Application of on-line versus off-line PD testing for stator insulation monitoring

Partial discharge testing can effectively diagnose insulation problems in the stator winding of high-voltage generators and motors, since partial discharges are a symptom of most insulation failure mechanisms. Partial discharge testing can be performed either when the machine is not operating (offline) or during normal machine operation (on-line). Off-line PD testing is a well-known method to assess the stator winding insulation. Over ten years, on-line PD testing has proven to be a successful technique of monitoring stator insulation condition to forewarn plant personnel of possible machine failures and to help them implement a predictive maintenance program. This paper presents an on-line and off-line PD test on a hydro-generator using the same test instrument and sensors. A comparison of the two tests indicates that the off-line test results are consistent with the on-line test results. Similarities and differences of the on-line and off-line PD tests are discussed.