Aluminum corona rings in the top of HV surge arresters, grading rings/Corona rings Applicated in the Extra high voltage（EHV）equipments/EHV testing equipments.
Aluminum corona rings use for the substations nad transmission line network， at higher voltage such as in EHV transmission networks，corona discharge effect is prominent，corona rings distribution the electric potential gradient evenly and maintains the maximum value below the threshold limits of the corona effect，corona rings are employed at high altitude electric equipment where the density of the air is less.
a useful secondary role of corona ring 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，difference 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 3-4 insulators，a field reshaped 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 discharge damages insulators and may produce breakdown products which can cause catastrophic insulation failure，if electric field at a certain point in the air is strong enough，electrons will be stripped from the atoms and make the air conductive plasma instead of an insulator，an object wiith high voltage and a small curvature like wire，sharp points，enables a field of enough high strength，forming a conductive layer of air，which is then surrounded by normal insulating air，in this case，the conductive layer can increase in size （usually in one direction）until it forms a complete conductive path to another object，and then a huge arc-shaped current flow，thus it is very important to minimize corona discharge.
the role of the corona rings is to distribute the electric field gradient and lower its maximum values below the corona threshold preventing corona discharge，its also called an anti-corona ring.