Overhead lines mainly refers to the overhead line, erected on the ground above, the insulator is the transmission line will be fixed on the tower on the ground to transmit power transmission lines. Overhead lines mainly include AAAC conductor , AAC conductor ,ACSR conductor, Aerial Bundled Cables . They are in easy installation and maintenance, low cost, but vulnerable to weather and the environment (such as wind, lightning, pollution, snow, etc.) caused by the failure, while occupying the entire power transmission corridor more land area, easy to cause electromagnetic interference to the surrounding environment.
The most common conductor in use for transmission today is all-aluminum-alloy conductor (AAAC). Aluminum is used because it has about half the weight of a comparable resistance copper cable (though larger diameter due to lower specific conductivity), as well as being cheaper.Copper was more popular in the past and is still in use, especially at lower voltages and for grounding.
While larger conductors may lose less energy due to lower electrical resistance, they are more costly than smaller conductors. An optimization rule called Kelvin's Law states that the optimum size of conductor for a line is found when the cost of the energy wasted in the conductor is equal to the annual interest paid on that portion of the line construction cost due to the size of the conductors. The optimization problem is made more complex by additional factors such as varying annual load, varying cost of installation, and the discrete sizes of cable that are commonly made.
AAAC has mush advantages in overhead conductors:
(1) In the same unit weight, the DC resistance of the aluminum alloy conductor is small, the current carrying capacity is large, the pulling force is large, the tensile single weight ratio is large, and the like;
(2) Compared with the same current carrying capacity, aluminum alloy wire light weight, large pulling force, tensile single weight greater than other advantages. Aluminum wire as a single material wire, easy installation and construction;
(3) Aluminum alloy wire bearing low resistance level;
(4) Ordinary ACSR service life of about 50 years, aluminum wire life of 100 to 120 years.