Corona phenomenon in HV transmission lines can be defined simply as a partial ionization of air in vicinity of conductor which is exposed to high voltage. Actually, corona is a natural cause of high voltage and is inevitable for the high voltage overhead lines in interconnected transmission line system. One can identify the corona phenomenon by its very distinctive audible noise and emitted light that can not be seen directly by human eye. Since there is sound energy and ionization of air, someone can easily conclude that there should be energy loss due to corona. This loss is proportional to the magnitude of the voltage on the conductor.
It had better to continue with the benefits and harms of corona to electrical system and society. I prefer start with the benefits of corona since it has very few indeed. Since corona can be represented as a loss that is proportional to voltage magnitude in power system, it has a damping effect on temporary over voltages or lightning discharges. By this way harmful effects of over voltages or lightning discharges reduced significantly especially for the long transmission lines. Another benefit of corona can be seen as an audible noise, although it is a harmful feature. Any person who is working on high voltages can identify the high voltage by its corona noise and it can be a warning signal especially if the line of sight or illumination is weak. Let me continue with the harms of corona, this part is much more dominant on benefits. As said before, corona is a source of loss in power system and has very high values for high voltages, V > 200kV. This loss should be reduced since it reduces the efficiency of power system network significantly. Also, the audible noise has a psychological effect on creatures, including human society, living vicinity of HV transmission line. In addition to these harms, corona is a source of electromagnetic waves, which has a negative effect on electronic devices and human body, which is not proved but is under investigation. Let me continue with how can we reduce corona in high voltage transmission lines.
There are a couple of ways that is effective for reducing the corona, which are reducing the electrical field per unit area and conductor bundling. Ionization of air starts in a point that has the highest electric field per unit area. If the conductor has singularities (sharp ends, wedge shaped ends, burr), corona become effective at smaller voltages. The electrical field per unit area at these singularities is increased since the differentiation of the surface area is undefined, by this way the corona becomes effective even at low voltages if the conductor has sharp ends. Using this information it can be concluded that we should make the conductor surface and conductor ends smooth and eliminate the burrs on conductor in order to reduce the corona. Please note that none of the manufactured high voltage element has singularities on its surface, their surface is made smooth on purpose. You can check the conductor or busbar ends by your hand so that you can confirm that there is no burrs or sharp points on it. The other method for reducing the corona is bundling. Corona is effective for the transmission line voltages higher than about 200 kV, so bundling should be applied to higher voltages like 380 kV interconnected system. Bundling is a method that requires more than one conductor on a phase, by this way the current is shared equally among bundled conductors and the equivalent radius (Geometric Mean Radius GMR) is increased and electric field per unit area decreases. This provides reduced corona in HV transmission lines. The above figure,1, represents the bundling process.
Figure 1: (1) Single Conductor , (2) Bundled Conductor
Conductors (1) and (2) have the same cross sectional area for current conduction but the three bundled conductors (2) have reduced corona effect. The above photography, figure 2, shows the practical usage of bundled conductors in 380 kV transmission line network.
Figure 2: Two-bundled conductor in 380 kV connection of Kemerköy Thermal Plant Muğla/TURKEY