State of the Corona Camera Industry Past, Present and Future RIAAN ROSSOUW

State of the Corona Camera Industry Past, Present and Future RIAAN ROSSOUW

State of the Corona Camera Industry Past, Present and Future By Riaan Rossouw

History of Corona Detection Early on luminous glows were noticed on HV equipment. Due to its similarity to corona from ships masts (St Elmo s fire) it was also referred to as Corona. Soon it was noticed that sound and radio interference was associated with the corona glow.

Corona Camera Corona Camera refers to an optical device used to visualise the fluorescence phenomenon associated with electrical discharges. Most of the light from the fluorescence phenomenon is in the UV range (<400nm) and very faint. Therefore they are actually UV cameras. Light from the sun is much brighter than the light from discharges.

Solar Blindness Only the UVc band (<300nm) has no light from the sun. In order for UV cameras to work in full daylight a filter had to be created to reject all light from the sun. The processed UVc image has to be overlaid onto the visible in order to determine the location of the discharge.

Corona Images Alli DeGroot Long Exposure Photography with conven6onal camera T&D SWorld Night vision Camera Intensified UVa/b Camera

Corona Images Early Solar Blind UV Camera Modern Solar Blind UV Camera

Severity Once a discharge is located based on high intensity UV locations, the question is: How serious is this? Some users erroneously use the count as an severity indicator.

Corona Counting The Count feature was introduced to indicate the intensity of the UV light detected. The count of discharges can only be compared if they are recorded under the same conditions by the same camera. Some cameras use blob counting while others measure intensity. Blobs merge and the count reduces at high UV intensi6es. Intensity measurements grows linearly un6l saturated.

Corona Counting Most cameras designs have a count variance >20% between units of the same model. For this reason all modern CoroCAM s readings are normalised to measure to within 5% of each other. There is currently no standard unit or measurement method. Presently there is a CIGRÉ workgroup (D1.61) to develop this standard.

Maintenance Priority / Severity Rating The US EPRI has devised a protocol based on examples in a guide and user experience / insight according to which a faults maintenance priority can be assigned.

UV Radiometry To understand UV radiometry we reference the IR radiometry method. Infrared cameras detects the number of IR photons generated by a heat source converting photon count per pixel to temperature after a convoluted algorithm, which takes into account distance and other variables. Distance highlinedivision.com

UV Radiometry The exact same principal of measurement is applied to photons generated by a corona event emitting UV photons. Distance Instead of C, UV intensity is expressed in Watts or number of photons per second.

Factors Affecting Corona Generation There are a number of factors which can influence the onset or extinction of corona discharges: Voltage and load applied Humidity Supply frequency Wind velocity Geometry of the discharge location Airborne particles and type affects the electric field gradient Air density (based on Air Pressure The material of the discharge point and Temperature) The temperature of the discharge Ionisation events in the vicinity point Surface condition (smooth vs rough)

Factors Affecting Corona Generation These factors have to be measured and compared to the average for a site in order to convert the measured value to a representative value.

How Long Until It Fails? Electrical discharges cause damage to different materials at different rates. Once a viable UVc radiometry method has been determined, the rate of material decay due to UVc irradiation over a year and related effects can be studied. The outcome of these studies will be able to answer the question above. This is the challenge to industry.

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