I. Case Background
This test was conducted to troubleshoot a fault in a 10kV power cable. The cable was a newly laid line with a clear and traceable path, primarily responsible for core power transmission. The test aimed to accurately locate the fault point to ensure the line's rapid restoration to normal power supply and provide practical reference for similar cable fault handling.
II. Cable Basic Parameters
Cable Model: YJV 3*95 8.7/10kV
Installation Method: Direct burial
Labeled Length: 290 meters
Connections: One end connected to the distribution room, the other end connected to the power consumption terminal.
III. Preliminary Fault Detection
Insulation Test: A 2500V electronic megohmmeter was used for comprehensive insulation testing of the cable, measuring the insulation resistance between A-ground, B-ground, C-ground, and between phases A-B, B-C, and A-C. The test results showed that the insulation resistance between phase A and ground, phase C and ground, and between phases all met the standard requirements. Only the insulation resistance between phase B and ground was abnormally low, with a value of 0.12 MΩ.
Fault Nature Determination: Based on the insulation test data, it was preliminarily determined that the cable had a high-resistance fault between phase B and ground.
IV. Test Instrument Configuration
This test used a professional set of testing equipment to ensure accurate and efficient fault location, including: 5000V insulation megohmmeter, 5/50 test transformer, 40/6 capacitor, 503 fault locator, 502 main unit, and 507C cable locator.
V. Fault Location Process
(I) Rough Measurement Stage
Test Method: A combination of low-voltage pulse method and high-voltage flashover method was used for rough fault location.
Test Results:
The low-voltage pulse method measured the actual cable length as 287.2 meters, which is basically consistent with the labeled length, verifying the reliability of the basic test data;
Using a combination of test transformer, capacitor, and other equipment to output high-voltage pulses, a high-voltage flashover test was conducted using a cable fault tester, preliminarily locating the fault approximately 58 meters from the distribution room. Waveform Analysis: The low-voltage pulse waveform clearly displays data for the entire cable length, while the high-voltage impulse waveform provides crucial information for rough estimation of the fault distance. The two waveforms corroborate each other, narrowing down the search range for subsequent precise measurements.
(II) Precise Measurement Stage
Test Method: The acoustic-magnetic synchronization method was used for precise localization of the main insulation fault.
Test Process: After applying a 20kV voltage to the cable, a breakdown discharge occurred at the fault point. Within the approximately 58-meter range determined by the rough measurement, a 503 pinpointing instrument was used for detailed detection directly above the cable. The discharge signal at the fault point was clear, allowing for accurate localization.
VI. Summary of Key Points of the Case
Fault Characteristics: This fault was a typical single-phase-to-ground high-resistance fault. In insulation testing, this type of fault manifests as a significant decrease in the insulation resistance of a single phase to ground, while other phases and inter-phase insulation remain normal. The faulty phase can be quickly identified through targeted insulation testing.
Testing Techniques: The high-voltage flashover waveform of a near-end fault differs from conventional waveforms. Special attention should be paid to waveform analysis and data timing to avoid errors in fault localization due to waveform misinterpretation.
Advantages of the Solution: The combined testing scheme of "low-voltage pulse method for rough measurement of the entire length + high-voltage impulse method for rough determination of the fault distance + acoustic-magnetic synchronization method for precise localization" is a progressive and precise approach that significantly improves fault localization efficiency, especially suitable for handling faults in directly buried medium-voltage cables.
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