How Corona Discharge Effect Occur in Transmission Line?

How Corona Discharge Effect Occur in Transmission Line?

§  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 color 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.

5)    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.

What is electrical corona?

§  Corona is the ionization of the nitrogen in the air, caused by an intense electrical field.

§  Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage and is invisible during the day and requires darkness to see at night.

§  Arcing starts at a voltage and stops at a voltage about 50% lower and is visible to the naked eye day or night if the gap is large enough (about 5/8″ at 3500 volts).

5)    What are the indications of electrical corona?

§  A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind wavelengths on earth created by the shielding ozone layer surrounding the earth.

6)    What damage does corona do?

§  The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials. Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in internal flash-over.

§  The corona is the only indication. Defects in insulating materials that create an intense electrical field can over time result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators.

7)    How long does corona require creating visible damage?

§  In a specific substation the corona ring was mistakenly installed backwards on a temporary 500kV NCI insulator, at the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining 2/3 was grey.

8)    What voltage are corona rings typically installed at?

§  It varies depending upon the configuration of the insulators and the type of insulator, NCI normally start at 160kV, pin and cap can vary starting at 220kV or 345kV depending upon your engineering tolerances and insulators in the strings.