Cross-linked cables are typically defined as cables that use cross-linked materials for insulation, with the most common being cross-linked polyethylene (XLPE). This material is created by transforming the linear molecular structure of polyethylene (PE) into a three-dimensional network through a specific chemical or thermal process. As a result, the long-term operating temperature of the cable increases from 70°C to 90°C (or even higher), and the short-circuit temperature tolerance rises from 140°C to 250°C (or more). These improvements significantly enhance the cable’s electrical performance and overall reliability during operation.
When conducting partial discharge and high-voltage tests on medium-voltage cross-linked power cables using a series resonance test system, a false breakdown may occur unexpectedly due to various factors. Correctly identifying whether the breakdown is real or just an artifact is crucial for improving the efficiency of testing in cable manufacturing facilities.
During these tests, the cables are usually tested in a shielded room, one core at a time. If after replacing the insulated cores, the test passes, it suggests that the original issue was not a true breakdown. Alternatively, if the breakdown voltage gradually decreases, it indicates a real fault in the cable. However, if the breakdown voltage remains constant, it could mean that the fault occurred at the end of the cable. In such cases, manually closing the partial discharge switch, opening the shield room door, and increasing pressure can help quickly locate the breakdown point. If no fault is found at the end, multiple insulated cores and cables should be replaced, or the previously tested cable can be retested under no-load conditions. If the breakdown voltage remains unchanged, it is likely a false breakdown, and the equipment should be checked before further troubleshooting.
For long single-core medium-voltage cross-linked cables, it's also important to check if the test equipment's capacity is exceeded. If so, the system may trigger a detuning trip, which should not be mistaken for a cable breakdown. Another special case occurs when internal insulation shorts out, causing the series resonance system to fail to resonate. In this situation, simply replacing the faulty core with a good one for testing can confirm the actual condition of the cable.
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