How to identify stator insulation problems using on-line partial discharge analysis

On-line partial discharge (PD) testing of the stator winding of hydro generators, turbine generators, and large motors has been performed for half a century. However, questions remain on how to best use PD data to identify stator insulation problems. This paper analyses why one cannot rely on an absolute PD magnitude to assess the insulation condition of rotating machines. Some of the factors which affect the diagnosis of stator insulation problems include the following: 1) PD calibration problems in rotating machines; 2) Bandwidth of the PD detector; 3) PD types and locations; 4) Differences among machines and PD measurements. The paper describes how these factors influence the assessment of stator insulation condition. PD trend analysis is recommended as the best way to identify machine insulation problems and is well demonstrated by some case studies. Precautions in using a PD database to assess the likelihood of insulation failures are given. The paper concludes that a significant increase in PD activity within a certain period of time is a sign of severe insulation deterioration.