In the following experiment, the system experiences DC faults at different locations. In the first case, a DC fault is initiated at 50 km from the rectifier side (VSC1). The fault time is from 0.7 to 0.75 second. Under this condition the variation of the actual DC line current is shown in Fig 3.19. The magnitude of the current is 2000 A when the system is in normal operation. But after the fault the magnitude of the current is increased to 4000 A. After the processing by the wavelet transform in five levels the absolute maximum value of the DC current coefficients is 450. The 5th level wavelet coefficients plot can be seen in Fig 3.20. From this analysis it is clear the coefficients value varies from 40 to 450 which clearly indicate departure from the normal operating condition.
Fig 3.19 Actual DC line Current for DC fault at 50 km
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In the second case a DC side fault is initiated at 100 km away from the rectifier side (VSC1). The fault time is the same as before. When the system experiences this fault the variation of actual DC line current is shown in Fig 3.21. The magnitude of DC line current is varies from 2000 A to 4000 A when the system experiences the fault. This DC line current is processed by the wavelet transform and it is observed that the absolute maximum value of the DC current coefficients is 460. The corresponding 5th level coefficients plot can be seen in Fig 3.22. From this analysis it is observed that the variation from the normal value of the coefficients is varied from 40 to absolute maximum value of 460.
Fig 3.21 Actual DC line current for DC fault at100 km
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In the third case a DC side fault is initiated at 150 km away from the rectifier side (VSC1). The fault time is the same as before. When the system experiences this fault the variation of actual DC line current is shown in Fig 3.23. This DC line current is processed by the wavelet transform and it is observed that the absolute maximum value of the DC current coefficients is 1250. The corresponding 5th level coefficients plot can be seen in Fig 3.24. From this analysis it is observed that the variation from the normal value of the coefficients is from 40 to an absolute maximum value of 1250. The results show that when the system experiences a DC fault at a distance greater than 100 km, there is a clear distinction between the magnitude of the coefficients compared to a DC fault which occurs less than 100 km.
Fig 3.23 Actual DC line current for 150 km fault
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In the next case, a DC fault is initiated at 200 km away from the rectifier side (VSC1). The fault time is from 0.7 to 0.75 second. After the processing by the wavelet transform in five levels the absolute maximum value of the DC current coefficients is 1520. The 5th level wavelet coefficients plot can be seen in Fig 3.25. From this analysis it is clear the coefficients value varies from 40 to 1520.
Fig 3.25 5th level wavelet coefficients of DC line current for DC fault at 200 km
In the last case, a DC fault is initiated at 250 km away from the rectifier side. The fault time is from 0.7 to 0.75 second. After the processing by the wavelet transform in five levels the absolute maximum value of the DC current coefficients is 1650. The 5th level wavelet coefficients plot can be seen in Fig 3.26. From this analysis it is clear the coefficients value varies from 40 to 1650 which is clearly discriminated from the normal operating condition.
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Fig 3.26 5th level wavelet coefficients of DC line current for DC fault at 250 km
Table 3.2 shows the absolute maximum value of the DC current wavelet coefficients in the 5th levels. From this table it can be seen when the system experienced a fault at 50-
100 km the maximum coefficients is 460. But when the system is in the normal operation the wavelet coefficients range is 40-50. From these analyses it is clear that when the fault location is varied from 50-100 km, the variation of the coefficients does not significantly change. When a DC fault is applied at 150-200 km, the wavelet maximum coefficients are between 1250-1520. Here it is possible to see a significant variation of coefficients, specifically after 100 km. In the case of a fault at 250 km, the maximum value of the coefficients is 1650 as mentioned in the table. In the analysis it is observed that the wavelet transform has a capability to detect the distance of fault location in the system. Moreover the result shows that it is possible to find the distance to the fault location in the DC side from the ranges of the value of the coefficients.
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Table 3.2 Absolute maximum values of the Wavelet Coefficients of the DC line current Absolute maximum value of five levels wavelet coefficients of DC line current for DC fault at various
fault distances DC Fault Normal Operation 50 km 100Km 150 km 200 km 250 km Max. value of wavelet coefficients in 5th level 40 450 460 1250 1520 1650