Fig.1 Topology of the four-terminal MMC flexible DC grid
Analysis of Waveform Characteristics of Current Fault
3.1 Fault interval identification of multi-terminal DC grid
When the system is in normal operation, the current waveform changes regularly, and the load current is traversing current. It is defined that the positive direction of the current is from the converter station bus to the DC line. Because the current on the DC side has only polarity and no phase, the current waveform of the converter station at both ends has no change and the polarity is positively correlated.
Combined with the four-terminal MMC flexible direct current grid topology, it is still assumed that a positive ground fault (f1) occurs between B3 and F1 stations, and the difference of current fault waveforms between fault lines (L3) and non-fault lines (L2 and L4) is analyzed. In the dashed line interval shown in Fig.1, the equivalent diagram of the fault line is shown in Fig.2.