Corona discharge — UV camera and ultrasonic detection in medium-voltage installations

The guide on thermographic inspection covers detecting heat generation resulting from increased contact resistance. This article covers a different, complementary inspection phenomenon that occurs almost exclusively at medium voltage (typically from a few kV upward) and is not detected by thermography itself: corona discharge.

What corona discharge is

Corona discharge occurs when the local electric field around a conductor — for example at a sharp edge, a damaged cable termination, a contaminated or damp insulator surface, or a poorly connected earthing braid — locally exceeds the breakdown strength of the surrounding air, without leading to a full breakdown (flashover) between two conductors. The result is small-scale, sustained ionization of the air directly around that point, which emits both electromagnetic radiation in the ultraviolet spectrum and produces a characteristic, high-frequency acoustic sound.

Why corona is relevant for periodic inspection

Corona discharge is often not directly destructive by itself, but it is a reliable indicator of an underlying problem: a sharp edge that has not been rounded, a contaminated or damaged insulator surface, a poorly executed cable termination, or local contamination that attracts moisture. Left untreated, sustained corona can over time lead to erosion of the insulating material around the discharge point, increasing the risk of a full breakdown over the longer term. Corona detection is therefore particularly valuable as an early warning, well before the stage at which a thermographic camera would register a temperature difference.

UV camera: making the phenomenon visible

A corona discharge emits, among other things, radiation in a narrow band of the ultraviolet spectrum (around 240–280 nm) that is strongly attenuated by the earth's atmosphere, meaning this band is barely disturbed by sunlight — hence the term "solar-blind" for cameras specifically tuned to this band. A solar-blind UV camera detects this radiation and projects the location of the corona discharge as an overlay on a regular video or photo image, allowing an inspector to directly and visually pinpoint the exact location of the discharge — for example on a specific insulator or cable termination — even in daylight.

Ultrasonic detection: making the phenomenon audible

Besides UV radiation, corona discharge also produces a characteristic ultrasonic sound (typically on the order of several tens of kHz, well above human hearing), caused by the local air ionization. An ultrasonic detector converts this sound to an audible frequency or a numeric intensity value, and is less sensitive to exact direction than a UV camera, but typically covers a larger area at once — making it a suitable first-screening tool for quickly scanning an entire installation or switchgear room, before zooming in on a suspect point with a UV camera.

Complementary use: broad screening first, then targeted localization

In practice, both techniques are often combined: ultrasonic detection is used to quickly scan an entire medium-voltage room or switchgear installation and identify suspect zones, after which a UV camera is aimed at that specific zone to confirm and document the exact location of the discharge on an insulator, cable termination, or busbar connection.

Note: both UV and ultrasonic corona detection are primarily suited to problems related to voltage distribution and surface field strength, while thermography is primarily suited to problems related to current conduction and contact resistance — the two techniques are complementary, not mutually interchangeable, and a complete medium-voltage inspection ideally combines both.

Practical relevance

During a periodic inspection of a medium-voltage installation (switchgear, cable terminations, transformer room), it is worthwhile to include a corona screening (ultrasonic, possibly supplemented with a UV camera at a suspect location) alongside the regular thermographic inspection, particularly for installations with a longer service life or a history of contamination or moisture ingress — corona discharge can be present for months to years before a thermographically observable temperature rise occurs.

Common mistakes

Relying solely on thermography for a medium-voltage inspection and skipping corona detection — corona discharge at a clean, cold connection produces no thermographic signal, even though there may well be an underlying insulation problem.
Using a UV camera without accounting for detection distance and ambient conditions (rain, fog, or strong wind can disturb the measurement) — the manufacturer's specifications for maximum reliable detection distance should be leading here.
Not following up sustained corona at a specific point with further investigation, assuming it is "just a small noise" — corona by definition indicates a locally exceeded field strength and therefore an underlying, potentially worsening problem.
Treating ultrasonic and UV detection as a replacement for thermography rather than a complement — the techniques fundamentally detect different failure mechanisms.