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What is the difference between overhead cables and buried cables?


In power cables, the common laying methods of cables are mostly divided into direct burial or overhead. What are the differences and advantages of the two in use, we can simply understand.

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What is an overhead cable?

Aerial insulated cables are a new type of transmission circuit. An overhead line whose conductors are protected by insulation and sheathing. It uses special cables manufactured in a similar process to cross-linked cables. Overhead cable is mainly used in power grid construction and 10kV transmission project line transformation and other places. It is the most suitable series of products for line maintenance and safety.

                                                    

Advantages of overhead insulated cables

1. Simplify the line tower structure, which not only saves line materials, but also beautifies the city streets.
2. It is conducive to improving and enhancing the safety and reliability of the power distribution system, greatly reducing the risk of personal electric shock casualties, insulated wires can prevent phase-to-phase short circuits caused by foreign objects, reduce the number of power outages during the operation of the combined pole line, reduce maintenance workload, and improve Line power utilization.
3. Save the line power loss and reduce the voltage loss, the line reactance is only 1/3 of the ordinary bare wire line reactance.
4. Go deep into the load center at high voltage to improve voltage quality and reduce power loss.
5. Reduce the gravity requirements of the line, reduce the investment of fittings, and reduce the labor intensity of workers when setting up the wire.
6. It saves the space occupied by overhead lines, and facilitates the passage of overhead lines in narrow passages. The line corridor is reduced, compared with the overhead bare wire, the line corridor can be reduced by 1/2.
7. Reduce wire corrosion, thereby improving the service life of the line and the reliability of power distribution.
8. Reduce the insulation requirements for line supports and increase the number of lines on the same pole. Due to the improvement of the technical status of the line, the maintenance workload is reduced, the maintenance cycle is extended, and the time of power outage due to maintenance is reduced.


shortcoming


1. The allowable carrying capacity of high-voltage insulated overhead wires is smaller than that of bare wires, because after the plastic layer is added, the heat dissipation of the wires is poor, and the shape of overhead insulated wires such as triplex overhead wire should be higher than usual, and must be matched to 95mm2.

2. The wire diameter of the high-voltage insulated overhead wire is larger, and with the addition of plastic sheath, the wire diameter is one grade larger than that of the steel-cored aluminum stranded wire with the same cross-section. Although insulated wires have many advantages, the unit cost is higher than that of bare wires, about twice as high for medium-voltage lines, and about 25% higher for low-voltage lines.


Environmental requirements for laying overhead insulated cables


1. When laying cables, the ambient temperature should not be lower than -20°C. When laying below -20°C, the cables must be heated in advance.
2. When laying overhead cables, the bending radius of rated voltage 1kV overhead insulated cables should not be less than 6 times the outer diameter of the cable, and 101kV and 35KV overhead insulated cables should not be less than 15 times the outer diameter.
3. The erection method of the cable is the same as that of the bare overhead line. It still needs to be fixed on the insulator of the cross arm, but the distance can be appropriately reduced.
4. The cable should be used in conjunction with specially designed metal appliances for overhead insulated cables.
5. During laying construction and storage, attention should be paid to the sealing of the cable end to prevent the cable from entering water and reducing its performance.
6. The long-term operating temperature of overhead cables (that is, the long-term allowable operating temperature of the cable conductive core), XLPE insulation should not exceed 90 ° C, high-density polyethylene insulation should not exceed 75 ° C; polyethylene, polyvinyl chloride insulation should not exceed 70°C; cross-linked polyethylene insulation should not exceed 250°C during short circuit; high-density polyethylene, polyvinyl chloride insulation should not exceed 150°C, and the longest duration should not exceed 5 seconds.


 What is a buried cables?

Buried cables are made of one or more mutually insulated conductors covered with insulating layers and protective layers. They are used to transmit power or information from one place to another. They are often laid in cable trenches, tunnels, pipelines or indoor. With the development of society, the tension of urban land use and the impact of traffic pressure, underground cable transmission is widely used in big cities. Compared with overhead lines, cables have the advantages of small footprint, reliable power transmission, and strong anti-interference ability.

                                             
The difference between overhead cables and buried cables

 

1. Different laying methods

Overhead cable is cables that hang from utility poles. This laying method can make use of the original overhead open line pole road, saving construction cost and shortening the construction period. But the overhead cables are required to be able to adapt to various natural environments. Due to the influence of various natural disasters, optical cables are extremely vulnerable to damage by external forces, so the failure rate of overhead optical cables is higher than that of direct buried and ducted optical cables. It is generally used for long-distance lines at or below the second level, that is, it is not an important location.
The direct buried optical cable is armored with steel tape or steel wire and buried directly in the ground. They require resistance to external mechanical damage and protection against soil corrosion. A qualified sheath structure can be selected according to the requirements of the use environment, and it is safer to be placed in the groove.


2. Different materials


Aerial cables generally use steel-cored aluminum strands as conductors. Steel has high strength and is not easy to break. Aluminum has a low specific gravity and is easy to install. It can also be used for overhead interception. If the distance is not long, the steel core can also be used. Use aluminum wire directly. If there is insulation, it is generally weather-resistant black polyethylene or cross-linked polyethylene, generally without a sheath.
Underground cables generally use aluminum as the conductor, polyethylene as the insulation, and polyvinyl chloride or polyethylene as the sheath layer.