Relative risks of end arm discharges on stator bars

Long term aging tests of stator bar end arms were carried out over two years. Six specimens affected by one of three types of defects were studied. The defects were corona activity around the junction of the armor and the end arm stress grading system, surface tracking and bar-to-bar discharges. All the specimens were submitted to an aging voltage of 20 kV AC (2.5*U₀). Phase resolved partial discharge (PRPD) and spectral analyses were performed periodically, to characterize each type of activity. After more than two years of aging, the authors noted that some types of activities were very sporadic, while others were more permanent. Three failures occurred up to now, two of them after less than 240 days of aging on specimens with corona activity at the junction. The third failure occurred on a specimen affected by bar-to-bar discharges after 680 days. Despite the fact that all failures occurred in the straight portion of the bars, the authors believe that the corona activity was, at least partially, involved in the failure mechanism on each setup