Ultrasonic Spectrum Signatures of Under-Oil Corona Sources

Ultrasonic techniques are useful for the detection and location of partial discharge or corona sources associated with HV power apparatus, particularly oil-filled power transformers. Measurements are commonly made using narrow-band transducers operating in the frequency range from ~20kHz to ~300 kHz, and occasionally at higher frequencies for which there is little published information. Because of the lack of available data on the spectra of ultrasonic emissions from discharges in oil-insulation systems, an investigation was initiated to determine if there was an optimum measuring frequency and whether discharges could be identified by their spectral characteristics. It was discovered that insulation voids and gap-type discharges, for example, have clearly identifiable ultrasonic frequency characteristics, and the actual physical size of voids may be estimated from their frequency spectra.