Have you ever seen a ring around the Surge Arrester or other EHV equipment? What is the purpose of the Ring around the Extra High Voltage (EHV) equipment? What is the use of big rings at EHV testing centers?
This Metal Ring what we see in the substations and other transmission line network is “Corona Ring“. At higher voltages such as in EHV transmission networks, corona discharge effect is prominent. Corona Ring distributes the electrical potential gradient evenly and maintains the maximum value below the threshold limits of the corona effect. Corona Rings are employed at high altitude electrical 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. 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.
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 with 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 ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing corona discharge. It is also called an anti-corona ring.
How Corona Ring Works?
Corona discharges occur when the electric field i.e. potential gradient at the surface of conductors exceeds a Critical value called Disruptive Potential Gradient. It is roughly 30 kV / cm (peak) for air but varies with atmospheric pressure, so Corona is more 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 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.
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 for dry air and at mains frequencies.
Effect of Corona Tube Diameter
The more increase in the corona tube diameter, the less voltage percentage on the bottom-most insulators. On the hand, increasing the corona tube diameter will increase voltage sharing for uppermost insulators.
Effect of Corona Ring Diameter
The higher the diameter of the ring, the lower the voltage percentage of the lowermost insulator. However, the decreasing rate is slow. On the other hand, higher ring diameters result in increases in voltage of uppermost units.