A mysterious cable bureau on the excess of the phenomenon

- Apr 30, 2018-

There is a three-core XLPE power cable The first day in the Bureau of the inspection of the insulation wire core has been a partial discharge of the phenomenon, at that time by locating the fault location in the vicinity of the cable end, after careful examination of the two ends of the cable, found that the cable inside the end of a phase of insulation shielding scratches, insulation exposed, According to the location and inspection conditions to confirm that the cable partial discharge over the cable in the end of the head root. At that time, the inspectors will be the product of the inspection mark, day by day by the workshop will be the cable hoisting fork to the outdoor open warehouse temporary disposal. The next morning, workshop on the plate products in the condition of not doing any processing, the request for a retest, after the workshop will be the plate cable to be lifted to the shielding room to carry out partial discharge retest, the disk cable of the bureau volume only 1.1pC, retest results Cable Bureau volume Detection qualified. Re-check the root of the cable inside the end, found that the cable insulation core surface shielding damage point is still in, and white insulation has been exposed. According to the local Test experience and the basic theory of the test, such a large quality defect, in the actual bureau of the measurement process will certainly lead to exceeding the standard. However, the actual test results are not in conformity with the theory of local discharge testing.

The problem in the end is where the inspectors are more confused. Just beginning to suspect that the equipment is a problem, the cable outside the end has done a qualified one core for the retest, the Bureau put qualified, but also artificial will end of the shield cut a small piece, so that the insulation exposed, testing bureau put out of the standard, this means that the test equipment condition intact. After a careful look at the cable look analysis, we found the reason why the cable was produced in winter. In the North winter outdoors at night the phenomenon of fog and frost. In such a weather situation, the cable without head will be placed outdoors for a day and night, the entire cable and cable the outermost surface and cable end of the insulation core surface is actually attached to a thin layer of frost mist. When the cable is transferred from outside to the workshop the next day, the frost mist attached to the cable surface is not forced to be dried, the frost mist attached to the outside of the insulated core surface of the cable end head will be due to the difference of temperature between indoor and outdoor (workshop indoor heating), condensation into a thin layer of water vapor attached to the insulating core surface, Because the resistivity of the water is close to the resistivity of the conductive shielding layer in the XLPE cable, the surface water film attached to the damaged point of the half conductive shielding layer of the cable is equal to the compensation repair of the outer half conductive layer of the point. As a result, the partial discharge of cable has appeared abnormal qualification in the process of retest. To verify our analysis, we then use a dry large cloth will be tested cable end of the outer shielding layer of the surface of the damage point of moisture attached to all dry, re-detection of local discharge is still excessive, positioning to determine the location of the wrong place on the cable end, and our first day of the test results coincide. Through this measurement, in the later phase of XLPE cable partial discharge test, if through the positioning of the inspection confirmed that the board is due to the cable test head stripping process end of the shield scratch or damage, we can completely touch a layer of water film as a half conductive layer, the cable for partial discharge test, The partial discharge test of the cable is sure to pass. This reduces the amount of trial work and cable waste from the two-time stripping cable test head.