Discussion on Thermal Expansion and Countermeasures during Power Cable Wiring

With changes in load current and environmental temperature, power cables undergo thermal expansion and contraction, resulting in significant thermal mechanical forces due to the thermal expansion and contraction of the core. The larger the cross-section of the cable core, the greater the thermal mechanical 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 poses a significant threat to the operation of power cables, causing displacement, slippage, and even damage to cables and accessories. The maximum cable cross-section currently used in China is 7 X 1600 mm, so it is necessary to pay attention to the thermal expansion problem of large cross-section cables.

Here is a brief analysis of the threats posed by cable thermal expansion to safe operation under various laying methods:

(1) When directly buried, 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 thermal mechanical force, causing end displacement and posing a great threat to the safety of cable accessories.

(2) When laying the cable, it will bend and deform under the action of thermal mechanical force because it is not subject to lateral constraints; As the temperature of the cable changes, bending deformation repeatedly occurs, causing fatigue strain in the metal sheath of the cable

(3) When laying in tunnels, cables are generally placed on supports and not rigidly fixed, so the thermal expansion and contraction of cables are relatively large, and they are prone to slipping when laid on slopes; Severe displacement is prone to occur at cable bends; With the continuous change of cable temperature, cables will repeatedly bend and deform, causing fatigue strain on the metal sheath of the cable.

(4) When laying in a vertical shaft, the self weight and thermal mechanical force of the cable may cause excessive strain on the metal sheath, thereby shortening the service life of the cable.

(5) When laying municipal bridges, if the cables are laid in the pipes inside the bridge, there are the same problems as laying pipes; If the cable is laid in the box girder of the bridge, there are the same problems as tunnel laying. In addition, the cable laid on the bridge will also be affected by the expansion and vibration of the bridge, thereby accelerating the damage of the cable metal sheath.

Corresponding measures should be taken to address the above-mentioned hazards, starting from the design and production of cables and accessories, cable line design, construction, and other aspects.

(1) Cables and accessories. To reduce the thermal expansion and contraction of large cross-section cables, it is advisable to use split conductors for the cable core. This not only reduces the loss of the core, but also generates less thermal mechanical force per unit area compared to other forms of conductors. The design of cable accessories must consider being able to withstand the thermal and mechanical forces of the cable without damage.

(2) There are currently two types of cable metal sheaths, aluminum sheath and aluminum alloy sheath, with significant differences in their performance: aluminum sheath can improve the operational performance of cables compared to aluminum alloy sheath. Therefore, except for projects with particularly high anti-corrosion requirements, it is generally advisable to choose aluminum sheath for cable metal sheath.

(3) Directly buried cables can be laid in a serpentine pattern near the terminal, such as in the cable layer of a substation, to absorb deformation and reduce end thrust. Rigid fixation should be applied at the support to prevent damage to the terminal due to cable displacement.

(4) When laying large section cables in ducts, bentonite can be filled into the ducts containing the cables to prevent bending deformation. Flexible fixation can be used at the outlet of the drainage pipe in the well, and rigid fixation is required on both sides of the cable joint to protect the safety of the cable joint.

(5) The cables inside the tunnel can be laid in a serpentine pattern to absorb deformation caused by thermal mechanical forces. When laid on a slope, 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 joint.

(6) The large cross-section cables inside the vertical shaft can be laid in a serpentine pattern using clamps, and suspended at the top of the shaft to absorb deformation caused by thermal mechanical forces.

(7) The cables laid on municipal bridges must use aluminum sheaths to reduce the fatigue strain caused by bridge vibration on the metal sheaths of the cables. The laying method can refer to conduit or tunnel. It should be noted that while considering the thermal expansion and contraction of the cables, the expansion and contraction of the bridge must also be taken into account. Flexible fixation must be adopted at the expansion and contraction joints of the bridge and the upper and lower bridges, or bent frames that can allow the cables to expand and contract freely must be selected.