Several common sources of discharges activity occurring on generators have been replicated in the laboratory under well-controlled conditions. Each source was evaluated individually and recorded with a phase resolved acquisition system and with a spectrum analyzer. The dominant features of each respective phase re-( ) solved partial discharge PRPD pattern are presented. The frequency content of the discharge signal at the detection coupler was also investigated. The association of each well-defined type of discharge source, with its specific PRPD pattern, constitutes the basis of our database used for the discharge source recognition during generator diagnostics. The comparison of laboratory results with actual field measurements gathered over the last decade is summarized in this paper.

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To improve our understanding of slot partial discharge (PD) mechanisms, an accelerated aging test was initiated two years ago. This long-term experiment is being performed on six stator bars subjected to slot PDs under electrical, thermal and mechanical stresses. It is well known that slot PD activity in air-cooled generators is harmful to the stator winding ground insulation. The degradation induced by slot PDs will modify the physical properties of the surfaces of the cavity where PDs take place and this in turn will influence the slot PD process. To understand the evolution of slot PD activity and the changes in its PRPD (Phased Resolved Partial Discharge) pattern, it is therefore essential to recognize and understand the interdependence between surface modifications and PD signals. This paper presents results of visual observations of stator bar surface degradation, changes in surface conductivity and the effect on the evolution of PD signals in the presence of slot PD activity under conditions of electrical, thermal and mechanical stresses.

Ozone emission in hydro generators is a sign of surface partial discharge (PD) in the stator windings. Surface PD activity can take different forms: slot PD, corona at the junction of the slot semiconductive coating with the voltage grading coating outside the slot, PD between adjacent coils in the end-winding, PD between adjacent connections and surface tracking. An ozone concentration above the daily exposure limit, for which there is a health hazard for workers, was found at several locations in a 6-hydro generator plant. The ozone level in the vicinity of the generators was in the same range for all of them. This case study presents how this problem was identified and addressed.