corona
A luminous appearance caused by corona discharge, often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages.
corona: electrical breakdown that glows blue at high voltage
A corona is a self-sustaining electrical discharge that occurs in air or other gases surrounding a high-voltage conductor. It manifests as a faint luminous halo, often blue or violet, visible in darkness around pointed or curved metallic surfaces, typically at potentials above 3 kV/cm. The glow happens because the electric field ionizes gas molecules, which recombine and emit light without the conductor and ground completing a full conductive path between them.
In transmission lines and substations, corona forms most readily at sharp edges, conductor splices, or damaged insulation where the electric field concentrates. A single conductor strand with a 12 mm diameter at 100 kV line voltage might initiate corona; bundled conductors with larger effective diameter distribute the field and suppress it. The phenomenon gets worse in wet conditions, at high altitude where air is thinner, and during fog or rain when water droplets lower the breakdown threshold.
Why corona matters to system operators
Corona discharge causes real problems: it generates radio frequency interference that degrades communications, produces audible hissing or crackling noise in calm weather, and represents energy loss, perhaps 1 to 3 kW per kilometer of transmission line under adverse conditions. More seriously, it slowly erodes conductor surfaces and nearby insulation through ion bombardment and ozone formation, shortening equipment life. Utilities combat corona by smoothing conductor surfaces, using corona rings (toroidal conductors) around component terminals, increasing phase spacing, or switching to smooth-surface cable designs in new installations.
The term comes from the Latin for crown, referring to the crown-like appearance the glow makes around a conductor. It is distinct from arcing, which is a continuous conducting path with much higher current and heat, and from tracking, which is degradation of solid insulation by a conductive path. Understanding where and when corona appears is essential for designing reliable high-voltage systems and diagnosing faults in existing infrastructure.