Ultrasonic visualization method of electrical trees formed in organic insulating materials

The only practical method to visualize electrical trees formed in organic insulating materials has been to section a specimen with a microtome or a diamond cutter, especially in translucent or opaque materials. In our experiment, a 25 MHz ultrasonic transducer was used to scan a specimen within which an electrical tree had been formed. Ultrasonic echo signals reflected from the treeing region were detected by the same transducer and fed to a electronic measuring system. By determining the horizontal coordinates at which each echo signal from the tree was detected, two-dimensional ultrasonic images were obtained for various types of electrical trees in polyethylene. Furthermore, three-dimensional ultrasonic images of bush-type trees were obtained by means of a new detection or measurement method developed by the authors. These images agree well with those obtained microscopically by sectioning the specimen with a microtome.