig.3 (c) and (d) can be seen, when a double pole across the fault (f 3) or double pole ground fault (f 4) occurs in the DC line, the currents at both ends of positive line flow from the converter to the fault point, the polarity of the waveform taken is positive; the currents at both ends of negative line are flowing from the fault point to the converter, the polarity of the waveform taken is negative. Positive and negative waveforms are positively correlated, and the current fault waveform distance is smaller than the setting value.
Therefore, from the above analysis can be obtained that when an internal fault occurs, the fault pole current waveform at protection installation 4 and 5 will change, the current fault waveform of both sides is positively correlated and the similarity is high. 4 and 5 fault current polarity and similarity are shown in Table 1.
Table 1 Polarity and correlation of the current fault component at the protection installations of 4 and 5 when an internal fault occurs