Brief introduction: with the change of load current and ambient temperature, power cable will have thermal expansion and contraction, in which the thermal expansion and cold contraction of the core will produce a very large thermal mechanical force. The larger the cross section of the cable core is, the greater the thermal mechanical force will be generated. At the same time, the core and metal sheath will also creep due to multiple cycles of thermal expansion and cold contraction. Thermal expansion poses a great threat to the operation of power cables, which may cause the displacement, sliding and even damage of cables and accessories. At present, the largest cable section has been selected in China is 7x1600mm =. Therefore, attention must be paid to the thermal expansion of large section cable.
1、 With the change of load current and ambient temperature, power cable will have thermal expansion and contraction, in which a very large thermal mechanical force is generated due to the thermal expansion and cold contraction of the core. The larger the cross-section of the cable core is, the greater the thermal mechanical force will be generated. At the same time, the core and metal sheath will creep due to the multiple cycles of thermal expansion and cold contraction. Thermal expansion poses a great threat to the operation of power cables, which may cause the displacement, sliding and even damage of cables and accessories. At present, the largest cable section has been selected in China is 7x1600mm =. Therefore, attention must be paid to the thermal expansion of large section cable.
This paper makes a simple analysis on the threat of cable thermal expansion to safe operation under various laying modes
(1) When the cable is directly buried, due to the limitation of the surrounding soil, the whole cable can not produce displacement, so the core will produce a great thrust at the two 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 the cable is laid in the pipe, because the cable is not constrained laterally, the cable will produce bending deformation under the action of thermal mechanical force; with the constant change of cable temperature, the bending deformation occurs repeatedly, which causes the fatigue strain of the cable metal sheath.
(3) During tunnel laying, the cable is generally placed on the support without rigid fixation, so the thermal expansion of the cable is large, and it is easy to slip when laying on the inclined plane; serious displacement is easy to appear at the bending part of the cable; with the constant change of the cable temperature, the cable will repeatedly appear bending deformation, causing fatigue strain of the cable metal sheath.
(4) When the cable is laid in the shaft, the dead weight and thermal mechanical force of the cable may cause excessive strain on the metal sheath, thus shortening the service life of the cable.
(5) When municipal bridge is laid, if the cable is laid in the inner pipe of the bridge, it will be very difficult. If the cable is laid in the box girder of the bridge, it will have the same problem as tunnel laying. In addition, the cable laid on the bridge will be affected by the expansion and vibration of the bridge, which will accelerate the damage of the metal sheath of the cable.
2、 Corresponding countermeasures should be taken for the above hazards, which must start from the design and production of cables and accessories, the design and construction of cable lines, etc.
(1) Cables and accessories. In order to reduce the thermal expansion of large cross-section cable, the cable core should be split conductor, which can not only reduce the loss of core, but also produce less thermal mechanical force per unit area than other types of conductor. Cable accessories must be designed to withstand the thermal mechanical force of the cable without damage.
(2) At present, there are two kinds of cable metal sheath: aluminum sheath and aluminum alloy sheath, and their performance is quite different: compared with aluminum alloy sheath, aluminum sheath can improve the operation performance of the cable, so in addition to the engineering with special high anti-corrosion requirements, aluminum sheath is suitable for general cable metal sheath.
(3) The cable directly buried near the terminal, such as in the cable layer of substation, can be laid in serpentine shape to absorb deformation and reduce the terminal thrust; the support shall be fixed rigidly to prevent the terminal from being damaged due to cable displacement.
(4) In order to prevent the cable from bending and deforming, bentonite can be filled into the cable laying duct. In order to protect the safety of the cable joint, it is necessary to make rigid fixation on both sides of the cable joint.
(5) The cables in the tunnel can be laid in a serpentine shape to absorb the deformation caused by thermal mechanical force. When laying on the inclined plane, the cables need to be fixed, and the cables on both sides of the joint also need to be rigidly fixed, so as to protect the safety of the cable joint.
(6) The large cross-section cable in the shaft can be laid in a serpentine shape with the help of a clamp, and suspended at the top of the shaft to absorb the deformation caused by thermal mechanical force.
(7) The cables laid on municipal bridges must use aluminum sheath to reduce the fatigue strain caused by bridge vibration on the metal sheath of cables. The laying method can refer to row pipes or tunnels. It should be noted that while considering the thermal expansion of cables, the expansion and contraction of bridges should also be considered. Flexible fixation must be adopted at the expansion and contraction joints of bridges and the upper and lower bridges, or the flexible cable can be selected Bent.