Corona ring

A corona ring, also called an anti-corona ring, is a toroid of conductive material, usually metal, which is attached to a terminal of high voltage equipment. The role 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 is an ionization of air adjacent to high voltage conductors. It is sometimes visible as a dim blue glow in the air next to high voltage equipment. The high electric field ionizes the air, allowing current to leak from the conductor into the air. In electric power transmission lines and equipment, corona results in an economically significant waste of power. In devices such as electrostatic generators, marx generators, and 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 embrittlement of nearby structures such as insulators, and 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 kilovolts per centimeter, but varies with atmospheric pressure, so corona is more of a problem at high altitudes. The electric field at a conductor is greatest where the curvature is sharpest, and therefore corona discharge occurs first at sharp points, corners and edges. The terminals on very high voltage equipment are frequently designed with large diameter rounded shapes such as balls and toruses called corona caps, to suppress corona formation. However, some parts of high voltage circuits require hardware with exposed sharp edges or corners, such as the attachment points where wires or bus bars are connected to insulators. Corona rings are installed at these points to prevent corona formation.

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