10kV direct buried cable high resistance fault detection

10kV direct buried cable high resistance fault detection

Part 1 On-site situation

1. Laying method: direct burial
2. Position of both ends: one end is in the power distribution room, the other end is in the excavated area in the middle of the road
3. Insulation test: three-phase insulation is above 10MΩ
4. Middle joint: two joints, approximately 180 meters and 80 meters away from the power distribution room
5. On-site construction: excavation in the middle of the road
6. Laying environment: direct burial
7. Laying for 5 years

Part 2 Equipment used for testing

(1) Insulation test tool: XHMR-5000V insulation resistance tester
(2) Rough test equipment: XHGG502 cable fault pre-locator
(3) Path finding: XHGX507 Cable fault comprehenvise tester
(4) Precision test equipment: XHDD503 cable fault pinpointer
(5) High voltage equipment: XHYB-5/50 high voltage test transformer + XHCC-6/40 pulse energy storage capacitor
(6) Others: XHSC-60Q digital burn-through bridge XHZG-200/2 DC high voltage generator

Part 3 Testing Process

1. Use the electronic megohmmeter at 5kV to measure the insulation resistance of A-ground, B-ground, C-ground, A-B, B-C, and A-C. It is found that the resistance of A to ground is 30MΩ, that of B to ground is 13MΩ, and that of C to ground is 17MΩ. The resistance between phases is about 40MΩ. It is initially judged to be a high-resistance fault.

2. Then, each phase is subjected to a withstand voltage test. The current limiting current is very large. The withstand voltage reaches 30kV and the current limiting current reaches more than 2000μA. The fault type is judged to be a high-voltage flashover fault.

Cable length test: The cable length was tested using the low voltage pulse method of the XHGG502 cable fault pre-locator and was 305 meters.

Rough measurement of fault distance:
Use XHZG-200/2 DC high voltage generator for sampling, the voltage reaches 30kV without breakdown, use XHSC-60Q digital burn-through bridge to burn through the fault point, after 2 minutes of burning through, the fault point is broken through.
Then use (XHYB-5/50) test transformer + XHCC-6/40 pulse energy storage capacitor high voltage pulse output, XHGG502 cable fault pre-locator to perform high voltage flashover test, the initial fault distance is about 160 meters from the distribution room.

Fault path search: Use XHGX507 cable fault comprehensive tester to search for the path. The path direction is: after coming out of the power distribution room, go straight down to the factory gate, which is all cement road, then turn right angle to the south gate of the factory, walk along the wall for 80 meters after leaving the factory, and turn right angle to the middle of the road.

Accurate fault location: After the voltage was increased to 28kV, the fault broke through. Then, 503 was used to accurately locate the fault just above the cable (within the rough measurement range). The discharge at the fault point was obvious. The fault point was determined and excavated.

Part 4 summary

1. This fault search mainly adopts the traditional high-voltage breakdown plus acoustic magnetic synchronization method to locate the fault.

2. The special feature of this fault is that the fault resistance is high, resulting in a withstand voltage of 30kV, and it is an intermediate joint fault, so the XHSC-60Q burn-through instrument is needed to reduce the resistance of the fault point.

3. The path is the most difficult to find, because it is old and the cable has been moved several times, resulting in differences between the path described by the customer and the actual measured path. If you trust the customer and do not search for the path, it will lead to the wrong direction of finding the fault.