Characterization of Medium Voltage Equipment Ageing by Monitoring of Partial Discharges Chemical and Acoustical Emission

During the ageing of in-service medium voltage equipment, electrical losses are mainly due to partial discharges. These partial discharges can lead to the electrical failure of industrial equipment [1]. Partial discharges include (i) bulk discharges occurring in insulating material voids, (ii) corona discharges affecting or not the insulator and (iii) surface discharges developing on an insulator surface from a triple point (conductor/insulating material/air). Bulk and surface discharges can respectively lead to a final volume or surface breakdown whereas corona discharges may only induce a long term chemical surface ageing. Non-intrusive diagnostic methods are then required for predictive qualitative analysis that can warn of a potential upcoming system failure of medium voltage electrical equipment. Within these methods, electrical partial discharge analysis [2,3] (fast transient current signal associated the discharge phenomena measured at the medium voltage side using capacitors or Rogowski coil sensors) is currently applied to detect any damage or insulation failure which may have occurred during shipment and installation. Although this electrical method provides accurate recordings of partial discharges in laboratory conditions, it is difficult to implement for in-service equipment, such as power transformers, because of the high level of electromagnetic interference [3]. Two alternative nonintrusive methods have been investigated in this study: acoustical detection and gas phase chemical detection. Acoustic emission detection techniques have been applied to transformer partial discharge detection since the 1950s (liquid medium transformers) [2]. Electrical and acoustical procedures are often employed jointly, the acoustic technique being more effective for the location of the discharge sites [2]. In the present study, ultrasonic acoustic emission was directly investigated on medium voltage equipment subjected to accelerated ageing; collected experimental data were correlated to simultaneous electrical partial discharge analyser recordings. Gas phase detection of chemical species produced and released in atmospheric air is limited to the specific cases of surface discharges and corona discharges. These gaseous products can be identified and their formation rate evaluated by a periodic sampling of the ambient air in the vicinity of the discharge zone. For this method, preliminary laboratory-scale investigations were first realized. In view to implement both of the two methods on inservice equipment, chemical and acoustic measurements were simultaneously performed on a power transformer.