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.