Significant progress has been made in the area of PD interpretation. Interpreting stator winding PD is fundamentally different than PD interpretation from other types of HV equipment, since PD is almost always present in stator windings 3.3 kV and above. To some extent, PD can be tolerated indefinitely because of the presence of mica in the insulation system. In other types of equipment, any PD at operating voltage will eventually lead to failure, and thus one makes sure no PD is occurring in service. The question regarding rotating machine PD is what level can be withstood before one has concern about the viability of the insulation. The rotating machine PD standards do not address this. In the past decade one organization has proposed limits based on correlating PD levels with actual visual inspection of the windings.
The trend in PD magnitude over time has long been the primary method of determining if insulation maintenance is needed. However, there is now recognition that in severely deteriorated insulation, the PD will level off eventually. That is, PD will not increase indefinitely until the insulation fails.
As with PD in other types of equipment, the pattern of the PD with respect to the AC cycle may provide clues about the root cause of the PD. Researchers at IREQ have performed extensive experiments where they simulate various PD processes and observe the associated PD pattern. This approach to determining the failure processes occurring in a winding seems to be effective as long as there is a single dominant failure process. Of course research is on-going to find reliable ways of automatically determining the failure process from the PD patterns. Neural networks, time frequency maps, statistical analysis and fractal analysis are popular. However, much work remains to validate the pattern recognition a before motor and generator owners will adopt these methods.