Measurement strategies for PD testing and pulse propagation in stator windings

The acquisition storage and processing of partial discharge (PD) signals is widely used as a condition monitoring tool to determine the integrity of the solid insulating systems of high power plant. Although improvements have been made to the storage and processing facilities of PD instrumentation, little attention has been directed to the accuracy with which PD instrumentation capture the PD pulse, i.e. taking account of the modification which PD pulses undergo in propagating in distributed impedance systems (e.g. transformers and rotating machines) from their sites of origin to the detection terminals. Calibration strategies currently do not address this problem. Efforts have been made by various groups in recent gears to understand the PD pulse propagation mechanism in distributed systems and, in particular, in rotating machine stator windings, but comparison among results from different groups is poor. As a consequence, little progress has been made. In this paper, it is contended that this lack of consistency among the results from different groups is due to differences in experimental procedure and technique and the lack of control of critical factors. Through a series of experimental studies on a stator winding, the importance of experimental procedures to the obtained result is demonstrated and the critical factors which must be controlled to ensure an accurate and reproducible result are indicated. The relative importance of individual elements is detailed and the mechanisms/theory underlying the obtained results is explained with critical analysis