Limitations of magnitude guidelines for PD Measurements on Stator Windings

Partial discharge (PD) measurements have been performed on rotating machines for many decades and have proven to be a reliable and useful tool to detect potential issues in their insulation system. Ever since PD measurements were established, engineers have been looking for ways to distinguish between an asset in a good condition or a deteriorated one. Several possibilities of interpretation have been investigated and some of them are described in the relevant documents IEEE-1434 and IEC-60034-27 such as a trending over time or the phase resolved pattern. An absolute global limit of the PD magnitude in rotating machines is not within these possibilities for good reasons. However, in 2016, IEEE-3004.8 published PD-level guidelines for stator windings for 10 kV to 15 kV industrial motors. Unfortunately, very limited information is provided to explain how, and under which circumstances these guidelines should be applied. This paper presents the results from an experiment that was performed on a 12.5 kV induction motor to expose the limitations of using limits of magnitude for PD measurements on stator winding. In addition, the results of other experiments performed in the past and the influence of using different sensors are also briefly discussed. It is shown that many factors influence the PD measured quantity in a stator winding. Thus, PD-Ievel guidelines should only be used under specific circumstances and should not be applied to a wide range of high-voltage motors and generators. Therefore, the presence of these guidelines in an IEEE document is questionable.