Test time: October 26, 2020
Test site: Construction section of Metro Line 9 of Zhijiang Road, Jianggan District
Test item: 10kV cable fault finding
Preparation before the test:
The two ends of the cable head are respectively located in the power distribution cabinet at the construction site of Line 9 and the four cabinets at Qibao Old Street Station. The two ends and the three phases were suspended in the air before the test, and the armor and copper shield were still grounded. This test is mainly carried out in the power distribution cabinet at the construction site. The three-phase insulation voltage test of the cable is carried out first, and the high-voltage megohmmeter is used for the test. The insulation resistance of the three cables A, B, and C relative to the main insulation is measured, and then used The multimeter measures the conduction between the three phases of the cable and the phase to the phase, which is convenient for preliminary judgment of the type of cable fault. According to the preliminary judgment of the test results and phenomena, the cable A phase and B phase have faults, but the fault points and fault phenomena are still not obvious, so further testing and judgment are needed.
Initial test of low voltage pulse method:
Using the low-voltage pulse method and path tester, the full length test and path identification of phases A, B, and C of the cable are carried out. The actual total length of the cable is about 2150 meters, which is basically the same as the actual number of meters marked on the sign. The fault waveforms of phase A and phase B are similar, and there are two suspected fault points, located at 870 meters and 350 meters respectively. It is more likely that the fault point is located at 870 meters through waveform and field environment analysis, but it does not rule out that the cable has two fault points.
Cable length waveform:
Cable length waveform
Suspected failure point 1 (350 meters away)
Suspected failure point 2 (at 870 meters)
In order to ensure the precise location of the fault point, verification was carried out by the high-voltage flashover method ball gap DC discharge method, and the specific fault point location was determined through on-site sound monitoring of the high-voltage flashover discharge. Before the high voltage test, it is necessary to identify the path of the cable. The construction site environment is complicated, and the original cable path cannot be confirmed through the site reference objects during the rectification of roads and fences. Through the identification of the cable routing instrument, the approximate fault point location range (850 meters to 900 meters) was confirmed.
Accurate positioning by high pressure flashover method:
Through the high-voltage flashover method, the ball gap DC discharge method, the test voltage is 12kV, and the cable is kept in the discharge state. The testers go to two places of 350 and 870 meters to monitor the discharge. No obvious discharge sound was heard at 350 meters, but there was obvious discharge sound at 870 meters, and the road surface was damaged by excavators. It was verified after excavation that this section of cable was protected by steel pipes and laid under the cement pavement, and the surface of the steel pipe and the internal cable core wires had been severely damaged.
After cutting, conduct insulation resistance tests on the distribution cabinets at both ends for A, B, and C respectively, and the test values are qualified. According to the test data analysis, it can be verified that the non-fault point at 350 meters does not have other obvious faults.
Summary: The faulty cable has a voltage level of 10kV and a diameter of 150mm2. The main reason for the failure is that the excavator accidentally penetrated the target cable during on-site construction work on the cement road, resulting in power outage. Through testing and identification, find out the cable path and locate the fault point smoothly.