What is what is corona ring and Why Do We Use Them?

Corona discharge on insulator string of a 500 kV transmission line Corona rings on insulator strings on a 225 kV transmission line in France (left) Grading rings on 1.4 MV X-ray generator at the US (right) Corona caps and rings on the

Grading rings on 1.4 MV X-ray generator at the US Bureau of Standards in 1947.Corona caps and rings on the Cockcroft–Walton particle accelerator from 1937, now in the Science Museum, London.

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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

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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

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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

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Grading rings along a linear accelerator beam tube at the University of Pennsylvania in 1940

A very similar related device, called a grading ring, is also used on high-voltage equipment. Grading rings are similar to corona rings, but they encircle insulators rather than conductors. Although they may also serve to suppress corona, their main purpose is to reduce the potential gradient along the insulator, preventing premature electrical breakdown.

The potential gradient (electric field) across an insulator is not uniform but is highest at the end next to the high voltage electrode. If subjected to a high enough voltage, the insulator will break down and become conductive at that end first. Once a section of the insulator at the end has electrically broken down and become conductive, the full voltage is applied across the remaining length, so the breakdown will quickly progress from the high voltage end to the other, and a flashover arc will start. Therefore, insulators can stand significantly higher voltages if the potential gradient at the high voltage end is reduced.

The grading ring surrounds the end of the insulator next to the high voltage conductor. It reduces the gradient at the end, resulting in a more even voltage gradient along the insulator, allowing a shorter, cheaper insulator to be used for a given voltage. Grading rings also reduce aging and deterioration of the insulator that can occur at the high voltage end due to the high electric field there.

In very high voltage apparatus like Marx generators and particle accelerator tubes, insulating columns often have many metal grading rings spaced evenly along their length. These are linked by a voltage divider chain of high-value resistors so there is an equal voltage drop from each ring to the next. This divides the potential difference evenly along the length of the column so there are no high field spots, resulting in the least stress on the insulators.

Uses

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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.[1]

Corona rings may also be installed on the insulators of antennas of high-power radio transmitters.[2] However, they increase the capacitance of the insulators.[3]

Grading ring on a Russian surge arrester

Grading rings on transformer bushings

Corona rings on switch gear

See also

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References

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Answer 4. Summary:

The role of the corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing the corona discharge.*

Refer to the "superb reference" below for an extremely good explanation of theoretical and practical aspects.

A corona ring is a conducting ring mounted on the end of or radially outside the end of an insulator in an extra high voltage system, (typically 200 kV+)

The role of the corona ring is to eliminate or reduce ionisation of the air with associated corona discharge. Corona discharge damages insulators and may produce breakdown products which can cause catastrophic insulation failure.

A corona ring works by modifying the shape of the electric field intensity so that the worst case rate of change of field on the insulator is reduced, thereby reducing the peak potential across surrounding air to below the breakdown voltage of about 10 kV per inch (dry air, mains frequencies).

A useful secondary role is to reshape the electric field distribution across a stack of insulators so that the potential drop per insulator is more even, thereby reducing the breakdown stresses on the insulator with highest voltage drop. A stack of symmetrical insulators used for EHV insulation will have far more voltage drop across the insulators at the "hot" end of the stack. Differences of more than 10:1 may occur in long insulator strings, with in some cases more than half the total EHV drop occurring across the first 3 or 4 insulators. The reshaping of the field by a corona ring can usefully reduce this imbalance - but a disproportionately large portion of the total voltage is still liable to be carried by the first few insulators.

Corona rings can be seen at either end of the insulators in the photo below.

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