The word “corona” means “crown,” and is derived from other types of electrical discharges (like that in a neon sign that creates a visible glow). Electrical breakdown of a gas occurs when the potential gradient between two conductors exceeds a threshold determined by the gas pressure, the dielectric present, and a number of geometric factors. For air at atmospheric pressure, that threshold is about 70v/mil. At 70,000 feet altitude, the threshold is about one-fourth of that.
Benjamin Franklin used the fact that the gradient, and not the absolute voltage, controls the discharge threshold (if not understood) when he developed the lighting rod. The sharp point on the lighting rod is a region of high potential gradient, enabling the electrical charge in atmosphere to dissipate into the rod and to conduct to ground before building up to a level that would cause a lighting strike.
In electrical equipment technology, the “sharp point” is a conductor of small radius such as a stray strand of shielding braid. These sharp points, of course, are to be avoided as much as possible to prevent corona. Also, since corona results from the ionization of air in vicinity of an electrode, one principle means of attacking the corona problem is to eliminate air.
Theoretical and experimental research has revealed much about the nature of corona-also called “partial discharge” –over the past 50 years. Partial discharge in an insulated high-voltage cable occurs when electrons within void gain enough energy to ionize air molecules, thus releasing additional electrons in an avalanche. The impact of electrons or ions erodes the surface of the insulation.
The erosion of insulation by corona is cumulative, and will cause a failure without warning. Unfortunately, a brief dielectric strength test will not reveal existing damage. Therefore much research has been conducted to determine the best methods to test for partial discharge.
Research has determined which insulation materials best resist both the mechanical erosion from bombardment and the chemical attack from ozone, also a product of the air ionizing. A general ranking of these materials in order of the best corona resistance is:
- Silicone rubber
- Ethylene Propylene rubber (EPR)
- High-density polyethylene
- Polyvinyl chloride
Theoretical research has developed relationships between corona inception voltage and the size and placement of void within a coaxial cable. Some of these relationships are: The higher the pressure in the void, the higher the voltage required to cause partial discharge. If the void is located at the surface of the inner conductor, partial discharge will begin at a lower voltage than if it were at the outer conductor. The smaller the void, the higher the voltage required to initiate corona. The thicker the insulation, the higher the corona inception voltage.