Evolution of Slot Partial Discharges under Electrical, Thermal and Mechanical Stresses

To improve the understanding of slot partial discharges (PD) activity in air-cooled generators, an accelerated aging test was conducted under different stress conditions on a total of six stator bars. The bars, which had been previously modified by the abrasion of a portion of their semi-conductive coating in order to create slot PD activity, were divided into three groups of two. The first group was electrically stressed at 16 kV AC while the second was both electrically and thermally stressed at 85degC by a circulating current. The last group was electrically and thermally stressed but, in addition, exposed to mechanical abrasion caused by lamination stacks vibrating on the bare insulation where slot PD were occurring. Partial discharge measurements were recorded at room temperature for each of the six bars separately as a function of aging time. Comparison of the change in the PD patterns reveals the differences in degradation when one stress or a combination of stresses is present.