For overhead transmission lines, besides the active loss of conductor resistance, there are corona losses and active losses caused by leakage of insulators. For cable lines, the main dielectric loss is.
Generally, in the overhead line, the active power loss caused by the leakage current of the insulator is very small, and it can be ignored. Therefore, the corona loss is mainly considered for overhead lines.
Because the insulation medium of overhead line is air, when the electric field strength of conductor surface exceeds that of air, the air layer will generate free discharge, and the movement of charged ions in air will form corona current. At the moment, we can hear the "discharge sound Zizi", and can smell the oxygen molecules by free and combined ozone (O3) smell at night can be seen around the light blue fluorescence wire, this phenomenon is called "corona". Corona not only produces active loss, but also interferes with radio and high frequency communication.
The generation of corona is mainly related to the transmission voltage. With the increase of voltage, corona usually starts from the local part of the electric field with strong intensity, which is called the initial corona. (the critical intensity of the initial corona is 30 thousand volts / cm). The corona is then visible and expanded to all surfaces to form a full corona.
In order to reduce corona loss, especially to reduce interference to radio and high frequency communications, try to avoid corona. The way to avoid corona is to increase the critical voltage of the corona and make it higher than the operation voltage of the line, so we must increase the distance between wires and the radius of conductor. But if the distance between lines is increased, the construction cost of the line tower must be increased, and the impact on the critical voltage is very small. So. Increasing the radius of the wire is an effective way to improve the critical voltage. Therefore, in UHV transmission lines, the expansion traverse or the split wire (the equivalent radius of the split wire plays the role of expanding the diameter).