Visualizing Method for Some Different Types of Electrical Tree using 20MHz Ultrasound

Abstract

The method to visualize an electrical treeing region in translucent or opaque electrical insulating materials by using ultrasound with a center frequency around 20MHz is presented.
The dependence of the spatial resolution on the frequency of ultrasound is investigated by means of actual measurements of velocity and attenuation of ultrasound for the test materials. On the basis of the results, sufficient resolution and intensity for the ultrasound echo signal reflected from the electrical tree are obtained by selecting the transducer with an appropriate center frequency to emphases the higher region of the frequency, and also by utilizing a Boxcar integrator for improving signal-to-noise ratio.
As the results, it is shown that the branch type tree in polymethylpentene, the bush and the ball type ones in polyethylene can each be visualized. The shapes and the sizes are agreed with those obtained by optical inspections.