![Corona rings on insulators](https://upload.wikimedia.org/wikipedia/commons/0/06/Corona_rings_on_insulators.png)
A corona ring, more correctly referred to as an anti-corona ring, is a
toroid
In mathematics, a toroid is a surface of revolution with a hole in the middle. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its ...
of conductive material, usually metal, which is attached to a terminal or other irregular hardware piece of
high voltage
High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant spec ...
equipment. The purpose of the corona ring is to distribute the
electric field gradient In atomic, molecular, and solid-state physics, the electric field gradient (EFG) measures the rate of change of the electric field at an atomic nucleus generated by the electronic charge distribution and the other nuclei. The EFG couples with the ...
and lower its maximum values below the corona threshold, preventing
corona discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown ...
. 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
Corona discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown ...
is a leakage of electric current into the air adjacent to high voltage conductors. It is sometimes visible as a dim blue glow in the air next to sharp points on high voltage equipment. The high
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
ionizes
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
the air, making it conductive, allowing current to leak from the conductor into the air in the form of
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s. In very high voltage electric power transmission lines and equipment, corona results in an economically significant waste of power and may deteriorate the hardware from its original state. In devices such as
electrostatic generator
An electrostatic generator, or electrostatic machine, is an electrical generator that produces '' static electricity'', or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest civi ...
s,
Marx generator
A Marx generator is an electrical circuit first described by Erwin Otto Marx in 1924. Its purpose is to generate a high-voltage pulse from a low-voltage DC supply. Marx generators are used in high-energy physics experiments, as well as to simulate ...
s, and tube-type 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
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
gas, which can cause aging and brittleness of nearby structures such as insulators. The gasses create a health hazard for workers and local residents. For these reasons corona discharge is considered undesirable in most electrical equipment.
How they work
Corona discharges only occur when the
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
(potential gradient) at the surface of conductors exceeds a critical value, the
dielectric strength
In physics, the term dielectric strength has the following meanings:
*for a pure electrically insulating material, the maximum electric field that the material can withstand under ideal conditions without undergoing electrical breakdown and becom ...
or ''disruptive potential gradient'' of air. It is roughly 30 kV/cm at sea level but decreases when atmospheric pressure decreases. Therefore, corona discharge is more of a problem at high altitudes. The electric field at a conductor is greatest where the curvature is sharpest, meaning that 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. Some parts of high voltage circuits have hardware with exposed sharp edges or corners, such as the attachment points where wires or bus bars are connected to insulators; corona caps and rings are usually 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 below the disruptive potential gradient, preventing corona from forming on the metal points.
Grading rings
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 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
Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating mate ...
.
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 the insulator at the end has electrically broken down and become conductive, 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 high voltage end due to the high electric field there.
In very high voltage apparatus like
Marx generator
A Marx generator is an electrical circuit first described by Erwin Otto Marx in 1924. Its purpose is to generate a high-voltage pulse from a low-voltage DC supply. Marx generators are used in high-energy physics experiments, as well as to simulate ...
s and
particle accelerator
A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.
Large accelerators are used for fundamental research in particle ...
tubes, insulating columns often have many metal grading rings spaced evenly along their length. These are linked by a voltage divider chain of high-value resistors so there is an equal voltage drop from each ring to the next. This divides the potential difference evenly along the length of the column so there are no high field spots, resulting in the least stress on the insulators.
Uses
Corona rings are used on extremely high voltage apparatus like
Van de Graaff generator
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct ...
s,
Cockcroft–Walton generator
The Cockcroft–Walton (CW) generator, or multiplier, is an electric circuit that generates a high DC voltage from a low-voltage AC or pulsing DC input. It was named after the British and Irish physicists John Douglas Cockcroft and Ernest Tho ...
s, and
particle accelerator
A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.
Large accelerators are used for fundamental research in particle ...
s, as well as
electric power transmission
Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission network''. This is ...
insulators, bushings, and switchgear. Manufacturers suggest a corona ring on the line end of the insulator for transmission lines above 230 kV and on both ends for potentials above 500 kV. Corona rings prolong the lifetime of insulator surfaces by suppressing the effects of corona discharge.
Corona rings may also be installed on the insulators of
antennas
In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies a ...
of high-power
radio transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s. However, they increase the
capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
of the insulators.
aerials for metre and decimetre wave-lengths
CUP Archive
See also
* Arcing horns
Arcing horns (sometimes arc-horns) are projecting conductors used to protect insulators or switch hardware on high voltage electric power transmission systems from damage during flashover. Overvoltages on transmission lines, due to atmospheric e ...
References
{{reflist
External links
Corona Ring Research
What is Grading Ring
Differences Between Corona Rings And Grading Rings
Electrical breakdown
Plasma physics
Dielectrics
Electric power systems components