corona discharge

Corona discharge is the ionization of gas surrounding a high-voltage conductor when the electric field strength exceeds the breakdown threshold of that gas. Unlike a full arc or spark, corona remains a partial ionization, creating a luminous glow or halo around the conductor rather than a complete conducting path. The phenomenon occurs in air at roughly 3 kilovolts per millimeter, though the exact threshold depends on gas density, humidity, conductor geometry, and polarity.

The visible corona appears as a faint violet or bluish glow, often accompanied by a faint hissing or crackling sound. The glow spreads from sharp points, edges, or rough conductor surfaces where the electric field concentrates most intensely. Positive corona and negative corona exhibit different characteristics: positive corona appears as a continuous glow with a crackling noise, while negative corona often shows a dimmer, more diffuse appearance with a quieter buzz.

Where it matters in practice

Corona discharge causes real problems in high-voltage transmission lines, insulators, and switchgear. Power dissipation through corona heating wastes energy and generates radio frequency interference that disrupts communications. Ozone produced by the discharge attacks rubber and polymer insulation. On transmission lines operating above 230 kV, corona loss becomes measurable in the megawatt range over long distances. Engineers suppress corona by using larger-diameter conductors, smoother surfaces, and bundled conductor arrangements that reduce the peak electric field at the conductor surface.

The condition is unavoidable in some applications and is merely managed rather than eliminated. Utilities accept minor corona on overhead lines as a trade-off against the cost and bulk of suppressing it completely. In enclosed switchgear or underground cables, corona must be prevented through insulation design, conductor spacing, and use of insulating fluids or gases with higher breakdown strengths than air, such as sulfur hexafluoride (SF6).

Corona discharge differs fundamentally from tracking, which is the formation of conductive paths across insulation surfaces. While corona ionizes only the surrounding gas and leaves the solid insulation intact, continued corona exposure degrades insulation through ozone attack and thermal stress, potentially leading to eventual failure.