Medium and low voltage cable accessories

The stress tube is a heat shrinkable tube with a moderate electrical resistivity (1010-1012 Ωcm) and a large dielectric constant (20--25). The electrical parameters are used to force the stress at the insulation of the cable insulation to break along the edge. The stress tube is more evenly distributed. This technology is typically used in cable accessories up to 35kV.

Keywords: low voltage cable accessories advantage parameters

Medium and low voltage cable accessories are currently used in a wide range of products, mainly heat shrinkable accessories, prefabricated accessories, and cold shrinkable accessories.

They have the following characteristics:

Heat shrinkable accessory

The materials used are generally composed of a blend of various material components such as polyethylene, ethylene vinyl acetate (EVA) and ethylene propylene rubber. This type of product mainly uses stress tubes to deal with the problem of electrical stress concentration. That is to say, the parameter control method is used to alleviate the electric field stress concentration. The main advantages are light weight, easy installation, good performance and low price.

The stress tube is a heat shrinkable tube with a moderate electrical resistivity (1010-1012 Ωcm) and a large dielectric constant (20--25). The electrical parameters are used to force the stress at the insulation of the cable insulation to break along the edge. The stress tube is more evenly distributed. This technology is typically used in cable accessories up to 35kV. Because the voltage level is high, the stress tube will heat up and cannot work reliably.

The key technical issues in its use are:

It is necessary to ensure that the electrical parameters of the stress tube must meet the values ​​specified in the above standards to work reliably. In addition, it is necessary to fill the air gap of the cable insulation semi-conductive layer with silicone grease to eliminate the gas, so as to reduce the partial discharge. When the cross-linked cable is poorly treated due to internal stress, it will shrink greatly during operation. Therefore, when installing the accessory, pay attention to the stress tube and the insulation shield cover not less than 20mm to prevent the stress tube from being separated from the insulation shield during shrinkage. Because of the low elasticity, the heat shrinkable attachment may cause an air gap at the interface during thermal expansion and contraction during operation. Therefore, the sealing technology is important to prevent moisture intrusion.

Prefabricated accessories

The materials used are generally silicone rubber or ethylene propylene rubber. The geometrical structure method, the stress cone, is mainly used to deal with the stress concentration problem. The main advantage is that the material performance is excellent, the installation is simpler and faster, the installation can be performed without heating, and the elasticity is good, so that the interface performance is greatly improved. It is the main form of medium and low voltage and high voltage cable used in recent years. The shortcoming is that the outer diameter of the insulation layer of the cable is required to be high. The normal interference is 2 to 5 mm (that is, the outer diameter of the cable insulation is larger than the inner diameter of the cable attachment by 2 to 5 mm), and the interference is too small. The attachment will fail; the interference is too large and the cable accessories are very difficult to install (high process requirements). Especially in the middle joints, the problem is prominent, the installation is not convenient, and 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 must match the size of the cable to be installed to meet the specified requirements. A silicone greased interface is also required to facilitate installation while filling the air gap at the interface. Prefabricated accessories generally have a certain sealing effect by their own rubber elastic force, and sometimes sealant and elastic clamp can be used to enhance the sealing.

Cold shrink attachment

The materials used are generally silicone rubber or ethylene propylene rubber. Cold-shrinkable attachments generally use geometric and parametric control methods to deal with electrical stress concentrations. The geometric structure method uses the stress cone to alleviate the concentrated distribution of the electric field, which is superior to the product of the parameter control method.

As with prefabricated accessories, the material has excellent performance, can be installed without heating, and has good elasticity, which greatly improves the interface performance. Compared with prefabricated accessories, its advantages are more convenient to install, just in the correct position. Pull out the cable attachment inner core tube to complete the installation. The material used is better than the prefabricated attachment in terms of mechanical strength, 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 less than 2 mm of the outer diameter of the cable insulation (this is the case with the prefabrication). The accessory requires 2 to 5 mm deviation - editor) to fully meet the requirements. Therefore, the construction and installation of the cold-shrinkable attachment is convenient.

Its biggest feature is that the installation process is more convenient and quick, and its installation performance is the same as that of prefabricated accessories.

The price is comparable to prefabricated accessories and slightly higher than heat shrinkable accessories, making it the most cost-effective product.
The key technical issues in its use are the same as for prefabricated accessories.

In addition, the cold-shrinkable accessory products can be divided into two types: factory expansion and on-site expansion. The cold-shrinkable accessories of 35kV and below are mostly factory-expanded, and the effective installation period is within 6 months. The maximum installation period must not exceed two years, otherwise the service life of cable accessories will be affected. The cold-shrinkable accessories of 66kV and above are mostly field-expanded, and the installation period is not limited, but special tools are required for installation. Special tools are generally available from the accessory manufacturer, which is very convenient to install and reliable in installation quality.