Evaluation of Change in Bow-tie Tree Degradation Due to Additional Voltage Application by Image Processing

It is important to investigate the change in bow-tie tree degradation with voltage application to clarify the mechanism of electrical-tree inception from bow-tie tree and influence of bowtie tree degradation on electrical insulation. The present study focused on the change in bow-tie tree degradation before and after voltage application. It is often difficult compare the optical microscope images before and after voltage application objectively, as the acquisition conditions of each image are different, e.g. brightness distribution. In order to unify the brightness distribution, methods for standardizing the background brightness distributions were developed in this study. However, since most of the image data is in the background, the changes in the tree region may have been cancelled out after standardization. Therefore, two approaches were made to reconstruct the background distribution by assuming that the background brightness distribution was approximated by a normal distribution. In addition, after standardizing the created brightness distributions, we compared the shapes and methods by differential extraction before and after the application of voltage. The use of these methods together with proper setting of brightness threshold enabled to observe the bow-tie tree region. By taking the difference between the bow-tie tree images before and after voltage application, the change induced by voltage application, e.g. tree extension, was detected. These method, however, require further development, as these are not applicable in the case that these original background distribution shows prominent asymmetry.