Thermal expansion is usually defined as the increase in the volume of most substances at increasing temperatures and the decrease in volume at decreasing temperatures with constant external pressure.
Thermal expansion is related to physical properties such as temperature, heat capacity, binding energy, and melting point.
The main factors affecting the expansion properties of materials are phase change, material composition and organization, and the effect of anisotropy.
The cable is the medium for the transmission of electric energy in the power system.
In essence, the cable is also a kind of electric load, except that due to the small resistance of the cable itself, the power lost in the cable under normal circumstances accounts for a very small proportion of the total transmission and distribution energy.
The following factors contribute to abnormal cable heating:
1 The cable is not produced in accordance with the standard cross-section specified in the national standards, resulting in the cable's own resistance exceeding the standard.
2 The copper used in the cable contains too many impurities and does not meet the purity standard of electrical copper, resulting in the cable's own resistance exceeding the standard.
3 The thickness of the cable insulation layer is not up to standard or the amount of rubber contained in the insulation material is not up to standard, resulting in the insulation resistance of the inter-phase cable not being up to standard.
4 The cable selection does not match the load current, which exceeds the load capacity of the cable during normal operation for a long time.
1 During construction, the terminal size is small or the terminal and cable are not crimped tightly, resulting in excessive resistance at the end of the cable.
2 The cable is laid too densely, resulting in the heat emitted during normal operation of the cable not being dissipated in time.
3 The cable is laid in a slot box or a closed bridge, which is not conducive to heat dissipation.
4 The cables are laid around heat sources, e.g., near heat pipes and without insulation measures.
5 Common cables are used for high-frequency loads, resulting in overheating of the cables.
6 Harmonic content in the grid is excessive and does not match the cable design and selection, resulting in overload.
The solution for the cable's own factors leading to heat generation
1 Strictly in accordance with the parameters set by the national standards require suppliers to provide test reports, including cross-sectional area, copper content, insulation layer thickness and insulation material adhesive content, and other indicators.
2 Strictly control the design selection, in accordance with the national standard requirements.
Cable selection principles are: cable load capacity should be greater than the rated current rectification value of this level circuit breaker.
In the project, the use of load calculation current as the basis for selecting the cable load capacity is wrong.
3 Before the installation of the cable, strictly in accordance with the operating procedures test the insulation resistance of the cable.
Solution measures for heat generation caused by laying environmental factors
1 Wiring terminals meet the requirements of the standard cross-section, crimping construction standards, in line with the requirements of the construction process.
2 Cable laying to avoid disorderly, stacking too dense and other irregularities, fixed in accordance with the requirements of the specifications, try to avoid multi-layer stacking, should be laid in a single layer, up to two layers.
3 For cables laid in slot boxes or enclosed bridges, set ventilation holes appropriately, or take necessary ventilation measures.
4 When the cable is laid adjacent to the heat source, conform to the spacing distance required by the specification, or take measures such as heat insulation plate.
5 High-frequency loads should use high-frequency special cables.
6 Harmonic control measures of the power grid are not only limited to the centralized control of substations but also increase the in-situ compensation appropriately according to the site conditions.
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