The corona event (also known as corona discharge) is an electrical discharge caused by the ionization of a liquid, such as air surrounding an electrically charged conductor. The corona effect will occur in high voltage systems unless sufficient care is taken to limit the power of the surrounding electric field. Corona discharge can cause an audible hissing or cracking noise, as it ionizes the air around conductors. This is common in high voltage electrical transmission lines. The corona effect can also create a purple glow, ozone gas production around the conductor, radio interference and loss of electrical energy.
What is the Corona Event?
The corona effect occurs naturally due to the fact that the air is not a perfect insulator, which under normal conditions contains many free electrons and ions. When an electric field is established in the air between the two conductors, the free ions and electrons in the air will experience a force. Due to this effect, ions and free electrons accelerate and move in the opposite direction. During their movement, charged particles collide with each other, as well as with slow-moving, unburdened molecules. Thus, the number of charged particles increases rapidly. If the electric field is strong enough, a dielectric air failure occurs and an arc is formed between conductors. Electrical energy transmission is related to the bulk transfer of electrical energy from production stations located miles away from the main consumption centers or cities. Therefore, long-distance transmission conductors are extremely necessary for effective power transfer, which leads to massive loss throughout the system. Minimizing these energy losses has been a major challenge for power engineers. Corona discharge can significantly reduce the efficiency of EHV (extra high voltage) lines in power systems. Two factors are important for corona discharge to occur:
- The potential difference in alternative electricity should be provided along the line.
- The range of conductors should be large enough compared to the line diameter.
When an alternate current is made to flow along two conductors of a transmission line with a large range relative to their diameter, the air surrounding the conductors (consisting of ions) is subjected to dielectric stress. At low values of the feeding voltage, nothing occurs, as the stress is too small to ionize the air outside. However, when the potential difference increases beyond a threshold value (known as critical destructive voltage), the field force becomes strong enough for the air surrounding conductors to decompsoe into ions and makes them conductive. This critical destructive voltage occurs at about 30 kv. Ionized air causes electrical discharge around conductors (due to the flow of these ions). This leads to a slight glowing glow, along with the hissing sound accompanied by the release of ozone. This phenomenon of electrical discharge occurring in high voltage transmission lines is known as corona effect. If the voltage along the lines continues to increase, the glowing and hissing noise becomes increasingly intense, causing a high loss of power to the system.
Factors Affecting Corona Loss
The line voltage of the conductor is the main determining factor for Corona discharge in the transmission lines. At low voltage values (less than critical destructive voltage), stress in the air is not high enough to cause dielectric degradation – and therefore no electrical discharge occurs. With increased voltage, the Corona effect on a transmission line is caused by the ionization of atmospheric air surrounding conductors – mainly affected by the conditions of the cable and the physical state of the atmosphere. The main factors affecting corona discharge are:
- Atmospheric Conditions
- Status of conductors
- Distance Between Conductors
The voltage gradient for dielectric disintegration of air was proven to be directly proportional to the air density. Therefore, on a stormy day, due to constant airflow, the number of ions surrounding the conductor is much higher than usual, and therefore, compared to a day with quite open air conditions, it is more likely to have an electrical discharge on transmission lines on such a day. The system should be designed with these extreme situations in mind.
Status of Conductors
This particular phenomenon depends greatly on conductors and their physical condition. It has an inverse proportionality relationship with the diameter of the conductors. that is, as the diameter increases, the effect of the corona on the power system decreases significantly. In addition, the presence of dirt or roughness of the conductor reduces critical failure voltage and makes conductors more prone to Corona losses. Therefore, in most cities and Industrial Zones with high pollution, this factor is of reasonable importance to counter the bad effects on the system.
Distance Between Conductors
As already mentioned, the corona must be much higher than its diameter in order to form effectively in the range between the lines, but if the length goes beyond a certain limit, dielectric stress in the air decreases, and as a result the effect of the corona decreases. If the range is too large, the Corona may not form at all for that area of the transmission line.
Reducing Corona Discharge
Corona discharge always causes a loss of power. It disappears in the form of energy, light, sound, heat and chemical reactions. Although these losses are small individually, over time they can cause significant power loss on high voltage networks. Corona discharge can be reduced as follows: Increasing the conductive size: A larger conductor diameter leads to a decrease in the Corona effect. Increasing the distance between conductors: Increasing the conductive range reduces the corona effect. Using packaged conductors: Packaged conductors increase the effective diameter of the conductor–therefore reduce the corona effect. Use of corona rings: Where the conductor has a sharp curvature, the electric field is stronger. Therefore, corona discharge occurs first at sharp points, edges and corners. Corona rings reduce the corona effect by 'rounding' conductors (i.e. making them less sharp). They are used in terminals of very high voltage equipment (for example, in the bushes of high voltage transformers). A Corona ring electrically binds to the high voltage conductor surrounding the points where the Corona effect is most likely. This siege significantly reduces the sharpness of the conductor's surface – distributes the load over a larger area. This reduces corona discharge.