2024-12-14
35kV cable fault at Inner Mongolia wind farm
1.On-site situation
The cable is cross-linked polyethylene 35kV, and the three phases ABC at both ends of the cable have been removed.
2.Equipment used for testing
(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: XHDD503E cable fault locator
(5) High voltage equipment: operation box + (XHYB-5/50) test transformer
XHCC-6/40 pulse energy storage capacitor
XHSC-60Q burn-through bridge
XHZG-200/2 DC high voltage generator
3.Testing Process
First, cut off the power supply and discharge the cable, and remove the cable ends at both ends.
1. Use the XHMR-5000V insulation resistance tester to measure the insulation resistance between the three phases and the ground. The resistance between phase A and ground is 4.6GΩ (4996V), the resistance between phase B and ground is 4.5GΩ (4973V), and the resistance between phase C and ground is 4.55GΩ (4985V); the resistance between phases is above 30GΩ.
2. The fault phase cannot be determined by the insulation test. Then the XHZG-200/2 DC high-voltage generator is used to perform DC withstand voltage on the three phases. It is found that when the withstand voltage of phase B reaches 60kV, a flashover breakdown discharge occurs. After multiple voltage increases, the flashover discharge voltage drops to 35kV. The withstand voltage of the other two phases is qualified.
3. Therefore, it is judged that phase B is a flashover high-resistance fault.
4. After determining the fault phase, use the low-voltage pulse method of the XHGG502 cable fault tester to measure the three-phase to ground and between the three phases. Under high-resistance faults, the low-voltage pulse method measurement only shows the full-length waveform of the cable open circuit state. Test: After turning on the XHGG502 cable fault tester, the device automatically enters the test page, selects the cable type, selects a length greater than 3 kilometers, selects the low-voltage pulse method for the detection method, and starts sampling. During sampling, adjust the waveform knob at any time to adjust the waveform. Use the cursor to the waveform period to display the cable distance. Significance: Because users are generally uncertain about the full length of the cable, if you listen to the user, it may cause deviations in the cable length and affect the precise positioning.
The full length of the marked cable is 17,000 meters, and the length tested using the low-voltage pulse method is 16,777.9 meters.
5. After confirming the fault in phase B of the cable, start preparing the placement of the booster equipment. The end of the cable should be kept suspended and at a safe distance of 30cm from the surrounding area. There should be personnel guarding the end or locking the door to ensure safety. After receiving the safety permit at the end, start placing the equipment. Because the cable is a high-resistance fault, the boost voltage is at least 50kV. One capacitor is 40kV+6uF, and the voltage of two capacitors in parallel is 80kV. The ball gap distance is adjusted to 15mm, and then the voltage is slowly increased, and the ball gap is discharged. Use the XHGG502 cable fault tester to connect the sampling box, place the sampling box next to the capacitor and the cable ground wire, select the high-voltage flashover method, and you can collect the fault waveform.
4.Accurately locate the fault distance
When the cable was tested for faults using the flash method, when the voltage rose to 45kV, a flashover discharge occurred at the fault point, and the fault distance was 7318 meters.
Use the XHGX507 pipeline locator to find the cable path. After completion, use the XHDD503 cable fault locator to find the approximate fault location of the cable. Start locating the fault point along the cable direction directly above the cable. The signal reception is strongest directly above the fault point. After digging up the soil pile on site, it was confirmed that this was the cable fault point.
5.Summary
1. The insulation resistance value can be used as a reference, but it cannot be used as the final judgment of the quality of the cable. The minimum requirement is to do a DC withstand voltage and leakage current test.
2. The flashover waveform of the ultra-long cable is too attenuated, and multiple equidistant cycles of the fault point waveform may not appear. Only 1-2 cycles can be analyzed, which increases the difficulty of rough fault measurement and judgment.
