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 of corona
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
It 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
It is a tool to verify the proper commissioning of electrical installation
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
How 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
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.
Measurement/Detection of corona
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.