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XLPE Insulated Power cables (referred to as XLPE cables) of rated voltages of 3.6kV or above shall be shielded by conductors and shielded by insulation. Conductor shielding is mainly to ease the internal and electric field stress concentration, eliminate the influence of conductor stranded, improve the internal electric field and radial distribution of cables to improve the electrical strength of the cable.
Due to the influence of various factors in the production process of xlpe cable, the defects such as the ﹑ bulge of the conductor Shield appear, resulting in the uneven distribution of the internal electric field, which seriously affects the electrical performance and service life of the cable.
1. Types and causes of conductor shielding defects
Cross-linked cable in the production process of the conductor shielding defects mainly include the following:
1.1 Hollows: The main shielding of the conductor is caused by the gap between the outermost line of the conductor. Because the conductor of the Xlpe cable is a tightly-pressed stranded conductor, the extruded die is used in the extrusion of the conductor shielding, and the conductor Shield sag will inevitably appear if the conductor structure is unreasonable or the tightness is not enough.
Tight-pressure conductor in the process of the release of the outer pine strands, the conductor in the melting of welded joints will also appear in the outer pine strands, which is leading to the cause of the shielding of the conductor.
1.2 Convex: In order to ensure uniform electric field inside the cable, the shielding surface of the conductor is smooth. If the shielding material in the extrusion process temperature disorder, resulting in the screening of the pre-crosslinking, in the extrusion layer surface to form a raised particles.
These raised particles in the cable for a long time to carry out the process of easy to trigger point discharge, and then in the insulation to form an electric tree, and eventually lead to cable breakdown.
2. Inspection of conductor shielding defects High-voltage XLPE cable insulation layer thicker, but also insulation shielding, so for the conductor shielding surface good inspection more difficult. The following methods can be used to inspect the conductor shielding. Method: Take the insulation core sample length 1m, one end of the 20cm long insulation shielding, insulation shielding peeling, try not to damage insulation, and then put it into the silicon fuel tank heating, silicone oil temperature of 120 degrees Celsius, placed 3h after the insulation layer becomes transparent, and then use a strong light source on the surface condition of the conductor shielding inspection. For insulated shielding can not be stripped or ultra-high voltage cables, can be used to the lathe insulation shielding or part of the car to go, and then to check the above method. The method can be used to visually inspect the surface condition of conductor shielding, but the geometrical dimension of the defect can not be measured accurately.