Cross-linked polyethylene insulated power cables (referred to as cross-linked cables) with a rated voltage of 3.6 kV and above shall have a conductor shield and an insulation shield. Conductor shielding is mainly to ease the internal cable and electric field stress concentration, eliminate the influence of conductor twisting, improve the internal electric field and radial distribution of the cable to improve the electric strength of the cable. Cross-linked cables in the production process due to a variety of factors, the conductor shield appears dents, bumps and other defects, resulting in non-uniform distribution of the internal electric field, seriously affecting the electrical performance and service life of the cable.
1. Types and Causes of Conductor Shielding Defects
The conductor shielding defects that occur in the production process of cross-linked cables are mainly the following:
1.1 depression:
The sag of the conductor shield is mainly due to the large gap between the conductors in the outermost layer of the conductor. As the conductor of the cross-linked cable is a pressed-stranded conductor, the extrusion of the conductor shield is carried out by a squeezing die and enters the cross-linked pipeline in an environment of high temperature and high pressure. If the conductor structure is unreasonable or the tightness is insufficient, Inevitably there is a conductor shield recess. The outer layer loose strand appears in the pressed conductor during the course of the pay-off process, and the outer layer loose strand may also appear when the conductor is not processed in the fusion welding head, which is also the reason of the conductor shield depression.
1.2 raised:
In order to ensure uniform electric field inside the cable, the conductor shielding surface is required to be smooth. If the temperature of the shielding material is abnormal during the extrusion process, pre-crosslinking of the shielding material occurs, and raised particles are formed on the surface of the extrusion layer. These raised particles are prone to cause tip discharge during the long-term implementation of the cable, which in turn forms an electrical tree in the insulation, ultimately leading to cable breakdown.
2, the conductor shield defect inspection
The high-voltage cross-linked cable has a thick insulation layer and an insulation shield, so it is difficult to perform a good inspection of the conductor shielding surface. Conductor shielding can be checked using the following methods. Method: take the insulated core sample length 1m, one end retain 20cm long insulation shield, insulation shield stripped, try not to damage the insulation, and then put it into the silicone oil tank heating, silicone oil temperature 120 degrees Celsius, after placing 3h transparent insulation layer Then, check the surface condition of the conductor shield with a strong light source. For insulation shielded non-peelable or extra-high voltage cables, insulation shields or partially insulated vehicles may be used on lathes and inspected as described above. With this method, the surface condition of the conductor shield can be checked visually, but the geometrical dimensions of the defect cannot be accurately measured.
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