Fig.4 Additional network diagram when an external fault occurs
As can be seen in Fig.4, when an external fault occurs beyond the line, protection devices are installed at 4 and 5 positions. The fault currents are flowing from the converter station to the fault point. However, for different types of faults, the polarity of the fault current waveform taken by the converter station on both sides will then change.
From Fig. 4 (a) and 4 (d), it can be seen that when the fault occurs atf 5 or f 8 point, the polarity of the positive and negative fault current waveforms at 4 locations is negative, and the polarity of the positive and negative fault current waveforms at 5 locations is positive. From Fig. 4 (b) and 4 (c), it can be seen that when the fault occurs atf 6 or f 7 point, the polarity of the positive and negative fault current waveforms at 4 location is positive, and the polarity of the positive and negative fault current waveforms at 5 locations is negative.
Therefore, from the above analysis it can be obtained that when an external fault occurs, two current fault waveforms taken at the protection device of 4 and 5 positions are negatively correlated, the fault waveform distance is larger than the setting value, similarity is low. 4 and 5 fault current polarity and similarity are shown in Table 2.
Table 2 Polarity and correlation of the current FAULT component at the protection installations of 4 and 5 when an external fault occurs