Corona: A problem you should not ignore
15 April, 2019 | Blog
Electrical corona discharges occur when the voltage applied to an insulated conductor is high enough to create current leakage, but not enough to cause a complete insulation breakdown (at least not yet). Still, corona discharges generate ozone, nitric oxide and acid gases, which are corrosive and can further deteriorate the insulation, up to complete failure. In some cases, corona discharges can represent significant power loss.
Thankfully, detecting corona discharges in open air is now easier than ever. Nowadays, it can be used as a warning before a major failure occurs to critical electrical equipment.
When dealing with internal partial discharge, many solutions are available on the market for continuous and on-line monitoring and for many types of apparatus. However, these won’t be discussed in this blog.
What causes corona?
Many factors can cause or increase corona discharges, such as:
- Cracked or excessively dirty insulators
- Damaged conductors
- Rusty hardware
- Sharp points or edges on a conductor
- Improper design or installation
- And more…
In high-voltage applications (generally 230 kV or higher), anti-corona devices are installed at sharp points or edges to eliminate the presence of corona by reducing the potential gradient at the conductor surface. These devices can be recognized by their rounded shape (i.e., corona rings, rounded connectors, etc.).
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.
Benefits of performing UV inspections
- Can be performed live, even on lightly or not loaded equipment
- Can pinpoint specific locations to investigate, allowing better planned and more efficient maintenance shutdowns
- Allows for early problem detection before a costly and major failure
- Helps find the source of power losses
If you want to learn more about UV inspections, feel free to contact one of our experts today!
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