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Analysis of inconsistent causes of partial discharge and alternating voltage test results of medium voltage cable
Apr 27, 2018

In the process of partial discharge and alternating voltage test of medium voltage XLPE power cable, there are sometimes inconsistency of side test results, and the cause and treatment of the problem are worth analyzing and discussing. The partial discharge and AC voltage test of intermediate voltage XLPE cable were successfully passed, but the partial discharge exceeded or breakdown occurred during the finished product test. Some of these phenomena are related to the accidental mechanical damage that has been suffered during the process of processing and handling in our factory. However, some defects are not caused by mechanical damage in the later process. such as the conductor of the core surface of some easily flaking adhesion burr, insulation inside some slight discoloration of the old gum (slightly yellow, or in the insulation condensed into white colloid block). These defects may not be detected during the partial discharge test of semi-finished products.


However, the defects may be further deteriorated and enlarged after the process of forming cable and sheath. The result is that the inspection process of the cable finished products has exceeded the standard and breakdown phenomenon. We found the above phenomenon in the actual test of the 10KV Xlpe cable. There is a plate of 8.7/10kv three-core finished cable in the process of partial discharge and alternating voltage test of semi-finished shielded insulated core, PD and AC voltage are detected. But in the finished product detection process has a yellow insulation core but has occurred the breakdown. There are no mechanical damage marks in the anatomical points of failure. There is a slight burr on the inner conductor surface of the breakdown electrical position, and the Burr has damaged the inner half conductive shielding layer. This type of problem is troubling and troublesome to cable manufacturers. Because the current cable manufacturing and supply process, are in accordance with the requirements of the contract orders for long scheduled production, on time bulk delivery. At the same time to reduce the cost of production considerations, almost no spot inventory. This way, once the cable product is produced, it may be nearing the delivery time. If the finished Xlpe cable products have partial discharge exceeding the standard and high pressure test breakdown, may result in rearrangement, and the rearrangement cycle at least about 6-7 days time. In this way, there is a contract default operating risk for cable manufacturers. At the same time, the occurrence of the above phenomenon will also cause the production department to test the partial discharge testing results of poor students questioned. This kind of flaw, according to the partial discharge examination method, may not be discovered in time in the semi-finished product inspection process.


The solution is to ensure the stability of the quality of XLPE cable raw material supply, mainly including the quality of copper used in conductor; On the other hand, the implementation of production process and quality control should be strictly managed. For example, XLPE cable 0+3 production line extrusion Machine Head, screw each area of the rational control of the process temperature; the stability control of nitrogen pressure; 0+3 in the boot process, the equipment must be ready for the process, the nitrogen pressure relief process,


After the abnormal condition of the continuous boot process equipment, the unqualified insulated core segment is removed and then flowed into the next procedure to ensure that there is no process manufacturing defect in the insulation core of the disk.


Some semi-finished cables, especially 35KV xlpe cables, sometimes occur during the actual test: semi-finished products in the partial discharge, AC voltage test due to the consideration of the finished product also to carry out alternating voltage test, the voltage time is relatively short, such as 26/35kv Xlpe cable semi-finished insulation shielding core, measuring local discharge 45KV, only 1.2pc. Applying AC voltage 65kv/10min smoothly. And in the finished product AC voltage 65kv/30min test in the process of applying voltage 5min after the breakdown occurred. Anatomical breakdown point of failure, no external mechanical damage traces. In view of the phenomena found in the test process. In order to ensure the quality of cable production, we in the later AC voltage test process for the inspection of semi-finished products and finished products are strictly according to the standard 65kv/30min test.


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