Main Types of Medium-and Low-voltage Cable Accessories(High Pressure Cable Accessories Production Technology)
At present, there are many kinds of products used in low-and medium-voltage cable accessories, including thermal contraction accessories, pre-standard accessories, and cold shrinkage accessories. They have the following characteristics:
Thermal contraction attachment
The materials used are generally composed of a mixture of polyethylene, ethylene-vinyl acetate(EVA) and ethylene propylene rubber. This kind of products mainly use stress tube to deal with the problem of electrostress concentration. That is, the parameter control method is used to relieve the stress concentration of the electric field. The main advantages are light, easy to install, good performance, cheap.
The stress tube is a thermal shrink tube with a moderate volumetric resistivity(1010-1012 Ω cm) and a large dielectric constant(20-25). The stress evacuation along the stress tube is more evenly distributed by forcing the cable insulation to block the fracture with electrical parameters. This technique is generally used in 35 kV and below cable attachments. Because the stress tube will heat up when the voltage level is high, it can not work reliably.
The key technical issues in its use are:
To ensure that the electrical parameters of the stress tube must meet the above standards can work reliably. In addition, attention should be paid to filling the air gap of the cable insulated semiconductor layer to eliminate gas and achieve the purpose of reducing partial discharge. Due to poor internal stress treatment, cross-linked cables will contract greatly during operation. Therefore, when installing accessories, note that the stress tube and insulation shield cover are not less than 20mm to prevent the stress tube from dissociating from the insulation shield during contraction. Thermal contraction accessories are less elastic and may cause air gaps in the interface when hot expansion and cooling occur during operation. Therefore, sealing technology is important to prevent tidal immersion.
The materials used are generally silicone rubber or ethyl-propyl rubber. The stress concentration problem is mainly dealt with by the geometric structure method, namely the stress cone. Its main advantages are excellent material performance, easier and faster installation, can be installed without heating, good elasticity, so that the interface performance has been greatly improved. It is the main form used by medium and low voltage and high voltage cables in recent years. The disadvantage is that the outer diameter of the insulation layer of the cable is required to be high, and the usual excess amount is 2 to 5 mm(that is, the cable insulation outer diameter is greater than the inner hole diameter of the cable attachment 2 to 5 mm). The excess amount is too small. Cable accessories will fail; Too much excess, cable attachment installation is very difficult(high process requirements). In particular, the problem is prominent in the middle joint. The installation is neither convenient nor often a fault point. In addition, the price is more expensive.
The key technical issues in its use are:
The dimensions of the attachment and the size of the cable to be installed shall conform to the specified requirements. In addition, silicone lubrication interface is also needed to facilitate installation and fill the air gap of the interface. Prefabricated accessories generally rely on their own rubber elastic can have a certain sealing effect, and sometimes the sealant and elastic fixture can be used to enhance the seal.
Cold shrink attachment
The materials used are generally silicone rubber or ethyl-propyl rubber. The cold shrinkage attachment generally uses the geometric structure method and the parameter control method to deal with the problem of electrostress concentration. The method of geometry structure is better than that of parameter control method.
Like the pre-standard accessories, the material has excellent performance, can be installed without heating, and is flexible, making the interface performance greatly improved. Compared with the pre-standard accessories, its advantages are more convenient for installation. The installation can be completed simply by pulling out the inner core of the cable attachment in the correct position. The material used is better in terms of mechanical strength than the pre-standard accessories, and the outer diameter size of the insulation layer of the cable is not very high, as long as the inner diameter of the cable attachment is smaller than the cable insulation outer diameter of 2mm(data). This is completely satisfactory with the pre-standard attachment requirement of 2 to 5 mm deviating-editor). Therefore, the construction and installation of cold shrink accessories is relatively convenient.
Its small size is the installation process is more convenient and quick, installation in place, its performance and the same as the pre-standard accessories.
The price is comparable to the pre-standard attachment, which is slightly higher than the thermal contraction attachment. It is a product with reasonable value for money.
The key technical problems in use are the same as those in the preform attachment.
In addition, cold-shrink accessories can also be divided into factory expansion and on-site expansion from the expansion status. Generally, cold-shrink accessories with a voltage level of 35kV and below are mostly factory expansion, and their effective installation period is within 6 months. The installation period must not exceed two years. Otherwise, the useful life of the cable attachment will be affected. The cold shrinkage accessories with a voltage level of 66kV and above are mostly on-site expansions. The installation period is not limited, but special tools need to be used for installation. Special tools can generally be provided by accessories manufacturers. The installation is very convenient and the installation quality is reliable.