What’s corona discharge?

What Is the Corona Effect?

The corona effect is a phenomenon of partial electrical discharge caused by the ionization of the air surrounding a conductor. It appears when the voltage exceeds a critical value, but whose conditions do not allow the formation of an electric arc. A corona effect produces a glow of bluely light, which forms around the cables carrying high voltage current, more frequently on aerial power lines.

How Does a Corona Discharge Occur?

A corona discharge occurs when a direct or alternating current is created between two electrodes brought to a high potential and separated by a neutral fluid like air, by ionization of this fluid. A plasma is then created and the electric charges propagate by passing from the ions to the molecules of neutral gases.

When the electric field at a point of the fluid is large enough, the fluid ionizes around this point and becomes conductive. If the fluid is air, the ionization is responsible for the bluely light emitted.

How Do Conductor Shape and Electric Field Affect Corona Discharge?

In particular, if a charged object has points or corners, the electric field will be greater there than elsewhere leading to a corona discharge: the phenomenon will tend to stabilize itself since the ionized region becoming conductive, the tip will apparently tend to disappear. The charged particles then dissipate under the effect of the electric force and are neutralized on contact with an object of opposite charge. Corona discharges therefore generally occur between an electrode with a small radius of curvature, such as a defect in the conductor forming a point, and a another with a large radius of curvature such as a metal plate or the ground.

From Corona Discharge to Electric Arc

If the geometry of the conductor and the value of the field are such that the ionized region expands instead of stabilizing, the current may eventually find a path to the reverse electrode and creates sparks or electric arc.

Impact of Weather Conditions

The corona effect depends on the superficial electric field of the conductor, but also on its diameter, its surface condition and then the density and humidity of the surrounding air. Indeed, when it is raining or under very humid conditions1, an aerial power line starts to vibrate at the natural frequency of the conductor in light winds on high voltage lines when drops of water are suspended under the conductors in rainy, wet snow or heavy fog2 or the presence of drops of water suspended on the underside of a cable3; the intermittent presence of the space charge4,5,6 and of the ionic wind located in the immediate vicinity of the drops of water suspended from the conductor are the main causes of this phenomenon2, the magnitude and location of which will depend on several parameters such as value and polarity of the electric field on the surface of the conductor, intensity of precipitation, speed of the transverse wind2,7.

Impact on High Voltage Lines

This effect is not desired on power lines since it causes power loss for the network. In addition to unpleasant crackles emitted by that phenomenon, the corona effect can interfere with radio and television signals. It also creates vibration on the power lines, which, combined with vibrations due to wind8, reduces the lifetime of power lines, accelerating aging of the conductor and wear of its anchor points and leading to an inevitable and more important corona effect. Dioxygen in the air is also transformed into ozone, which is a harmful gas for exposed animals living nearby.

Advanced Imaging Solutions for Power Line Inspection

Detecting Corona Discharge with SUV Imaging

Exosens offers advanced imaging solutions to detect corona discharge in high voltage power lines with SUV solutions. Exosens SUV (iNocturn SUV) solutions are based on the coupling of image intensifier and a CMOS high performance detector for high precision corona discharge detection.

Detecting Heat-Related Power Line Defects with LWIR Imaging

Exosens offers advanced imaging solutions to detect power line defects with LWIR solutions. Exosens LWIR cameras and cores consists of SWaP features and are available in industry standard interfaces of MIPI CSI-2, USB and UVC for power line defect detection.

Frequently Asked Questions About Corona Discharge

Is corona discharge harmful to humans?

Corona discharge is not normally a direct health risk when observed from a safe distance. However, it occurs around energized electrical equipment that may present serious shock or arc hazards. It can also produce ozone, which may irritate the respiratory system at elevated concentrations. Inspections must therefore follow electrical safety rules and be performed by trained personnel.

Are corona discharge and partial discharge the same?

Corona discharge is a type of partial discharge that occurs in a gas, such as the air surrounding a conductor. However, partial discharge can also occur inside insulation or along its surface. The two terms are therefore related but are not always interchangeable.

