On-line partial discharge (PD) testing has been used for over 50 years to assess the condition of motor and generator stator winding insulation systems. In the past decade, advanced hardware and software have enabled superior methods of converting the I'D data into practical information about the relative condition of the insulation, as well as determining the root causes of any insulation deterioration. This helps machine owners plan appropriate maintenance. By performing a statistical analysis of the test results in our large database, it is clear the type of measuring system, the rated voltage of the stator, as well as if the machine is air or highpressure gas cooled, has a significant impact on what I'D readings may be considered high. Tables are presented which allow, through gross comparisons to similar machines, for a single measurement on a stator to be classed as having high or low PD. In addition, a case study is presented which shows new methods to identify phase-to-phase insulation problems.

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