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ARTICLES

Factors Affecting UV Inspection Results

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 a remote non-destructive testing equipment (NDT) has an outstanding corona detection performance that is due to:

  1. Noise recognition reduction
  2. Pinpointing high resolution
  3. UV-Visible superimposing accuracy
  4. Highest sensitivity to UVc radiation

DayCor® cameras are used to capture UV radiation emitted by corona and use the UV photon pulses counting as a severity assessment. Corona is expected to develop in locations with irregular high electrical fields that are usually indicative of defects, incorrect design, bad installations workmanship, contaminations, precipitations etc.


The relation between coutning and inspection distance

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 rain drops and icicles. Whatever the factors are, there should be awareness to them so that they are documented and mentioned in inspection reports.

Leaving all other factors the same and changing just the distance between the lens receiving surface of the camera and the corona source it was found that the UV count rate decreases according to the inverse-square law that specifies that a physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. The gain settings of the camera must be kept in the camera's plateau level. In that level changing gain does not affect the counting rate but affects the resolution of the display.

Environmental conditions affect both the corona generation and the discharge quantity. For a given corona situation, as temperature rise and atmospheric pressure falls, the dielectric strength of air decreases resulting in lower corona inception voltage and increase in 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 same distance. Component: outgoing line of a main transformer


Based upon the theoretical analysis and practical tests results obtained during field tests, CEPRI concluded as follows:

  • Factors that affect UV imaging are humidity, temperature, distance
  • Increasing the inspection distance decreases the counting
  • With the increase of humidity corona is restrained and the counting drops, yet background noise increase
  • It recommended to perform inspections on sunny days with relatively low humidity
  • For comparative and trending plans 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)

UVolle with TRH meter
Luminar with TRH meter
Scalar with TRH meter