The main types of products that are currently used for medium and low voltage cable accessories are heat shrinkable accessories, prefabricated accessories, and cold shrinkable accessories. They have the following characteristics:
1. Heat shrinkable accessories
The material used is generally a blend of polyethylene, ethylene-vinyl acetate (EVA), and ethylene-propylene rubber. This kind of product mainly uses stress tube to deal with the problem of electric stress concentration. That is, the parameter control method is used to relieve the electric field stress concentration. The main advantages are light weight, easy installation, good performance and low price.
The stress tube is a kind of heat-shrinkable tube with special electrical parameters with moderate volume resistivity (1010-1012Ωcm) and large dielectric constant (20-25). The electrical parameters are used to force the stress at the abc cable insulation and shielding fracture to evacuate. The stress tubes are more evenly distributed. This technology is generally used in cable accessories of 35kV and below. Because the stress tube will heat up when the voltage level is high and cannot work reliably.
The key technical issues in its use are:
To ensure that the electrical parameters of the stress tube must reach the values specified in the above-mentioned standards, it can work reliably. In addition, attention should be paid to filling the air gap at the fracture of the cable insulation semi-conductive layer with silicone grease to eliminate gas and achieve the purpose of reducing partial discharge. Cross-linked cables will shrink greatly during operation due to poor internal stress handling, so when installing accessories, pay attention to the stress tube and the insulation shield to cover not less than 20mm to prevent the stress tube from separating from the insulation shield during shrinkage. Due to the small elasticity of heat-shrinkable accessories, air gaps may occur at the interface during thermal expansion and contraction during operation. Therefore, the sealing technology is very important to prevent moisture intrusion.
2. Prefabricated accessories
The material used is generally silicone rubber or ethylene propylene rubber. The geometric structure method is mainly used to deal with the stress concentration problem. Its main advantages are excellent material performance, easier and faster installation, installation without heating, good flexibility, and greatly improved interface performance. It is the main form adopted by low-voltage and high-voltage cables in recent years. The disadvantage lies in the high requirements for the outer diameter of the cable insulation layer. The usual interference is 2-5mm (that is, the outer diameter of the cable insulation is larger than the inner hole diameter of the cable accessory by 2-5mm). The interference is too small, and the cable The accessories will malfunction; the interference is too large, and the installation of the cable accessories is very difficult (high process requirements). Especially in the middle joint, the problem is prominent, the installation is not convenient, and it often becomes the point of failure. In addition, the price is more expensive.
The key technical issues in its use are:
The size of the accessory and the size of the cable to be installed must meet the specified requirements. In addition, it is necessary to use silicone grease to lubricate the interface to facilitate installation and fill the air gap of the interface. Prefabricated accessories generally rely on their own rubber elasticity to have a certain sealing effect, and sometimes sealants and elastic clamps can be used to enhance the sealing.
3. Cold shrinkable accessories
The material used is generally silicone rubber or ethylene propylene rubber. Cold-shrinkable accessories generally adopt geometric structure method and parameter control method to deal with the problem of electric stress concentration. The geometric structure method that uses the stress cone to relieve the concentrated electric field distribution is better than the product of the parameter control method.
Like prefabricated accessories, it has excellent material properties, can be installed without heating, and has good flexibility, which greatly improves the interface performance. Compared with prefabricated accessories, its advantage is that it is more convenient to install and only needs to be in the correct position. The installation can be completed by pulling out the inner lining core tube of the cable accessory. The material used is better than prefabricated accessories in terms of mechanical strength, and the requirements for the outer diameter of the cable insulation layer are not very high, as long as the inner diameter of the cable accessories is less than the outer diameter of the cable insulation 2mm (the data is like this, this The type attachment requires a deviation of 2～5mm (Editor) to fully meet the requirements. Therefore, the construction and installation of cold shrinkable accessories are more convenient.
Its biggest feature is that the installation process is more convenient and quicker. After the installation is in place, its working performance is the same as the prefabricated accessories.
The price is equivalent to that of prefabricated accessories, and slightly higher than heat shrinkable accessories. It is the most cost-effective product.