At higher voltages, the localized high electric fields ionize the air surrounding the surface of the conductor giving a violet visible discharge, called corona.
The physics of this process sees it as excitation of nitrogen molecule in the air which emits ultra violet radiation, generally in the wave length range of 200 to 405 mm.
It is an external phenomenon dependent only on voltage.
It is accompanied by hissing sound, vibrations, power loss and radio interference.
The phenomenon of corona is accompanied by predominant third harmonic current which cause radio interference in the adjacent communication circuits.
Corona causes increase in effective conductor diameter and reduces surface stress.
Location
It is observed on hard ware, at the point of connection between the conductors, along the busbar, equipments, etc
It is often observed on the first insulator in a string where the electric field tends to be the greatest
Looped excess wire around the conductor at critical point generates high electric fields due to uneven surface. Hence high corona discharge
Uneven/rough surface generates high electric fields, hence high corona discharge.
Significance
Corona can be an indicator of poorly designed or defective component or poor workmanship.
Magnitude of corona can be used to decide the best time to wash the insulators.
Corona is a tool to verify the proper commissioning of electrical installation.
What is the effect of corona on the transmission lines? How is it to be reduced ?
Effects of Corona
The ozone and other by products of corona on a cap and pin type insulator will cause corrosion of the pin and may pit the cement, leading to pin separation.
The nitric acid formed from the nitrogen oxide and water vapour generated by corona can damage the polymeric material leading to cracks or puncture in the housing.
The moment the housing is damaged, the moisture can penetrate to the internal fiber glass core.
This moisture combined with acid not only degrades the rod but also reduces the dielectric properties of the insulator leading to eventual failure.
Progressive degradation of the polymer near the end fittings.
To Reduce corona
To reduce corona on the line
- The insulator strings to be equipped with grading ring / corona ring ( 400KV and above lines)
- Proper washing and cleaning of insulators on the line
- Proper construction and commissioning of the line
- Proper installation practice to avoid uneven or rough surfaces
What are the factors affecting corona ?
Rain, moisture, fog , humidity, temperature, pressure ,etc affect the critical disruptive voltage stress of air
The fair weather has negligible corona effect as its critical disruptive value is high
Rain causes increase in corona
How corona is measured/detected ? Is it possible to detect corona during day time?
An Israeli-based firm ‘Ofil’ developed Corona Camera which can be used to detect corona discharge to monitor degradation and condition of composite insulators/housing material in electrical network.
Yes, with the use of Corona Camera, it is possible to detect corona even during daytime in ambient solar conditions.
The wave length of the emitted UV radiation due to corona discharge corresponds directly to that of background solar radiation and therefore is basically blocked from normal view.
Technology using special optical filters to block out background solar radiation thereby only that radiation emanating from the corona source is viewed even in full daylight.
