Evidence of partial discharge activity is seen in the dusting of white power coating the surface of this stator winding. The development of severe PD activity is attributed to the original winding design, which lacked a semi-conductive coating.Partial Discharge
Problem: The issue of partial discharge (PD) is getting more attention from the owners of some of the newer air-cooled generators which have experienced failures after less than ten years of operation. PD, also known as corona, is caused by a partial voltage breakdown within the generator coil insulation. This may occur in the gaps between the coil and the stator core or in the end turns when the coils are in close proximity. Because the breakdown of the insulation system is not complete, the condition does not cause a full electrical ground. However, over time, discharges can cause deterioration of the insulating system to the degree that a full ground occurs and trips the unit offline.
Evidence of partial-discharge (PD) activity often appears as a white powder dusting the surface of the stator winding as shown in photo on this page. Severe PD activity can also totally destroy the resins and binders of slot filler materials.
Prevention: PD activity can be monitored while the machine is operating, but special equipment is required. However, the detection of PD activity alone is not enough. It is important to trend PD activity over time, because different machines have different baseline values.