Corona discharge

A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona occurs at locations where the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages and emits light by the same mechanism as a gas discharge lamp.

In many high voltage applications, the corona is an unwanted side effect. Corona discharge from high voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high voltage equipment like cathode ray tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone (O₃) and nitric oxide (NO), and in turn, nitrogen dioxide (NO₂), and thus nitric acid (HNO₃) if water vapor is present. These gases are corrosive and can degrade and embrittle nearby materials and are toxic to humans and the environment.

Corona discharges can often be suppressed by improved insulation, corona rings, and making high voltage electrodes in smooth rounded shapes. However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.

Introduction

A corona discharge is a process by which a current flows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid to create a region of plasma around the electrode. The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules.

When the potential gradient (electric field) is large enough at a point in the fluid, the fluid ionizes and becomes conductive. If a charged object has a sharp point, the electric field strength

around that point will be much higher than elsewhere. Air near the electrode can become ionized (partially conductive), while regions more distant do not. When the air near the point becomes conductive, it increases the apparent size of the conductor. Since the new conductive region is less sharp, the ionization may not extend past this local region. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.

Along with the similar brush discharge, the corona is often called a "single-electrode discharge", as opposed to a "two-electrode discharge" – an electric arc. A corona only forms when the conductor is widely enough separated from conductors at the opposite potential that an arc cannot jump between them. If the geometry and gradient are such that the ionized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric spark or electric arc, depending upon the source of the electric field. If the source continues to supply current, a spark will evolve into a continuous discharge called an arc.

Corona discharge only forms when the electric field (potential gradient) at the surface of the conductor exceeds a critical value, the dielectric strength, or the disruptive potential gradient of the fluid. In air at atmospheric pressure, it is roughly 30 kilovolts per centimeter, but this decreases with pressure, so corona is more of a problem at high altitudes. Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations so that the air breaks down and forms plasma there first. On sharp points in the air, corona can start at potentials of 2–6 kV. To suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large-diameter rounded shapes like balls or toruses, and corona rings are often added to insulators of high voltage transmission lines.

Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly curved electrode. If the curved electrode is positive concerning the flat electrode, it has a positive corona; if it is negative, it has a negative corona. The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures.

An important reason for considering coronas in the production of ozone around conductors undergoing corona processes in air. A negative corona generates much more ozone than the corresponding positive corona.

Applications

Corona discharge has several commercial and industrial applications:

• Removal of unwanted electric charges from the surface of aircraft in flight and thus avoiding the detrimental effect of uncontrolled electrical discharge pulses on the performance of avionic systems
• Manufacture of ozone
  
• Sanitization of pool water
• In an electrostatic precipitator, removal of solid pollutants from a waste gas stream, or scrubbing particles from the air in air-conditioning systems
• Photocopying
• Air ionizers
• Production of photons for Kirlian photography to expose photographic film
• EHD thrusters, lifters, and other ionic wind devices
• Nitrogen laser
• Ionization of a gaseous sample for subsequent analysis in a mass spectrometer or an ion mobility spectrometer
• Static charge neutralization, as applied through antistatic devices like ionizing bars
• Refrigeration of electronic devices by forced convection

Coronas can be used to generate charged surfaces, which is an effect used in electrostatic copying (photocopying). They can also be used to remove particulate matter from air streams by first charging the air, and then passing the charged stream through a comb of alternating polarity, to deposit the charged particles onto oppositely charged plates.

The free radicals and ions generated in corona reactions can be used to scrub the air of certain noxious products, through chemical reactions, and can be used to produce ozone.

Problems

Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. Therefore, power transmission equipment is designed to minimize 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
  • Possible distress in animals that are sensitive to ultraviolet light
• Electrical components such as transformers, capacitors, electric motors, and generators:
  • Corona can progressively damage the insulation inside these devices, leading to equipment failure
  • Elastomer items such as O-rings can suffer ozone cracking
  • Plastic film capacitors operating at mains voltage can suffer progressive loss of capacitance as corona discharges cause local vaporization of the metallization

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

Electrical wind

Ionized gases produced in a corona discharge are accelerated by the electric field, producing a movement of gas or electrical wind. The air movement associated with a discharge current of a few hundred

microamperes can blow out a small candle flame within about 1 cm of a discharge point. A pinwheel, with radial metal spokes and pointed tips bent to point along the circumference of a circle, can be made to rotate if energized by a corona discharge; the rotation is due to the differential electric attraction between the metal spokes and the space charge shield region that surrounds the tips.