A corona ring, also called an shielding 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.

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.
 

Why does Grading ring use in the Transmission line?
Grading ring always provides with the insulator in the transmission line. The Design of the Grading rings is similar to corona rings and they encircle insulators of the Transmission line. The potential gradient or electric fields across the line insulator is not uniform. The highest gradient observed at the endpoint insulators towards the high voltage conductor. Due to this continuous grading at high voltage, the insulators near the high voltage conductor are easily deteriorated and breakdown. The line observes short circuit earth fault because of the breakdown insulator.

grading ring in transmission line
The top 4 reasons to use grading rings in the Transmission Line.

It reduces the potential gradient along the insulator and prevents electrical insulator breakdown. The grading ring covers the surrounds ends of the insulator near the high voltage side and creates a uniform distribution of voltage and reduces the stress on the insulator.
The grading ring reduces the aging effect and deterioration of the line insulator.
During high voltage conditions, it prevents the insulator from the breakdown.
It also serves the purpose of the corona ring and prevents corona discharge of the line.
The grading ring is electrically connected with the conductor. The Ring shape grading rings are used in the insulator of the transmission line and the surface of the ring is very smooth for uniform distribution of the field.

Design of Grading Ring
The design of the grading ring depends on the following factors:
Rated voltage of the Transmission line
Maximum over voltage rating.
Type of transmission line insulator,
Weather condition,
Testing,
Tests for checking the quality of Grading Ring before use in Transmission Line,
Mechanical Strength,
Resistance,
Smoothness of the ring surface.