Corona on Overhead Lines: Causes & How Corona Rings Help
Corona is a partial electrical discharge into the air that occurs at high-voltage points where the electric field is intense enough to ionize the surrounding air, typically at sharp edges and the metal ends of insulator strings. It causes audible noise, radio/TV interference, energy loss, and long-term degradation of fittings. A corona ring (grading ring) is a smooth toroidal ring fitted at the high-field points that spreads the electric field over a larger, rounded surface, lowering the peak field below the corona threshold. Corona rings are standard on high-voltage lines above a design-set voltage.
On high-voltage lines, you can sometimes hear a faint crackle or hiss on a damp evening that’s corona. It’s not just a sound; it’s wasted energy and slow damage. This guide explains what corona is, why it matters, and how corona rings control it.
What Is Corona?
Corona is a partial discharge a small, localized ionization of the air around an energized conductor or fitting. It happens where the electric field is intense enough to break down the air immediately around a surface, but not intense enough to cause a full flashover. The air molecules ionize, glow faintly (visible as a bluish glow in the dark), and the discharge produces sound, radio noise, and a small energy loss.
The field intensifies at sharp edges, points, and small-radius surfaces and at the metal end fittings of insulator strings, where the geometry concentrates the field. That’s why corona tends to appear at those points first, especially at high voltages and in humid or foul weather (moisture lowers the threshold).
Why Corona Is a Problem
Corona isn’t dramatic, but its effects add up:
- Audible noise the characteristic crackle/hiss, a nuisance near lines.
- Radio and TV interference the discharge emits electromagnetic noise across radio frequencies.
- Energy loss corona consumes power continuously; across a long high-voltage line, it’s a real loss.
- Degradation of fittings the discharge and associated chemistry (ozone, nitric acid in moisture) can, over time, degrade insulator surfaces and metal fittings.
For high-voltage transmission especially, controlling corona is part of good line design it reduces losses, interference complaints, and long-term wear.
How Corona Rings Help
The cause of corona is a too-intense electric field at sharp points. The fix is to reduce that peak field and that’s exactly what a corona ring (also called a grading ring) does.
A corona ring is a smooth, rounded, toroidal (doughnut-shaped) metal ring fitted around the high-field point typically at the energized end of an insulator string. By presenting a large, smooth, curved surface in place of a sharp edge, it:
- Spreads the electric field over a bigger, rounded area, so the peak field at any point is much lower.
- Grades the voltage distribution along the string more evenly (hence “grading ring”).
- Brings the peak field below the corona threshold, so the discharge doesn’t start.
In effect, the ring replaces the sharp geometry that causes corona with a smooth one that doesn’t. It’s a simple, passive, highly effective solution.
When Do You Need Corona Rings?
Corona rings become necessary above a certain voltage, set by the line design lower-voltage lines generally don’t need them, while high-voltage transmission lines typically do, fitted at the string ends. The exact threshold and ring size depend on the voltage, the fitting geometry, and the design standard. As a rule: the higher the voltage, the more important corona control becomes, and the more likely corona rings are specified. Confirm the requirement and ring specification for your voltage with your design.
Corona rings work alongside the rest of the string hardware the suspension insulators, the couplings, and the yoke plates on bundled arrangements as part of a properly graded high-voltage string.
Corona on Overhead Lines: FAQ
What is corona in a power line? Corona is a partial electrical discharge into the air at high-voltage points where the electric field ionizes the surrounding air typically at sharp edges and the metal ends of insulator strings. It produces noise, radio interference, energy loss, and slow degradation of fittings.
What causes corona discharge? An electric field intense enough to break down the air right at a surface which happens at sharp edges, points, and small-radius fittings where the field concentrates, especially at high voltage and in humid or wet weather. Smooth, large-radius surfaces reduce it.
What does a corona ring do? A corona ring (grading ring) is a smooth toroidal ring fitted at the high-field points of an insulator string. It spreads the electric field over a large rounded surface, lowering the peak field below the corona threshold so the discharge doesn’t start, and it grades the voltage more evenly along the string.
Why is corona a problem? It causes audible noise, radio/TV interference, continuous energy loss, and over time degradation of insulator surfaces and fittings. On high-voltage lines, controlling corona reduces losses, interference, and long-term wear.
At what voltage are corona rings needed? Above a threshold set by the line design lower-voltage lines usually don’t need them, while high-voltage transmission lines typically do, fitted at the string ends. The exact threshold and ring size depend on the voltage, geometry, and standard; confirm for your line.
Do corona rings prevent flashover? Corona rings control corona (partial discharge) by grading the field, which is different from preventing flashover (a full discharge across the insulator). By grading the voltage distribution, they contribute to a well-behaved high-voltage string, but flashover is governed by the insulation and creepage.
The Bottom Line
Corona is a partial discharge at intense-field points on high-voltage lines, causing noise, interference, energy loss, and slow degradation. The solution is to reduce the peak field with a corona ring a smooth toroidal grading ring at the string ends that spreads the field below the corona threshold. On high-voltage lines above a design-set threshold, corona rings are standard high-voltage string hardware.
DAPENG Power (DP Power Fitting) manufactures corona rings and the complete high-voltage string suspension insulators and strings, couplings, and yoke plates. Tell us your voltage and string configuration, and we’ll specify the corona rings and grading hardware for your line.
