INTRODUCTION
Partial discharge (PD) measurement is widely used for all types of high voltage equipment for both factory quality assurance and insulation system condition assessment. For the latter, the PD may be measured off-line or on-line. For motor and generator stator windings in particular, on-line PD testing has been implemented on tens of thousands of machines using a wide variety of methods. How to perform such on-line tests is described in IEC and IEEE standards [1,2]. The usefulness of on-line testing has been described in many case study papers, and in particular, reference [3] summarizes the results of about 200 PD case studies presented by machine owners where stator failures were anticipated by the test, or where the findings of the on-line PD results were confirmed by visual inspections, therefore allowing proactive maintenance.