How can corona discharge be detected?

Corona discharge can be detected using ultraviolet imaging, acoustic or ultrasonic instruments, and electrical or electromagnetic measurements. UV imaging is particularly useful for locating external corona discharge on overhead lines and substation equipment.

Can corona discharge be detected during daylight?

Yes. Solar-blind UV imaging systems can detect the ultraviolet emissions produced by corona discharge during daylight. These systems can combine the UV signal with a visible image to show the exact location of the discharge.

Can a thermal camera detect corona discharge?

Not directly. Thermal cameras detect temperature differences, while corona discharge may not generate enough heat to be visible in a thermal image. UV imaging is used to detect corona emissions, while LWIR imaging can identify heat-related defects. Combining both technologies provides a more complete inspection.

Does corona discharge always indicate faulty equipment?

Not necessarily. Corona activity may be related to the design or normal electric field distribution of the equipment. However, it can also indicate contamination, surface damage, sharp edges, or an incorrectly installed component. The location, intensity and operating conditions must be assessed before deciding whether maintenance is required.

Can corona discharge damage electrical equipment over time?

Yes. Continuous or repeated corona activity can contribute to the gradual deterioration of insulation and nearby materials. The level of damage depends on factors such as discharge intensity, duration, material properties and environmental conditions. Detecting corona does not always mean that failure is imminent, but the equipment should be assessed and monitored.

How can corona discharge be reduced or prevented?

Corona discharge can be reduced by limiting concentrations of the electric field. Possible measures include using smooth and rounded components, larger or bundled conductors, and corona rings or other field-grading devices. Cleaning contaminated surfaces and repairing damaged or incorrectly installed components can also help. The appropriate solution depends on the equipment and the cause of the discharge.

Can corona discharge inspections be performed on energized equipment?

Yes. Non-contact technologies such as UV imaging can inspect energized power lines and electrical equipment from a suitable distance. This allows potential problems to be located without necessarily shutting down the equipment. Inspections must always be performed by qualified personnel following the applicable safety procedures.

Explore further below to learn more about corona-discharge, our technology, benefits, and our cutting-edge solutions:

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https://www.exosens.com/news/high-voltage-line-maintenance-which-solutions

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References

1 M.Farzaneh, "Effects of the intensity of precipitation and transverse wind on the corona induced vibration of HV conductors", IEEE, Vol. 7, No. 2, April 1992

2 Gourbi, Abdelkader (2008) Mécanisme des vibrations induites par effet de couronne ; Université Djillali Liabes de Sidi bel Abbes - Magister 2008

3 Akazaki M (1965), "Corona phenomena from water drop on smooth conductors under high direct voltage", IEEE, Janvier 1965

4 Carl Potvin, " Comportement et effet de la charge d'espace sur le mécanisme des vibrations induites par effet de couronne ", Mémoire comme exigence partielle de la maîtrise en ingénierie, université de Chicoutimi, Canada, mars 2000.

5 Farzaneh M & Teisseyre Y (1988), "Mechanical vibration of HV conductors induced by corona: roles of the space charge and ionic wind", IEEE, VOL, 3, 3 July 1988

6 Farzaneh M & Phan LC(1985) "On the Role of the Space Charge Produced by Hanging Water Drops in the Mechanism of the Corona-Induced Vibration of H.V. Conductor", J. of electrostatics, Vol. 17, pp. 235-244

7 Myriam Hamel, "Influence de la variation de la température ambiante sur les vibrations induites par effet de couronne", Mémoire comme exigence partielle de La maîtrise en ingénierie, université de Chicoutimi Canada, août 1991.

8 Sayah H, Brahami m, Bendaoud A & Rahli M (2004) "Vibrations éoliennes des câbles aériens", La 1re Conférence Nationale sur l’Électrotechnique et ses Applications, Sidi Bel-Abbès, Algérie, Mai 2004.

A & Rahli M (2004) "Vibrations éoliennes des câbles aériens", La 1re Conférence Nationale sur l’Électrotechnique et ses Applications, Sidi Bel-Abbès, Algérie, Mai 2004.