In a research conducted lately by CEPRI – China Electric Power Research Institute that was presented at the SPIE 2014 conference, the use of Ofil’s DayCor® Superb was analyzed. The purpose of the study was to establish operating principles taking into account the influence of external factors such as inspection distance, inspection angle, camera gain settings and ambient conditions. The study concluded that when performing comparative corona inspection of electrical apparatuses conditions should be kept similar (if not identical) and always documented.
The DayCor® Superb as remote non-destructive testing equipment (NDT) has an outstanding corona detection performance that is due to:
- Noise recognition reduction
- Pinpointing high resolution
- UV-Visible superimposing accuracy
- Highest sensitivity to UVc radiation
DayCor® cameras are used to capture UV radiation emitted by corona and use the location of the emission to zoom into the fault. Corona is expected to develop in locations with irregular high electrical fields. These are usually indicative of defects, incorrect design, bad installations workmanship, contamination and other. The camera can also display, upon calling the function, a count of corona events per second that reached the camera, The displayed value can be helpful in laboratories where all parameters that affect the results are controlled.
When conducted outdoors UV imaging is influenced by environmental conditions. Some of the conditions relate to optical laws, such as inverse-square law, some relate to quantum chemistry of particles, such as temperature, barometric pressure, humidity and some to electrical and electromagnetic fields such as raindrops and icicles. Whatever the factors are, they should be documented and mentioned in inspection reports.
The relation between counting and inspection distance
Leaving all factors but the distance constant, will result in different UV count rate for the same source. The expected change will follow the inverse-square law that states that a physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity.
Environmental conditions affect both the corona generation and the discharge quantity. For a given corona situation, as the temperature rises and atmospheric pressure falls, the dielectric strength of air decreases resulting in lower corona inception voltage and in an increase of the discharge. At the same time, when air density drops, absorptions coefficient of UV radiation in air decreases and there is an increase in UV pulse propagation.
Regarding humidity: moisture, on the one hand, restrains corona generation because it absorbs free electrons and diminishes the avalanche process of ionization that leads to corona, but on the other hand, saturation may lead to droplets on the electrodes, and these may change the electrical field by creating protruding edges that induce corona formation.
Left: 50% humidity, counting 4110 events/second. Right: 76% humidity counting 0. Inspection performed from the same distance. Component: outgoing line of the main transformer
Based upon theoretical analysis and practical tests results obtained by CEPRI the following applies:
- Factors that affect UV imaging are humidity, temperature & distance
- Increasing the inspection distance decreases the counting
- With the increase of humidity, the corona is restrained and the counting drops, yet background noise increase
- It is recommended to perform inspections on sunny days with relatively low humidity
- For comparative and trending detection should be conducted at the same distance with a free line of sight
Xiao Han, Ning Yang, Shuai Yuan, Jiangang Bi “An Analysis on Effecting Factors of Ultraviolet Imaging Appliance in Corona Discharge”, International Symposium on Optoelectronic Technology and Application (2014)