2024-11-01
10kV cable fault detection-Underground cable corridor + pipe penetration
1. On-site situation
The laying method is underground cable corridor + cable pipe laying, and most of the cables are laid in trenches.
(1). The cable is 10kv 3*150, which is the user's backup incoming line. The end is under the jurisdiction of a certain university. The end cable head cannot be disassembled. The end switch has been separated, but there are lightning arresters and CT links, and the end cannot be tested.
(2). The cable is measured with an insulation megohmmeter of 2500V:
Phase A to the ground: the voltage is 55V, insulation is 0;
Phase B to the ground: the voltage is 258V, insulation is 0;
Phase C to the ground: the voltage is 2173V, insulation is 1.73MΩ;
The replacement test voltage is 5000V, the residual voltage and insulation value of Phase C to the ground are about -3200V and 1MΩ; Phase A and B to the ground are low resistance faults , Phase C to the ground is high resistance fault , and it is initially suspected to be a cable joint fault.
2. Test equipment used
(1) Insulation test tool: XHMR-5000V insulation resistance tester
(2) Rough test equipment: XHGG502 cable fault tester
(3) Path finding: XHGX507 pipeline locator
(4) Precision test equipment: XHDD503 cable fault locator
(5) High voltage equipment: XHYB-5/50 high voltage test transformer + XHCC-6/40 pulse energy storage capacitor
3. Testing Process
(1) Cable length test:
The cable length from phase A, B, and C to the ground was measured using the low-voltage pulse method and was 1746.9 meters. The cable length between the three phases was 1746.9 meters, which was basically consistent with the construction drawings and was confirmed to be the full length of the cable.
(2) Rough measurement of fault distance:
Use XHYB-5/50 high-voltage test transformer and XHCC-6/40 pulse energy storage capacitor to apply high-voltage pulse signal to phase A, and use high-voltage flashover method to collect waveform. At first, the voltage rises to 15kV, and the fault point is brokendown, but the waveform is too chaotic to analyze the cable fault distance. Then the voltage rises to 22kV, and the analysis waveform shows that the fault distance is 1379.1m.
(3) Accurately locate the fault:
From the rough measurement, it can be known that the distance of the fault point is about 1379.1 meters. The cable is pressed by the soil pile and other cables in the trench. The sound of discharge is small and the human ear cannot hear the discharge sound. It is necessary to go down to the cable trench and use XHDD503 cable fault locator to locate the point. (Note: When going down the cable trench, it is best to wait until the inside of the trench is ventilated to ensure safety before entering.)
After entering the trench, you can hear obvious electrical stress sound from the cable. There is no obvious discharge sound at the first joint before and after the rough measurement distance. Continue forward 40m in the trench to another cable joint (the cable is completely covered by soil), and use a positioning instrument to hear obvious discharge sound. (Note: This is the cable head remade after the subway relocation, so the distance between the two cable heads is close.)
After the cable joint personnel went to the site to dig up the soil pile, they confirmed that this cable joint had a fault.
4. Summary
If there is no obvious fault point discharge sound at the cable joint at the rough measurement position, the fault phase should be replaced in time; if there is still no sound, the positioning instrument should be used to re-position near the front and rear joints in time. If the path is not clear, the path should be found in time to eliminate other interferences such as manhole covers or direct passage through pipes, so that the fault point can be accurately located as soon as possible.
