Corona discharge is an electrical discharge brought about by the ionization of a fluid, such as air, surrounding a conductor that is electrically charged. It occurs when the electric field strength (potential gradient) around a conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects.
The process initiates when a high-voltage conductor has a non-uniform electric field (usually at sharp points or thin wires).
Ionization: The intense electric field accelerates free electrons in the air to speeds high enough to knock electrons out of neutral gas molecules upon collision.
Electron Avalanche: This creates a chain reaction of ion-pair production known as an "electron avalanche" or Townsend discharge.
Plasma Region: A region of low-density plasma is formed around the conductor. In this region, ions carry the charge to the neutral fluid.
Visual: A faint blue or violet glow (luminous corona) visible in darkness. This is caused by electrons returning to lower energy levels in nitrogen molecules.
Audible: A characteristic hissing or crackling sound caused by the rapid expansion of air molecules.
Chemical: Production of Ozone (O3) and various nitrogen oxides (NO). These gases have a distinct, pungent odor and are chemically corrosive.
Corona discharge is generally considered a parasitic phenomenon in high-voltage engineering:
Power Loss: It dissipates energy into the atmosphere, reducing the efficiency of power transmission.
Electromagnetic Interference (EMI): The discharge generates high-frequency pulses that cause "radio noise," interfering with radio and television signals.
Material Degradation: The chemical by-products and ultraviolet radiation can degrade the insulation of high-voltage components over time.
Despite being a loss in power lines, corona discharge is utilized in several technologies:
Electrostatic Precipitators (ESP): Used in factory chimneys to remove dust and smoke particles from exhaust gases by charging them.
Photocopiers and Laser Printers: Uses a "corona wire" to apply a uniform static charge to the photoreceptor drum.
Surface Treatment: Used to increase the surface energy of polymers and plastics to improve the adhesion of inks and glues.
Ionizers: Used in air purifiers to charge particles so they stick to collection plates.
The onset of corona discharge is governed by several variables, often calculated using Peek’s Law:
Curvature: Smaller radii (sharper points) increase the local electric field and trigger corona at lower voltages.
Air Density: Lower air pressure (high altitude) or higher temperatures decrease the voltage required for corona.
Surface Condition: Dirt, water droplets, or scratches on a conductor's surface act as local points of high field intensity, promoting discharge.
