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Concept
Corona ring
Summary
A corona ring, more correctly referred to as an anti-corona ring, is a toroid of conductive material, usually metal, which is attached to a terminal or other irregular hardware piece of high voltage equipment. The purpose of the corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing corona discharge. Corona rings are used on very high voltage power transmission insulators and switchgear, and on scientific research apparatus that generates high voltages. A very similar related device, the grading ring, is used around insulators.
Corona discharge
Corona discharge is a leakage of electric current into the air adjacent to high voltage conductors. It is sometimes visible as a dim blue glow in the air next to sharp points on high voltage equipment. The high electric field ionizes the air, making it conductive, allowing current to leak from the conductor into the air in the form of ions. In very high voltage electric power transmission lines and equipment, corona results in an economically significant waste of power and may deteriorate the hardware from its original state. In devices such as electrostatic generators, Marx generators, and tube-type television sets, the current load caused by corona leakage can reduce the voltage produced by the device, causing it to malfunction. Coronas also produce noxious and corrosive ozone gas, which can cause aging and brittleness of nearby structures such as insulators. The gasses create a health hazard for workers and local residents. For these reasons corona discharge is considered undesirable in most electrical equipment.
Corona discharges only occur when the electric field (potential gradient) at the surface of conductors exceeds a critical value, the dielectric strength or disruptive potential gradient of air. It is roughly 30 kV/cm at sea level but decreases when atmospheric pressure decreases. Therefore, corona discharge is more of a problem at high altitudes.
Official source
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