Corona discharge is a luminous partial discharge from conductors and insulators due to ionization of the air, where the electrical field exceeds a critical value.

A high local electric field ionizes the air and causes a discharge. This process is accompanied by excitation of Nitrogen molecules, leading to emission of UV radiation. Buildup of ionization occurs only if electron is going fast enough, i.e. if electric field exceeds a critical value.

Thus a problem or defect in a component creating a local high electric field will show corona activity. Some molecules are not ionized but excited – emitting photons on relaxation.

Corona Creates Corrosive Materials: Ozone, Nitrogen oxides which in presence of water vapor yield Nitric acid.

Utilities are typically made aware of corona by complaints of faulty radio or television signals. Because corona are invisible in daylight with the naked eye, maintenance crews will investigate by aiming devices such a corona camera or radio antenna at suspected areas, and track corona.

Corona has the following damaging and environmental disturbing effects:

• Generation of corrosive materials, like ozone and nitrogen oxides that yields nitric acid under conditions of high humidity. These corrosive materials shorten the life span of high voltage lines and substations components.
• Corona causes damage to HV insulators, especially non-ceramic ( NCI) insulators.
• Radio interference (RI/ RFI) mainly to AM transmissions.
• Audio noise.
  Radio interference and audio noise may raise customer complaints.
  
  

Corona generates only little heat and therefore is not detected with thermovision cameras.

Corona Light Emission

The corona discharge emits radiation in the 280nm-405nm spectral range, mostly in the ultraviolet (UV) and therefore is invisible to the human eye, though relatively weak emission at about 400 nm might be observed at night under conditions of absolute darkness. The corona emission in the 280nm-405nm spectral range can not be detected during daytime due to the highly disturbing background of the solar radiation.
The DayCor® corona camera is a bi-spectral Solar Blind UV-Visible imager, designed to detect these very faint UV emissions in the solar blind UV band, with high signal to background ratio.

What is Partial Discharge?

Partial discharge is an electrical discharge that does not bridge the entire space between the two electrodes.