Laboratory study of slot discharge characteristic PRPD patterns

In order to reproduce typical slot discharge activity, a full height section of a stator core was assembled in the authors' laboratory. Six stator bars were installed in three of the 14 slots section. Three levels of side packing were used: tight, intermediate and loose. The radial wedging was constant with ripple springs compressed at 80%. The six bars were initially characterized without any defect of their semi-conductive coating. The bars were then removed, and defects were made by abrasion in specific areas of the coating. The bars were fitted back into the same slots, with the same tightness as before and discharge tests were performed once again. The presence of slot discharges was observed visually with an ultra-violet camera, before radial wedges were installed. Some features of the phase resolved partial discharge (PRPD) features associated with slot discharge activity were identified. For instance, some asymmetry was observed with regard to the ratio of positive and negative discharges.