Corona ring

From Wikipedia, the free encyclopedia
  (Redirected from Grading ring)
Jump to: navigation, search
Corona discharge on insulator string of a 500 kV transmission line.
Corona rings on insulator strings on a 400 kilovolt transmission line in France.
This Cockcroft–Walton voltage multiplier was part of one of the early particle accelerators responsible for development of the atomic bomb. Built in 1937 by Philips of Eindhoven, it is now in the National Science Museum in London, England. Note the silver corona rings around the insulators.

A corona ring, also called an anti-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. A very similar related device, the grading ring is used around insulators.

Corona discharge[edit]

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. It is caused by the leakage of current from the conductor into the air, and represents a significant waste of power in electric power transmission lines and equipment. 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.

How corona rings work[edit]

Corona discharges only occur when the electric field (potential gradient) at the surface of conductors exceeds a critical value, the disruptive potential gradient. It is roughly 30 kV / cm, but varies with atmospheric pressure, so corona is more of a problem at high altitudes. The electric field at a conductor is greatest where the curvature is sharpest, and therefore corona discharge occurs first at sharp points, corners and edges. The terminals on very high voltage equipment are frequently designed with large diameter rounded shapes such as balls and toruses called corona caps, to suppress corona formation. However some parts of high voltage circuits require hardware with exposed sharp edges or corners, such as the attachment points where wires or bus bars are connected to insulators. Corona rings are installed at these points to prevent corona formation.

The corona ring is electrically connected to the high voltage conductor, encircling the points where corona would form. Since the ring is at the same potential as the conductor, the presence of the ring reduces the potential gradient at the surface of the conductor greatly, below the disruptive potential gradient, so corona does not form on the metal points.

Grading rings[edit]

A very similar related device, called a grading ring is also used on high voltage equipment. Grading rings are similar to corona rings, but they encircle the ends of insulators rather than conductors. Although they may also serve to suppress corona, their main purpose is to reduce the potential gradient along the insulator, preventing premature electrical breakdown.

The potential gradient (electric field) across an insulator is not uniform, but is highest at the end next to the high voltage electrode. If subjected to a high enough voltage, the insulator will break down and become conductive at that end first. Once a section of insulator at the end has broken down, the full voltage is applied across the remaining length, so the breakdown will quickly progress from the high voltage end to the other, and a flashover arc will start. Therefore insulators can stand significantly higher voltages if the potential gradient at the high voltage end is reduced.

The grading ring surrounds the end of the insulator next to the high voltage conductor. It reduces the gradient at the end, resulting in a more even voltage gradient along the insulator, allowing a shorter, cheaper insulator to be used for a given voltage. Grading rings also reduce aging and deterioration of the insulator that can occur at the HV end due to the high electric field there.

Uses[edit]

Corona rings are often seen on Van de Graaff generators, electrical insulators, Cockcroft–Walton generators, particle accelerators, Lightning Arresters, high voltage bushing terminals, and other high voltage equipment.

Corona rings are typically installed on very high voltage power line insulators. Manufacturers suggest a corona ring on the line end of the insulator for above 230 kV and on both ends for above 500 kV. Corona rings prolong lifetime of insulator surfaces by suppressing the effects of corona discharge.[1]

Corona rings may also be installed on the insulators of antennas of high-power radio transmitters.[2] However, they increase the capacitance of the insulators.[3]

Grading ring on Russian surge arrester
Grading rings on transformer bushings
Corona rings on switchgear

See also[edit]

References[edit]