Cable thermal expansion and its solutions

As the load current changes and the ambient temperature changes, the power cable will undergo thermal expansion and contraction. The thermal expansion and contraction of the core produces a very large thermomechanical force. The larger the cross section of the cable core, the greater the thermomechanical force generated. At the same time, the core and metal sheath will also experience creep due to multiple cycles of thermal expansion and contraction.

Thermal expansion and contraction pose a great threat to the operation of power cables, which can cause the running cables to move, slip off, and even damage the cables and accessories. Therefore, attention must be paid to the thermal expansion and contraction of large cross-section cables.

Now let’s make a simple analysis of the threats to safe operation caused by the thermal expansion and contraction of cables under various laying methods:
1. When laying directly, the cable is limited by the surrounding soil, and the entire cable cannot be displaced. Therefore, the core will generate a large thrust at the two ends of the line under the action of thermo-mechanical force, causing the end to be displaced. It poses a great threat to the safety of cable accessories.
2. When the lingering pipe is laid, the cable will not be constrained laterally. Under the action of thermo-mechanical force, the cable will produce bending deformation. With the continuous change of the cable temperature, the bending deformation will occur repeatedly, causing the cable metal sheath to produce fatigue strain.
3. When laying in a tunnel, cables are generally placed on supports without rigid fixation, so the thermal expansion of the cables is large, and they are prone to slipping when laying on an inclined plane. Serious displacement is likely to occur at the bend of the cable. Changes and repeated bending deformations will cause fatigue strain on the cable metal sheath.

The corresponding countermeasures to the above hazards must start from the design and production of cables and accessories, cable circuit design, and construction.

1. Cables and accessories. In order to reduce the thermal expansion and contraction of large cross-section cables, the cable core should be split wire, which can not only reduce the loss of the core, but also the thermo-mechanical force generated per unit area is smaller than that of other types of wires. The design of cable accessories must be considered to be able to withstand the thermo-mechanical force of the cable without damage.

2. At present, there are two types of cable metal sheath: aluminum sheath and aluminum alloy sheath. Their performance is quite different. Compared with aluminum alloy sheath, aluminum sheath can improve the running performance of the cable, so the anti-corrosion requirements are particularly high. For engineering, it is generally advisable to choose aluminum sheath for cable metal sheath.

3. Directly buried cables can be laid in a serpentine shape near the terminal, such as in the cable layer of a substation, to absorb deformation and reduce the end thrust. The bracket should be rigidly fixed to prevent the terminal from being damaged due to cable displacement.

4. When laying large cross-section cables in the piping, in order to prevent the cable from bending and deforming, the piping with the cable can be filled with bentonite. It can be disturbedly fixed at the exit of the pipe of the work well, and rigidly fixed on both sides of the cable joint to protect the safety of the cable joint.

5. Cables in the tunnel can be laid in a serpentine shape to absorb the deformation caused by thermomechanical forces. When laying on an inclined plane, the cables need to be fixed, and the cables on both sides of the joint need to be rigidly fixed to protect the safety of the cable joints.