Corona ring
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.[1]
Corona discharge
[edit]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.
How they work
[edit]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. The electric field at the surface of a conductor is greatest where the curvature is sharpest, so 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. 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 and rings are usually installed at these points to prevent corona formation.
The corona ring is electrically connected to the high voltage conductor, encircling 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 metal points.
Grading rings
[edit]
A very similar device, called a grading ring, is also used on high-voltage equipment. Grading rings works in a same way that corona ring does but their main difference lies in how and where they are used. Corona rings mainly works on conductor while grading rings are used on insulators where its purpose is to reduce potential gradient along the insulator, to prevent the electrical breakdown.[2]
Across an insulating column, the electric‑field intensity is never perfectly uniform—it peaks near the terminal that sits at the highest potential. If the applied voltage exceeds the material's withstand capability, breakdown begins right where that field is strongest. As soon as the end segment becomes conductive, the full system voltage is forced onto the remaining length, so the failure propagates rapidly toward the grounded side, producing a flashover. Limiting that initial field spike therefore lets the same insulator tolerate a much higher operating voltage.[2]
Enter the grading (corona‑control) ring. By wrapping a toroidal conductor around the high‑voltage end, we enlarge the effective electrode diameter and pull equipotential lines outward. The local field drops, the axial gradient flattens, and the entire string sees a smoother potential distribution. The net result: 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.[2]
At extreme potentials—Marx generators or accelerator drift tubes—engineers scale this idea. Multiple rings are spaced at set intervals, each tied into a chain of mega‑ohm resistors that acts as a built‑in voltage divider. Every ring then sits at a predetermined intermediate potential, eliminating hot spots and spreading the stress uniformly down the stack. This equalization strategy minimizes the chance of premature puncture, maximizing system reliability while keeping component size and cost in check.[2]
Uses
[edit]Corona rings are used on extremely high voltage apparatus like Van de Graaff generators, Cockcroft–Walton generators, and particle accelerators, as well as electric power transmission insulators, bushings, and switchgear. Manufacturers suggest a corona ring on the line end of the insulator for transmission lines above 230 kV and on both ends for potentials above 500 kV. Corona rings prolong the lifetime of insulator surfaces by suppressing the effects of corona discharge.[3]
Corona rings may also be installed on the insulators of antennas of high-power radio transmitters.[4] However, they increase the capacitance of the insulators.[5]
See also
[edit]References
[edit]- ^ "Corona rings and Grading rings". Retrieved 2025-04-24.
- ^ a b c d gd-admin. "China FJP Grading Ring for Insulator factory and suppliers | Beili". www.beiliele.com/. Retrieved 2025-04-24.
- ^ Electric power generation, transmission, and distribution, Volume 1 By Leonard L. Grigsby, CRC Press, 2007, ISBN 0-8493-9292-6
- ^ The Handbook of antenna design, Volume 2 By Alan W. Rudge, IET, 1983, p. 873, ISBN 0-906048-87-7
- ^ aerials for metre and decimetre wave-lengths, CUP Archive