Ultrasonic NDT for defects/degradation in insulation

The partial discharge test is the major tool at the disposal of the engineer to determine the integrity of a sample of insulation on production and its probable in-service performance. The test, when properly interpreted, yields information about the size of the discharge, but gives no indication as to the nature, position or geometry of the source. All of these parameters may affect the in-service performance of a sample. We have employed two ultrasound techniques which may be used in conjunction with the discharge test to give more information about the discharge source. The use of acoustic emission is similar to the partial discharge test in that a single reading is provided by the whole sample for each test. The result of the test varies, however, with the position of the acoustic transducer. Acoustic emission may thus be used to gain information about the position of the discharge source, and must be performed while the sample is `live´. Further investigation, via the use of active ultrasound, of an area of interest may indicate the size of the discharge source. Interpretation of the signals produced by both ultrasound techniques yields information about the nature of the discharge source. Examples of flaw detection in commercially produced samples are presented