Corona discharge is an electrode-outer air current flow that happens (among other) in high-voltage power lines. One way to reduce corona discharge is using corona rings - a ring made of thick metal rod positioned orthogonal to the wire, earthed and mounted to the power line post.

Corona discharge damages insulators and may produce breakdown products which can cause catastrophic insulation failure. If electric field at a certain point in air is strong enough, electrons will be stripped from the atoms and make the air a conductive plasma instead of an insulator. An object with high voltage and a small curvature (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. Even a non-arc-shaped field can form a corona discharge. For that a conductive layer of plasma stops expanding and stays around the wire, ions in the plasma layer are repelled into neutral air and drift slowly in the air until discharged on another object, which is still a current flow yet much smaller.

Corona rings can be seen at either end of the insulators

The role of a corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, thus prevent corona discharge. Application of corona rings creates a "wire-to-ring" current flow Instead of "wire-to-air" current flow that helps to suppress corona discharge. It works by modifying the electric field intensity to reduce the worst case rate of field change on the insulator, thereby reducing peak potential across air surrounded to lower breakdown voltage to 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. 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.