HOW IT’S DONE

A corona discharge starts when a free electron is accelerated by an electric field and impacts on a Nitrogen molecule with enough energy to knock 2 or more electron out of their orbits, ionizing the molecules. The liberated electrons result in an electron avalanche.

When the electrons colliding with the molecule no longer have enough energy to knock an electron out of its orbital part, the electrons energy is absorbed resulting in an excited molecule. The excited molecule will return to its ground state after emission of the excess energy as photons. This is called fluorescence.

In the case of Nitrogen, the emitted photons are mainly in the UV range (100 – 400nm). Other air molecules may also fluoresce, adding their light.

The UV emission is shown below, overlaid onto the natural sunlight spectrum. The visible range is from 400nm to 700nm.

The fluorescence spectrum of air is so faint that even after 10 000x amplification it barely peaks above the natural sunlight spectrum.

Below 300nm the sunlight spectrum drops to near zero intensity, this is due to the fact that the O-Zone layer absorbs these frequencies. The air fluorescence spectrum continues down to ~240nm.

UV below 300nm (UVc) is generated by a few sources:

  • Fires
  • Lightning
  • Grinding
  • Welding
  • Lighting
  • Lasers
  • Corona
  • Arcing

PROBLEMS CAUSED BY CORONA DISCHARGES

Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. They also represent a power loss, and their action on atmospheric particulates, along with associated ozone and NOx production, can also be disadvantageous to human health where power lines run through built-up areas. Therefore, power transmission equipment is designed to minimise the formation of corona discharge. Corona discharge is generally undesirable in:

  • Electric power transmission, where it causes:
    • Power loss
    • Audible noise
    • Electromagnetic interference
    • Purple glow
    • Ozone production
    • Insulation damage
  • Inside electrical components such as transformers, capacitors, electric motors and generators. Corona progressively damages the insulation inside these devices, leading to premature equipment failure. One form of attack is ozone cracking of elastomer items like O-rings.
  • Situations where high voltages are in use, but ozone production is to be minimised
  • Static electricity discharge
  • Lightning

Coronas can be suppressed by corona rings, toroidal devices that serve to spread the electric field over larger area and decrease the field gradient below the corona threshold.

And there are likely to be a number of other possible applications that have not yet been realised or quantified.