The use of power cables has been more than hundred years of history. Since 1879, American inventor T.a Edison has wrapped the bronze rod in jute and put it into an iron tube, then filled the iron tube with a mixture of bitumen and other mixtures to make a cable, creating an underground transmission; by 1889, the British s.z. Phelan Ladder with a 10,101,010,000-volt oil-soaked paper insulated cable laid between London and Detford; and 20,000 in the United Kingdom by 1908 Volt cable network, since this power cable in the power construction has been more and more widely used. Now, with the development of China's economy, the continuous improvement of cities and roads, people's awareness of urban beautification is increasing, and the strength of government planning has been strengthened. In the urban and road reconstruction and beautification, overhead lines have become an obstacle to their development, and power cables because of the concealment of their laying and good performance, the favorite of urban transformation.
The pace of transformation goes further and further, the variety of cables is more and more, and the technology of cable use is becoming more and more mature. Most of the cables used in China's 10KV power grid are three-core cables, and because they are buried, this has a higher insulation requirement for the cable itself and its accessories. At this stage, the main cable accessories used in Guangdong are: Cable terminal head, cable Middle head and cable T connector. These accessories are divided into pouring type, cold shrinkage type and thermal shrinkage three different specifications, where the three kinds of accessories materials and use for a simple analysis and discussion.
First, the material of three sizes of cable accessories is analyzed:
1.1, Hot shrinkage cable accessories in the power construction is widely used, it uses cross-linked polyethylene and other thermal shrinkage materials, it uses nucleon to radiation materials such as polyethylene, so that the original linear structure of the polymer material into a mesh structure, into a memory function of the material, and then through sulfonation treatment and other processes. That is, in the heating state of its expansion, through nuclear radiation after cooling down, if heated again, the material will return to the original state.
1.2, cold shrinkage cable accessories is a new use of cable accessories this year, it uses elastic materials (commonly used silicone rubber and ethylene propylene rubber, etc.) vulcanization molding, and then mechanically stretch its inner diameter to expand, and then the prefabricated spiral plastic cylinder into the tensile expansion of the inside of the accessories, support the tensile elastic accessories, Thus constitute a variety of cable accessories of various parts. When installing on site, it is only necessary to set these pre-expanded fittings at the end of the finished cable or at the joint, and then pull the helical plastic cylinders inside the fittings out of the end of the fittings, as these fittings are made of elastic materials, after pulling out the helical plastic cylinders for support, The cable fittings automatically tighten the cable accessories that are formed on the cable insulation.
1.3, pouring cable accessories is the use of a corresponding mold to fix the need for construction of the cable parts, into the mold pouring epoxy resin and other insulating materials, until these insulating materials dry and remove the mold on the line pull. This kind of cable accessory installation process is relatively simple, but the weather and other natural and geographical environment requirements are high, can not be poured in wet weather or place, it is not appropriate to pour in the dusty site, so as not to reduce the cable accessories damp, contaminated and lower cable insulation. Because the weather in the south is relatively humid, coupled with the construction site is inevitably dusty, the installation quality of pouring cable accessories has a greater impact, so the use of such accessories in Guangdong is less. The pouring cable accessories are no longer analyzed in detail here.
Second, secondly, in the installation of the process of heat shrinkage and cold shrinkage cable accessories have their own characteristics and advantages and disadvantages:
2.1, heat shrinkage Cable accessories Installation: First will need to install accessories of the cable terminal stripping, and according to the cable accessories installation requirements for processing, and then with heat shrinkage materials made of the main insulation casing cover cable, the use of heat shrinkage principle of the casing heating, so that the main insulation casing tightly shrinkage in the cable semiconductor tube, and then package metal shielding layer, Finally, the external casing is thermally shrunk. Component heating shrinkage is the key link in the quality of hot shrink, it is best to put the cable on before heating, which is conducive to heating operation and uniform shrinkage of components. When heating, be aware of:
① Heating shrinkage temperature is Ll0℃ 120 ℃.
② Adjustable Spray lamp flame is yellow soft flame, beware of high temperature blue flame.
Disadvantages: The construction process is more complex. Installation heating requires the use of a hairdryer or open fire heating, which requires a temporary power supply or open fire heating device in the construction site. If the installation of temporary power supply, may be a lot of human and material resources, so the thermal shrinkage cable accessories installed generally using natural gas spray gun heating. The use of open fire heating requires high safety measures. In the use of natural gas for outdoor heating, there are many fire safety problems, to the construction of the project brought fire and other safety risks, it is necessary to increase a variety of fire safety measures to prevent, which increases the construction difficulties and construction costs.
2.2, cold shrinkage Cable Accessories installation: First according to the requirements of the attachment installation of the cable peel off and clean up, and then in order to the cable attachment into the corresponding cable area, from the inside out of the support of the spiral plastic cylinder, so that the cable accessories using their own material elastic tight pressure attached to the cable, Thus to the attachment installation site of the cable to play an insulating and protective effect, to achieve the installation of the attachment effect and requirements. The cold shrinkage method eliminates the hassle and unsafe factors that heat shrinkage products need to heat, makes the operation of the accessory installation relatively simple, and the cold shrinkage material is easy to use and quick to install, greatly reduces the time of construction preparation and safety measures implementation, and saves valuable time for power outage tasks and repair work to complete cable installation and access within a limited power outage time.
At the same time cold shrinkage cable accessories installation is not affected by the geographical environment and natural and social environment, suitable for a wide range of cable engineering use. The disadvantage is that the cold shrinkage cable accessories on the storage conditions require a higher heat shrinkage, generally required to be used in the factory half a year, the physical and chemical properties of the accessory material relative to the heat shrinkage accessories to be lower, in the high temperature and dry environment storage easy aging and elasticity weakened.
It must be pointed out that before installing to the cable, the cold shrinkage cable attachment is in a high tension state, so it is necessary to ensure that during the storage period, the cold shrinkage parts should not have obvious permanent deformation or elastic stress relaxation, otherwise installed on the cable can not be guaranteed to have sufficient elastic compression force, so that good interface characteristics can not be guaranteed.
Third, from the operation of the Economic and security analysis:
1) Cold shrinkage cable accessories raw materials and production costs are high, so the market price of cold shrinkage cable accessories to heat shrinkage cable accessories a lot higher. In terms of service life, cold and hot shrinkage cable accessories are the same as the cable, so in terms of economy thermal shrinkage cable accessories are superior to cold shrinkage cable accessories.
2) Cold Shrinkage cable accessories have a good elasticity, its strong resilience is the attachment has enough grip force, no matter how the ambient temperature changes, accessories can be guaranteed to firmly hold the holding area. This ensures that the water and moisture are not inhaled, thus ensuring a safe internal crawl distance of the cable. The shrinkage temperature of the heat shrinkable cable accessory is 100~140℃, that is, when the heating is installed to meet the shrinkage conditions of the attachment, if the ambient temperature is reduced, due to the different expansion coefficient of the cable and heat shrinkage material and the formation of a gap between the attachment and the cable, then the water and moisture and other substances through the respiration of the cable into the gap , which destroys the insulation of the cable and endangers the safe operation of the cable.
To sum up, thermal shrinkage cable Accessories Although the installation process is complex, but because of its cheap, in Guangdong and other places with little temperature difference is still widely used, and cold shrinkage type of cable accessories because of its simple construction technology and good operation performance, in the power system is also more and more widely used, but its storage period and material performance need to be improved, Production costs need to be reduced.
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