Partial discharge activity at the intercept between solid and air insulation: Qualitative interpretation of the ultrasonic signatures

A discharge regime could be described based on the recording and analysis of digitized real-time sequences of the electrical signals. These signals were compared with the ones obtained using an ultrasonic diagnostic. The ultrasonic transducer easily detected partial discharges at corona threshold having a charge in the range of 200 pC. Discharge events consisting of a single pulse had a very consistent and reproducible ultrasonic signature. A single oscillating wave was produced exhibiting a fast damping over periods ranging from 30 to 50 ms. The ultrasonic method does-not permit the discrimination of multiple discrete isolated discharge events consisting of several single pulses occurring over the 1-ms frame. However, pulse sequences were observed to greatly affect the signatures.