The cable is mainly composed of the following 4 parts.
Conductive wire core: made of high conductivity material (copper or aluminum). Depending on the requirements of the softness of the cable, each wire center may be made of a single wire or multiple wires.
Insulating layer: The insulating material used as a cable should have high insulation resistance, high breakdown electric field strength, low dielectric loss and low dielectric constant. Commonly used in cables are oil-impregnated paper, polyvinyl chloride, polyethylene, cross-linked polyethylene, rubber, etc. Cables are often classified as insulating materials, such as oil-immersed paper-immersed insulated cables, polyvinyl chloride cables, cross-linked polyethylene cables, etc.
Sealing sheath: protects the insulating wire core from damage from machinery, moisture, moisture, chemicals, light, etc. For insulation that is prone to moisture, lead or aluminum extruded sealing sheath is generally used.
Protective cover: Used to protect the sealing sheath from mechanical damage. Generally, galvanized steel strips, steel wires, copper strips, copper wires, etc. are used as armor to wrap around the sheath (called armored cables). The armor layer also acts as electric field shielding and prevents external electromagnetic wave interference. In order to avoid corrosion of steel strips and wires from surrounding media, they are usually coated with asphalt or entangled with impregnated jute layer or extruded polyethylene and polyvinyl chloride sleeves.
According to their purpose, cables can be divided into power cables, communication cables and control cables. Compared with overhead lines, the advantages of cables are that they have small insulation distances between lines, small spaces for land, underground laying without occupying space above the ground, are not affected by surrounding environmental pollution, have high power transmission reliability, and have little interference to personal safety and surrounding environment. However, the cost is high, construction and maintenance are both troublesome, and manufacturing is also more complicated. Therefore, cables are mostly used in densely populated areas and dense grid areas and busy traffic areas; when laying across rivers, rivers and underwater, large-span overhead lines can be avoided. Cables can also be used where overhead lines need to avoid interference to communications, and where aesthetics need to be considered or exposed to the target.
As the load current changes and the ambient temperature changes, the power cable will undergo thermal expansion and contraction, and a very large thermal mechanical force is generated due to the thermal expansion and contraction of the wire core. The larger the cross-section of the cable core, the greater the thermal mechanical force is generated; at the same time, the core and metal sheath will also cause creep due to multiple cycles of thermal expansion and contraction. Because thermal expansion poses a great threat to the operation of power cables, it will cause the operation cable to displace, slide, and even damage the cables and accessories. Therefore, it is necessary to pay attention to the thermal expansion and contraction of large-section cables.
The threats posed by thermal expansion and contraction of cables to safe operation under various laying methods are:
(1) When laying directly, the cable is restricted by the surrounding soil, and the entire cable cannot be displaced. Therefore, the wire core will generate a large thrust at both ends of the line under the action of thermal mechanical force, causing the end displacement, which poses a great threat to the safety of cable accessories.
(2) When laying a bending pipe, the cable will be bending and deformation under the action of thermal mechanical force; as the cable continues to change in the cable temperature, bending deformation will occur repeatedly, causing fatigue strain in the cable metal sheath.
(3) When laying the tunnel, the cables are generally placed on the bracket without rigid fixing, so the cables are hot and stretched, and slipped when laid on the inclined surfaces are prone to occur; severe displacement is prone to occur at the bend of the cable; as the cables continue to change in the cable temperature, they will also repeatedly bending and deform, causing fatigue strain to occur in the metal sheath of the cable.
(4) When laying the shaft, the self-weight and thermal mechanical force of the cable may cause excessive strain to the metal sheath, thereby shortening the service life of the cable.
(5) When laying a bridge, if the cable is laid in the inner discharge pipe of the bridge, there is the same problem as the laying of the drain pipe; if the cable is laid in the box girder of the bridge, there is the same problem as the tunnel laying. In addition, the cables laid on the bridge will also be affected by the expansion and vibration of the bridge, thereby accelerating the damage to the cable metal sheath.