Image 1: Corona rings: 400 kV insulators (Source:https://en.wikipedia.org/wiki/Corona_ring)
Image 2: High-voltage surge arresters (ABB)
How to detect corona discharges?
Corona can manifest itself in various ways, such as:
- Radio-frequency interference (RFI)
- Audible noise (crackling heard near substations and high-voltage transmission lines)
- Radiation of ultra-violet (UV) light (bluish glow)
UV-sensitive cameras are frequently used to detect corona. Most of them are now bi-spectral, meaning they have two sensors. The first sensor only detects UV light while the second sensor captures visible light to produce a background picture, which helps locate the corona discharges on the equipment.
Such UV inspections are performed from a safe distance, when equipment is energized, and during favourable weather conditions (no rain and little wind). Contrary to infrared (IR) measurements, which are intended to locate hot spots, UV inspections do not require the equipment to be under load.
Many corona cameras are now equipped with a solar blind feature, which allows for daytime use. A special filter blocks UVA and UVB light from the sunlight, but does not block UVC light from the corona discharges. Some also have an infrared sensor for a more complete inspection.
Below are some examples of corona discharges using a bi-spectral camera.