How to reasonably choose the conductors of overhead lines？
1. The principle of conductor selection
The conductor and ground wires of power transmission lines operate in the wilderness, mountainous areas or the edge of lakes and seas for a long time, and need to withstand the effects of external loads such as wind and ice, drastic changes in temperature and attack by chemical gases, etc., and are also subject to national resources and line costs And other factors. Therefore, in the design, especially for large spans, the material and structure of the wires must be carefully selected.
Generally, the following principles should be considered when selecting the material and structure of the wire:
⑴ The conductor material should have high conductivity. However, considering national resources, copper wires should not be used in general.
⑵The conductor and ground wire should have high mechanical strength and vibration resistance.
⑶ The conductor and ground wire should have certain chemical resistance and oxidation resistance.
⑷ When choosing the wire material and structure, in addition to meeting the transmission capacity, the cost of the line should be economical and technically reasonable.
2. Selection of conductor cross section
The cross section of the overhead transmission line conductor is generally selected according to the economic current density, and should be checked according to the heating conditions, voltage loss, mechanical strength and corona in the event of an accident. When necessary, it is determined through technical and economic comparison; but for lines of 110KV and below, corona is often not the decisive factor in selecting the conductor cross section.
1) Choose the wire section according to the economic current density
According to the economic current density to select the transmission capacity used for the conductor cross-section, the development plan of the power system should be considered for 5-10 years after the line is put into operation. In the calculation, the maximum load that frequently recurs under the normal operation mode must be adopted. But when the system is not clear, care should be taken not to make the wire cross section too small.
2) Check the cross-section of the wire according to the corona condition
With the continuous increase of operating voltage in my country, the probability of corona and discharge of wires, insulators and fittings has increased. For the cross-section of wires of 220KV and above voltage lines, corona conditions often play a major role.
The corona generated by the wire will bring about two undesirable consequences: ① It increases the power loss of the power transmission line; ② It interferes with radio communication and carrier communication.
Regarding the corona loss, if the corona loss of the transmission line is directly calculated, the advantage is that the concept of quantity is very clear, but the disadvantage is that the calculation is cumbersome. This method is rarely used at present. It tends to be measured by the ratio of the maximum working electric field strength Em (unit: KV/cm) of the wire to the critical electric field strength E0 of the overall corona. The ratio of Em/E0 should not be greater than 80%-85%.