Improvement of acoustic detection and localization accuracy by sensitive electromagnetic PD measurements under oil

Abstract: Partial discharge (PD) diagnosis for large power transformers as one of the most powerful tools to evaluate insulating quality and characteristics should be carried out as permanent monitoring or onsite measurements with adequate sensitivity. This allows conditioned based maintenance strategies, serves asset appraisements and permits continuous risk assessment. Online/onsite electric PD measuring procedures according to the IEC60270 standard so far show a limited reachable sensitivity e.g. caused by corona. Since PD also appear as source of mechanical waves in the ultra-sonic range and electromagnetic waves up to the UHF (ultra high frequencies) range additional acoustic or electromagnetic measurements are possible and helpful in online/onsite measurements. Comparative investigations with the acoustic and the UHF method revealed a very moderate damping of the UHF signals in oil and in the solid insulations in contrast to resulting attenuation of the acoustic signals. Thus a significantly higher sensitivity especially for hidden PD defects could be reached with the UHF method. Sensitive electromagnetic UHF PD measurements help to identify and localize faults which are not detectable with acoustic single impulses by simple averaging of the acoustic signals. Furthermore broad-band investigations of UHF spectra revealed cavity resonances which possibly make up the basis for advantageous narrow-band measurements