In the actual operation of power cable, there are quite a lot of accidents caused by COLD SHRINKABLE ACCESSORIES. Generally, the most likely place for cold shrinkable accessories to fail is in and near the middle connector and terminal head of the cable, especially the manufacturing process of the middle connector is more complicated, so it is relatively more likely to have potential accidents.
1. It is caused by the imperfect metal shielding grounding of COLD SHRINKABLE ACCESSORIES. There are two points grounded on the general cross-linked cable, and the grounding resistance value should be less than the specified value, so as to effectively protect the cable. If the grounding resistance value exceeds the standard a lot, a better voltage will be induced when the cable and connector receive electric voltage, resulting in aging and breakdown of the insulating part.
2. Because the installation quality of COLD SHRINKABLE ACCESSORIES is not high, it mainly refers to that during the laying and installation of cables, the bottom of cable trench is the auxiliary point of sand or soft soil, etc. As a result, the cable accessories are squeezed by stones or heavy objects in the cable trench, resulting in mechanical trauma. This kind of mechanical trauma is a very obvious fault point of cable accessories.
3. It is caused by the poor manufacturing quality of the intermediate connector and terminal of the COLD SHRINKABLE ACCESSORIES. If the creepage distance of the semi-conductive layer is not treated enough during the manufacturing process, the internal impurities, sweat and air will be contained when the cold shrinkable cable accessories shrink. For this reason, the impurities in the cable will be free under the action of a strong electric field after operation, Discharge occurs. In addition, German standard machinery pointed out that in the manufacturing process of cold shrinkable cable accessories, if the wire crimping quality is not good, it will lead to excessive contact resistance and heating of the connector, or insulation aging due to excessive shrinkage, so that the insulation layer is aged and broken down, which will also lead to cable grounding short circuit, goods phase to phase short circuit, and will also hurt other nearby cables.
The electric field of cable joint is a distorted electric field. At the cut-off of wire core and shielding layer, there will be electric stress concentration. The electric field intensity is very strong, which is the weak link of the whole joint. Through the electric field simulation analysis, when the semiconductor layer is too long, the semiconductor layer is too short, the semiconductor layer has no chamfer, and the silicone grease is applied unevenly in the transition stage between the semiconductor layer and the main insulation, the electric field distortion intensity is 4 ~ 12 times higher than the maximum electric field intensity at the cable joint under normal conditions, especially when the semiconductor layer has no chamfer. When the electric field distortion intensity reaches a certain degree, insulation partial discharge breakdown will occur.
Therefore, it is suggested that the cable intermediate joint must be constructed in strict accordance with the construction quality control requirements and the specifications of the construction drawings to prevent the cable belt defects from being put into operation. For the cable with intermediate joint that has operated for a certain period of time, the cable oscillation wave partial discharge test shall be carried out to find and eliminate the cable defects in time and ensure the reliable operation of the cable line.