The so-called "corona" often appears on overhead lines at night. In fact, there is a high voltage between the two wires, and the charge overcomes the resistance of air with a small current under the action of high-intensity electric field, making the air glow.

Air is an insulating substance, but no insulating substance can hinder the charge indefinitely. In other words, its resistance value is large, but it is not infinite. However, the conductivity of insulating materials varies with the applied electric field. When the voltage is low, the conductivity is poor, and only a small current called "leakage current" passes through; With the increase of applied electric field voltage, the "leakage current" gradually increases; When the electric field voltage increases to a certain value, the amount of "leakage current" gradually increases obviously, and finally "leakage current" becomes short-circuit current, at which time the insulating substance is called "breakdown". The breakdown of insulating materials is not only related to its insulation performance, but also related to its size.

For transmission lines of normal design, the distance between conductors is specified. Under normal circumstances, the "leakage current" between two wires is too low to generate corona visible to human eyes. But when the air humidity increases or the dust in the air increases, the insulation performance of the air decreases, and the "leakage current" increases at this time. If the "leakage current" increases enough to produce corona, people can see blue light between the wires of the transmission line at night, that is, corona appears. This is the mechanism of corona.

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