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How to effectively select the conductors of overhead cables?
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How to effectively select the conductors of overhead cables?
When selecting wires for overhead lines, aluminum stranded wires that meet national standards should be used, and single-strand aluminum wires, unstrand wires and iron wires are prohibited. When selecting the cross-section, it is necessary to take into account the mechanical strength of the wire to avoid uncertain factors such as wire breakage due to weather changes and external forces.
1. The selection principle of wire
The conductors and ground wires of power transmission lines run in the wilderness, mountains or the edge of lakes and seas for a long time, and they need to withstand the external loads such as wind and ice, the drastic changes in temperature and the invasion of chemical gases, etc. factor constraints. Therefore, in the design, especially in the large-span section, the material and structure of the wire must be carefully selected.
The material and structure of the selected wire should generally consider the following principles:
(1) The wire material should have a high conductivity. However, considering the factors of national resources and line prices, generally copper wires should not be used.
(2) Conductors and ground wires should have high mechanical strength and vibration resistance.
(3) Wires and ground wires should have certain chemical corrosion resistance and oxidation resistance.
(4) When selecting the material and structure of the wire, in addition to meeting the transmission capacity, it should also ensure that the cost of the line is economical and the technology is reasonable.
2. Selection of wire cross section
The conductor cross section of overhead power transmission lines is generally selected according to the economic current density, and should be checked according to the heating conditions, voltage loss, mechanical strength and safe current carrying capacity in the event of an accident. If necessary, it shall be determined through technical and economic comparison; but for lines of 110KV and below, corona often does not become the decisive factor for selecting the wire cross section.
1) Select the wire cross section according to the economic current density
The transmission capacity used for the cross-section of the wire is selected according to the economic current density, and the development plan of the power system for 5 to 10 years after the line is put into operation should be considered. The maximum load that often occurs repeatedly in the normal operation mode must be used in the calculation. However, if the system is not yet clear, care should be taken not to make the cross-section of the wire too small.
2) Check the cross section of the wire according to the corona condition
As the operating voltage in China continues to rise, the probability of corona and discharge in conductors, insulators and fittings increases, and the corona conditions often play a major role in the conductor cross-section of 220KV and above voltage lines.
The corona generated by the wire will bring two adverse consequences: ① increase the power loss of the power transmission line; ② interfere with radio communication and carrier communication.
What are the advantages and disadvantages of choosing overhead lines?
Advantage
1. High reliability, less affected by the surrounding environment, suitable for various harsh weather conditions;
2. The insulation distance between the wires is small, the area is small, and the interference is small;
3. It is more convenient to erect in open areas;
4. The construction period is short.
Shortcoming
1. It is not easy to change the branch of the cable line, and the operation and maintenance costs are relatively high;
2. It is difficult to detect and repair cable faults.