Acoustic mode transition of creeping discharge from oil–paper interface to interior of insulating paper

There is a complex mapping relationship between creeping discharge of oil paper insulation and acoustic wave it excites, which restricts the identification of discharge types and threat assessment based on acoustic detection of oil–immersed transformer. This study detects the electrical, acoustic, optical and schlieren characteristics of the discharge simultaneously, clarifying the relationship between acoustic waves and different creeping discharge modes. The results show that as the creeping discharge develops from oil–paper interface to the interior of insulating paper, the acoustic wave excited will convert from a dispersed pulse form to a continuous form. Specifically, when the electric field presents a weak vertical component or in initial stage of strong vertical component, the discharge occurs on the oil side of oil–paper interface. The acoustic wave is pulsed, with sound pressure peak increasing and equivalent frequency decreasing as the discharge energy rises, both following power laws. When the electric field presents a strong vertical component, the acoustic wave is spindle–shaped with the period consistent with AC voltage as the discharge develops to interior of insulating paper. The sound pressure peak is reduced from thousands to tens of Pascals and the frequency range is reduced from 10~400 kHz to 1~100 kHz compared to the aforementioned pulsed wave. From the energy perspective, the acoustic wave converting from pulse form to continuous form is caused by the increase in discharge repetition rate and the decrease in converted acoustic energy by the formation of carbonization channels. This paper discovered the discharge dependent acoustic characteristics caused by different discharge modes, confirming that the severity of creeping discharge cannot be simply judged by the sound pressure amplitude, which is helpful to improve the accuracy of acoustic detection and identification for oil–immersed transformers.