Corona rings are toroidal shaped metallic rings which are fixed at the end of bushings and insulator strings. They are also called as anti-corona rings. Corona rings are used to prevent corona discharge that occurs in high-voltage power lines. Corona discharge or corona loss is a significant issue with very high voltage power lines which causes power loss. One way to reduce corona discharge is using corona rings.
How corona rings work?Corona discharge occurs when the electric field (potential gradient) at the surface of conductors exceed a critical value, called as critical disruptive voltage. The value of critical disruptive voltage varies with atmospheric condition. Its value is roughly 30 kV/cm. The electric field greatest where the curvature is sharpest. Therefore, corona discharge occurs first at the points where the curvature is greatest - i.e. at suspension points, corners and edges. Corona rings are installed at these points to prevent corona formation.
|Corona discharge on a corona ring|
Image source: Wikimedia commons
A corona ring is electrically connected to the conductor, encircling the points where corona discharge may occur. Therefore, the corona ring distributes the electric field (or charge) across a wider area, because of its smooth round shape. Hence, it reduces the potential gradient below the critical disruptive value.