the difference between grading ring and corona ring?

General recommendations firstly
corona forms when the voltage gradient at the surface of a conducting material exceeds a critical value and ionises the surrounding air，corona becomes a great concern at extra high voltages where precautionary measures are necessary，effects of corona including radio and television interference，voltages loss and audible noise on the line，so factory produce a wide range of grading rings to suit EHV transmission line for both suspension and tension configuration，typical designs are shown below for demonstration of factory‘s ability to produce customer's requirements.

A grading ring is used to ensure a uniform voltage distribution along the length of an electrical device. This is important for surge arresters so each MOV disc in the arrester is energized at the appropriate voltage.

A corona ring is traditionally used to electrically shield external hardware to prevent corona from developing. This corona could lead to degradation of insulating materials or create interference to electronic communication.

Surge arresters below 500 kV system voltage do not typically need corona rings.