Corona Discharge Effect in Electrical Transmission Lines
Corona Discharge Effect
Corona Discharge Effect in the transmission lines is a phenomenon in which ionization of the surrounding air of power conductors takes place at voltages, greater than critical break down voltage. When the potential between the power conductors of transmission lines in a Extra High Voltage (EHV) system goes on increases hissing noise, followed by the violet glow and the production of ozone gas and finally electrical breakdown and flash over is observed, all these constitutes the Corona Discharge Effect Phenomenon. Corona Discharge Effect is affected by many things such as Conductor Diameter, number of conductors per phase, phase spacing, conductor surface condition, weather, altitude, and temperature and line voltage. Effects of Corona Discharge Effect constitute power loss in the transmission lines popularly called as corona loss, production of ozone gas, and interference with the neighboring Radio and Television communication lines.
Corona Discharge Effect is defined as Self Sustained Electric Discharge in which the field Intensified Ionization is localized only over the portion of the distance between the electrodes.
How Corona Discharge Effect Occur:
In a power system transmission lines are used to carry the power. These transmission lines are separated by certain spacing which is large in comparison to their diameters. In Extra High Voltage system (EHV system ) when potential difference is applied across the power conductors in transmission lines then air medium present between the phases of the power conductors acts as insulator medium however the air surrounding the conductor subjects to electro static stresses. When the potential increases still further then the atoms present around the conductor starts ionize. Then the ions produced in this process repel with each other and attracts towards the conductor at high velocity which intern produces other ions by collision. The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow of violet colour appears together along with hissing noise. This phenomenon is called virtual corona and followed by production of ozone gas which can be detected by the odor. Still further increase in the potential between the power conductors makes the insulating medium present between the power conductors to start conducting and reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon constitutes CORONA DISCHARGE EFFECT in electrical Transmission lines.
Factors Affecting Corona Discharge Effect:
Corona Discharge Effect occurs because of ionization if the atmospheric air surrounding the voltage conductors, so Corona Discharge Effect is affected by the physical state of the atmosphere as well as by the condition of the lines.
Corona Discharge Effect is considerably affected by the shape, size and surface conditions of the conductor
Corona Discharge Effect decreases with increases in the size (diameter) of the conductor, this effect is less for the conductors having round conductors compared to flat conductors and Corona Discharge Effect is concentrated on that places more where the conductor surface is not smooth.
Corona Discharge effect is not present when the applied line voltages are less. When the Voltage of the system increases (In EHV system) corona Effect will be more.
Breakdown voltage directly proportional to the density of the atmosphere present in between the power conductors. In a stormy weather the ions present around the conductor is higher than normal weather condition.
So Corona Breakdown voltage occurs at low voltages in the stormy weather condition compared to normal conditions
Spacing between the Conductors:
Electro static stresses are reduced with increase in the spacing between the conductors.
Corona Discharge Effect takes place at much higher voltage when the distance between the power conductors increases.
Methods to reduce Corona Discharge Effect:
Critical Breakdown voltage can be increased by following factors
By increasing the spacing between the conductors:
Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phases results in heavier metal supports. Cost and Space requirement increases.
By increasing the diameter of the conductor:
Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.
By using Bundled Conductors:
By using Bundled Conductors also corona effect can be reduced this is because bundled conductors will have much higher effective diameter compared to the normal conductors.
By Using Corona Rings or Grading Rings:
This is of having no greater significance but i presented here to understand the Corona Ring in the Power system. Corona Rings or Grading RIngs are present on the surge arresters to equally distribute the potential along the Surge Arresters or Lightning Arresters which are present near the Substation and in the Transmission lines.