Investigations On Detection Of Corona Discharges And Diagnosis Of Faults On External Insulation Of High Voltage Equipments Based On Ultra Violet Imaging Technology

Corona discharges on the surface of external high voltage insulation are generally weak under normal operating conditions. Corona discharges can be triggered by badly designed hardware of HV equipments.In recent years UV imaging technology has been used to detect the corona discharges on the external insulation surface of HV equipments at home and abroad. Associated with Hubei Electric Power Test and Research Institute, a series of laboratory and theoretical investigations were carried out in this project arming at solving the problems existed in the applications of UV imaging technology.Intersection of photon number detected within interval time during the streaming discharge stage was selected as the caution range-the caution value of the photon number detected within interval time, close to the flashover stage is 5000-9000 s-1, which was verified afterwards in the UV imaging test for corona discharges of 110 kV post porcelain. It was revealed by the UV imaging test for corona discharges of two 110 kV post porcelain insulators with different artificial contamination classes that the photon number detected by the UV camera within interval time could reflect and distinguish the severity of the contamination on the post porcelain insulators but hard to determine the contamination level. By observing the electrical discharging process of the clean, wet 110 kV post porcelain insulator in parallel with the other one covered with non-uniform artificial contaminations by UV camera, the image of corona discharges was fond initiating from the boundary between the contaminated and cleaned side, coinciding with theoretical analysis. Corona discharges process on the polymeric insulator without grading rings installed at both ends observed by UV camera showed us that the corona discharges were found happening at the intersection between the high voltage hardware fittings and the core rod, therefore it was suggested that grading ring be installed at the high voltage side of polymeric insulators. Corona discharges were very obvious under the conditions of wet polymeric insulators with longitude direction splitting on sheath and shed both, and the location of corona discharge was also very precise. Therefore detection of these two faults on polymeric insulators was very effective. It was also revealed by the laboratory investigations that as long as the distribution of splitting on polymeric insulators is in the longitudinal direction; however as for the horizontal splitting, the corona discharge was not obvious due to the small potential difference. Corona discharges on the deteriorated 110 kV porcelain suspension insulator string observed by UV camera revealed that the number of deteriorated insulators in a glass insulator string could be determined by the photon number within unit interval detected by UV camera if the number of the deteriorated ones exceeded three and the distribution is continuous.Analysis on electrical discharges on external insulation of HV equipments were carried out by utilizing the classic gas discharge theory. Based on the description on the plasma radiation when air discharging, explanation concerning several stages of electrical discharges on external insulation of HV equipments was performed. Associated with photonics and material science, the byproduct phenomena of corona discharges on external insulations were elucidated. Based on the analysis on related test and theory at home and abroad, the conclusion that the UV radiation could represent the potential distribution as well as discharging intensity on external insulation was drawn, which was also the theoretical background for the detection of corona discharges on external insulation of HV equipments.UV image processing flowchart was proposed. DV was used to record the corona discharges videos displayed by CoroCAMâ…£+synchronously; the saturation layer was extracted subsequently and the image then turned into 8-bit gray-scale image; median filter was employed then to treat the extracted 8-bit gray-scale image to eliminate the background noise; next we used clustering threshold to process the background noise removed image and the image became binary image composed of pixels inside the threshold interval with the value of "1" and the ones outside the interval with the value of "0"; and finally the sum of the "1" areas were statistically analyzed, the ratio of which to the total area was defined as the characterizing parameter of corona discharge intensity.Finally the investigations of corona discharges of rod-plane model under typical ambient conditions were carried out by utilizing the charactering parameter of corona discharge intensity proposed above. The attenuation regularities of corona UV images of the same discharging gap at different observation distances as well as that of different discharging gaps at the same observation distance were discussed, and the criterion concerning the application of UV camera to detect corona discharges on external insulation of HV equipments based on "standard transformation method" was proposed.