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In electrical engineering, 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.
The terminals on very high voltage equipment are frequently designed with large diameter rounded shapes such as balls and toruses called corona caps, to eliminate pointed shapes and suppress corona formation. Some parts of high voltage circuits have hardware with exposed sharp edges or corners, such as the attachment points where wires or bus bars are connected to insulators; corona caps or corona rings are usually installed at these points to prevent corona formation.
A corona ring is electrically connected to the high voltage conductor and encircles the points where corona would form. Since the ring is at the same potential as the conductor, the presence of the ring reduces the potential gradient at the surface of the conductor below the disruptive potential gradient, preventing corona from forming on the points it surrounds.
Across an insulating column, the electric field intensity is not uniform — it is greatest near the high potential end. If the applied voltage exceeds the air or the insulator surface's dielectric constant, breakdown begins at that end, where the field is strongest. As soon as the insulator's surface at that end becomes conductive, the full system voltage is applied across the remaining length of the insulator, creating a higher electric field, so the failure propagates rapidly toward the grounded side, producing a flashover arc. Limiting the electric field at the high voltage end therefore lets the same insulator tolerate a much higher operating voltage.
The grading ring is mounted around the high voltage end of the insulator, connected to the high voltage terminal. It's smooth electric field equipotential lines reduces the potential gradient and thus the electric field along that part of the insulator where it is highest. The result is a shorter or lower‑cost insulator can handle the same kV rating, and the usual erosion, corona cutting, and surface tracking at the energized end are dramatically reduced.
For insulating columns working at extreme potentials—Marx generator or particle accelerator drift tubes, multiple grading rings are installed, usually at equal intervals along the insulator, each connected to a series chain of megohm resistors that acts as a built‑in voltage divider. Therefore the potential across each section of the insulator between adjacent rings is equalized, so that the entire length of the insulator is subjected to a uniform electric field, and no section has to withstand an excessive field. This equalization strategy minimizes the chance of premature puncture, maximizing system reliability while keeping component size and cost in check.
Corona rings may also be installed on the insulators of antennas of high-power radio transmitters. The Handbook of antenna design, Volume 2 By Alan W. Rudge, IET, 1983, p. 873, However, they increase the capacitance of the insulators. aerials for metre and decimetre wave-lengths, CUP Archive
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