Corona rings are often added to insulators of high voltage transmission lines.Ĭoronas may be positive or negative. In order to suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large-diameter rounded shapes like balls or toruses. On sharp points in the air, corona can start at potentials of 2–6 kV. The high curvature causes a high potential gradient at these locations so that the air breaks down and forms plasma there first. Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. In air at sea level pressure of 101 kPa, the critical value is roughly 30 kV/cm, but this decreases with pressure, therefore, corona discharge is more of a problem at high altitudes. If the source continues to supply current, a spark will evolve into a continuous discharge called an arc.Ĭorona discharge forms only when the electric field (potential gradient) at the surface of the conductor exceeds a critical value, the dielectric strength or disruptive potential gradient of the fluid. 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. A corona forms only when the conductor is widely enough separated from conductors at the opposite potential that an arc cannot jump between them. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.Īlong with the similar brush discharge, the corona is often called a "single-electrode discharge", as opposed to a "two-electrode discharge" – an electric arc. Since the new conductive region is less sharp, the ionization may not extend past this local region. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor.
Air near the electrode can become ionized (partially conductive), while regions more distant do not. If a charged object has a sharp point, the electric field strength around that point will be much higher than elsewhere. When the potential gradient (electric field) is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules. Notice, especially in the last two pictures, how the discharge is concentrated at the points on the objects.Ī 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 so as to create a region of plasma around the electrode. However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.Ī variety of forms of corona discharge, from various metal objects. These gases are corrosive and can degrade and embrittle nearby materials, and are also toxic to humans and the environment.Ĭorona discharges can often be suppressed by improved insulation, corona rings, and making high voltage electrodes in smooth rounded shapes. In the air, coronas generate gases such as ozone (O 3) and nitric oxide (NO), and in turn, nitrogen dioxide (NO 2), and thus nitric acid (HNO 3) if water vapor is present.
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. Corona discharge from high voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In many high voltage applications, corona is an unwanted side effect. 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. 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 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. NIST laboratory in 1941Ī corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. Large corona discharges (white) around conductors energized by a 1.05 million volt transformer in a U.S.
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