Relevance of phase resolved PD analysis to insulation diagnosis in industrial equipment

The display of digitized PD pulses with respect to the power frequency AC cycle has been widely applied both in the laboratory measurement of PD, as well as within industrial PD measurement systems on equipment such as cables, transformers and rotating machines. Theory shows that PD will occur with a specific polarity and phase position. Furthermore, depending on the location of the PD with respect to the electrodes, the pattern will change. This well-proven phenomena obtained from experiments with single voids and defects has been applied to the industrial measurements of PD using a number of commercial PD detectors on HV apparatus, with the intention of identifying the root cause of any PD. PD data from many stator windings indicates that these basic principles are often valid. However, results from thousands of on-line PD measurements indicate that PD patterns will not always correctly predict the nature of the PD sources, as determined by visual examination of the insulation. Thus diagnosis based on pulse phase analysis in stator winding insulation systems should be done with some skepticism. We speculate that, unlike PD in most other apparatus, there tends to be hundreds of PD sites in stator windings, which may obscure the theoretical patterns.