Objective methods to interpret partial-discharge data on rotating-machine stator windings

Partial-discharge (PD) measurements have long been used to assess the condition of the electrical insulation in motors and generators rated 3.3 kV and above. There are many ways to measure PDs during normal service of the motor or generator. Unfortunately most of the measurement methods mix stator PD with electrical-interference signals from poor electrical connections, power tool operation, corona from transmission lines, etc. The result can be false indications of stator-winding problems, reducing confidence in PD measurements. Another issue with online PD testing is interpretation, i.e., identifying which machines are in good condition and which machines need maintenance. In the past decade, a database of over 60 000 test results has been assembled. In hundreds of machines, the condition of the insulation determined by a visual inspection has been compared to the PD levels. The result is a table that provides an objective means of determining the stator-insulation condition relative to other similar machines. An analysis of the results also shows that there are significant differences in PD activity between manufacturers. This paper gives a review of the methods that can reduce the risk of false indications, thus making the measurement less subjective.