Identification of the type of partial discharges in an operating 16kV/250 MVA generator

Fault diagnosis using partial discharge (PD) measured from operating generators is a continuing research in the past 25 years. This paper reports the diagnosed data from an operating 16 kV/ 250 MVA generator. Permanently installed capacitive couplers were used for tapping PD signals from 16 kV generator and busbar connected step-up transformer terminals. A four channel fast sampling oscilloscope was used to record the simultaneous behavior of PD signals at the selected 4 terminals. The signals were analyzed for collective PD distribution with the phase angle after filtering and phase referencing with 20 samples of 40 ms duration. Denoising was done either by wavelet, threshold and digital filtering techniques. It indicated the presence of minor surface discharges in all the 3 phases. Identification of the phase of PD origin was difficult as PD occurred simultaneously in most of the monitored terminals. Single pulse analysis indicates that the initiating pulse may be characterized by the time delay in occurrence in different phases, magnitude in time and frequency domains and various dominant frequency content. The measuring input impedance changed the sensitivity of measurement.