PD detection in rotating machines using a portable instrument based on an antenna probe

The availability of a fully-portable Partial Discharge (FD) instrument based of an antenna probe, is quite attractive but additional degree of freedom that is, the position of the probe and the distance from the tested object, have to be considered to optimize its use. The optimum location of the antenna probe has been investigated through both laboratory and on-site experiments carried out on coils, stator windings and complete rotating machines. Both calibrated pulse signals and PD patterns were recorded at different distances and positions along the tested object and the results were compared with those obtained using a PD instrument connected with a coupling capacitor and a current transformer. It was found that the diagnostic information remains almost the same when the sensor is moved along the tested object and it is comparable with that obtained using the different sensors. It was also found that the surface discharges can be located with accuracy and the internal discharges are more evident when the antenna is positioned in correspondence of conductive the edge of the slot-stress grading.