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The key problem of 110kV cable accessory installation

2022-12-12
110kV cable accessories, due to the improvement of the voltage level and the importance of the line, the technical consideration is more comprehensive, the technical requirements and technical difficulties are increased. Some problems that may be ignored in the cable accessories of medium and low voltage levels may become key technical problems in the installation of 110kV high voltage cable accessories. Therefore, to install and use 110kV cable accessories, we must understand and master its characteristics.

1. Performance of the insulation interface

In the insulation of cable accessories, there are many interfaces of various media, and the interface of different media becomes the interface. The interface can be thought of as a very thin layer of gap. Because the surface of the two layers of insulating materials is uneven, the gap contains small particles of the material, a small amount of water, gas and solvent and other foreign bodies which are distributed unevenly. Due to these factors and the effect of external pressure, the insulation performance of the interface is lower than that of the material itself. Electrical parameters vary with the foreign body condition and external conditions.

The seriousness of the problem is that these interfaces are often located in the middle and high field intensity of the insulation of the cable accessories, such as the stress cone of the straight through joint and the root of the stress cone of the termination.

In the high voltage cable accessories of 110kV voltage class, it becomes the decisive factor restricting the insulation performance of the whole cable accessories, and becomes the weakest link of cable accessories insulation. Although the insulation of cable accessories has been designed with appropriate margins to ensure use after normal installation, special attention must be paid to the treatment of cable insulation surfaces and interface pressure during installation.

2. The insulation retraction problem

When producing XLPE insulated cables, there will be stress inside the insulation of the cables. This stress causes the insulation near the conductor to contract toward the middle of the insulator. When the power supply is cut off, there will be a gradual retraction of the cable insulation and the core of the cable exposed. The stress in the cable insulation is slowly and confidently eliminated over time, but it often takes a long time to completely disappear.

During field installation, it is not possible to wait a long time for the residual stress in insulation to eliminate itself. The common way to eliminate the retraction stress is to wrap the heating belt around each phase cable and heat it to 80-90℃ for 8-12h. After this treatment, more than 95% of the retraction stress of the cable can be eliminated. It is safer to install the cable accessories after that. In fact, it is not possible to deal with the retraction stress in the entire cable insulation at once, and cable accessories have been designed with this fact in mind. 

In the design of prefabricated cable accessories, the semi-conductive shield near the conductor connection tube is designed to be longer, so that the two ends of the semi-conductive shield are respectively covered on the cable insulation by 10-15mm. After the cable accessories are installed, there will be some retraction of the cable insulation, and the semi-conductive shielding of the conductor can still overcome the defects caused by retraction.

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