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.