Fig .2 A fault line equivalent diagram when a fault at f1 position
between B3 and F1 occurs
As can be seen from Fig.2, when a fault of f1 position
occurs between the B3 station and the F1 station, the current waveform
and polarity of the six protection measurement installation points will
change, and the current of each measurement point will flow to the fault
point. Therefore, the positive and negative correlation of the current
measured at each line protection measurement installation point will
change accordingly. The current fault component waveforms extracted from
the two protection measurement installation points (4 and 5) on the
fault line have a large change and they are positive correlation, and
the distance between the two current fault component waveform is smaller
than the preset setting value. While the current fault component
waveforms extracted from the non-fault interval protection measurement
installation point (2,3 or 6,7) have a small change and they negative
correlation, and the distance between the two current fault component
waveform (2,3 or 6,7) is larger than the preset setting value.
Therefore, based on the difference of fault characteristics, it is
determined whether the fault occurs between the B3 and the F1 station,
that is, on the line L3. If the fault occurs on the line, the
corresponding protection element is activated.
3.2 Internal and External Fault Characteristic Analyses
The physical boundary of a DC line is defined as the flat wave reactor
on that line and the fault is classified as internal or external fault.
After determining the interval of the converter station where the fault
occurred, it is further determined whether the fault occurred on the DC
transmission line, i.e., the internal and external fault characteristics
are carried out. The type of fault is determined by the setting values
and comparing them with the collected current signals.
Therefore, combined with the superposition theorem, the fault additional
network can be obtained as shown in Fig. 3 and 4. UP4 and UN4 are the
positive and negative transient voltage components at the 4th protection
measurement installations respectively, UP5 and UN5 are the positive and
negative transient voltage components at the 5th protection measurement
installations respectively. iP4 and iN4 are the positive and negative
fault currents at the 4th protection measurement installations
respectively, iP5 and iN5 are the positive and negative fault currents
at the 5th protection measurement installations respectively. ZP4X,
ZP5X, ZN4X and ZN5X are the positive and negative equivalent impedances
on the AC side of the two converter stations, ZP4 and ZP5 are the
equivalent impedances at fault points 4 and 5 respectively, and Zh is
the non-fault line equivalent impedance.
3.2.1 Internal fault characteristics analyses : When a fault
occurs in the DC line, the additional network for each fault is shown in
Fig.3. if1 -if4 are the actual
fault currents in the line area after the occurrence of the fault,
respectively, and the current flows in the direction shown by the arrows
in Fig.3.