Insulation degradation analysis of stator bars subjected to slot partial discharges

As part of an extensive study on slot Partial Discharge (PD) activity, a long-term laboratory aging test has been conducted on six stator bars subjected to slot PDs. Two bars were electrically stressed, two others were electrically and thermally stressed, and the last two were electrically, thermally and mechanically stressed. The aging experiment lasted 25,000 hours, and all the bars were periodically characterized using PD measurements, dielectric response measurements, surface conductivity measurements and visual inspections. Following the aging test, microscopic observations were performed on cross-section slices taken from the slot PD sites on each of the six bars. Results confirmed that slot PD activity under a combination of electrical, thermal and mechanical stresses was the worst condition since all six slot PD sites of this group failed. This paper presents the evolution of the dielectric response measurements for every bar with respect to the stress condition and the difference in the groundwall insulation degradation.