Why Do Only High-voltage Insulators Need Corona Rings?

High-voltage insulators require corona rings (also called grading rings) to manage the electric field distribution and prevent corona discharge, which can cause power loss, audible noise, radio interference, and material degradation. Here’s why they are necessary:

1. Electric Field Control
At very high voltages (typically 230 kV and above), the electric field around insulators becomes highly non-uniform, with extreme concentrations near sharp edges (e.g., at the conductor attachment point).
Corona rings smooth out the electric field gradient, reducing localized high-stress areas that could lead to ionization of air and corona discharge.

2. Preventing Corona Discharge
Corona occurs when the electric field exceeds ~3 kV/mm (the breakdown strength of air), causing partial ionization.
Without corona rings, tiny irregularities (like dirt, scratches, or hardware edges) can trigger corona, leading to:

Power loss (inefficiency)
Ozone production (damages insulation)
Radio frequency interference (RFI)
Audible noise (hissing or crackling sounds)

3. Voltage Grading
Long insulator strings in EHV (Extra High Voltage) systems have uneven voltage distribution, with higher stress near the energized end.
Corona rings act as capacitive couplers, balancing the voltage distribution along the insulator string, preventing excessive stress on individual discs.

4. Why Only High Voltage?
Lower voltages (< 230 kV) have weaker electric fields, so corona is less likely to form naturally.
Insulator designs for lower voltages (e.g., porcelain or polymer) don’t need additional grading because the field stress is manageable without rings.
EHV (500 kV+) and UHV (1000 kV+) systems always use corona rings due to extreme field intensities.

5. Where Are Corona Rings Installed?
On the line end (energized side) of insulator strings.
Near hardware connections (clamps, fittings) where sharp edges exist.
Sometimes on both ends (HV side and grounded side) for very long strings.

6. Alternative in Some Designs
Some polymer insulators have integrated grading devices (e.g., molded sheds with conductive layers) instead of separate rings.

Corona rings are essential for EHV systems to control electric fields, prevent energy loss, and avoid damaging side effects of corona discharge. Lower-voltage systems don’t need them because the electric stress is naturally below the corona threshold.