On-line measurement of partial discharges on large motors in a generating station

For several decades, off-line and/or on-line partial discharge (PD) measurements have been used for condition assessment of stator insulation systems in motors and generators. Many large critical motors in fossil and nuclear generating stations have surge capacitors installed on their phase terminals. On-line PD measurements on eight such motors were done without contact with high voltage terminals in Ontario Power Generation (OPG, earlier Ontario Hydro) by installing high (or radio) frequency current transformers (HFCT or RFCT) and line frequency current transformers (LFCT) on the grounding leads of the surge capacitors. The PD and line frequency voltage signals were picked from the HFCT and the LFCT, respectively. Passive filters were used to eliminate interference from high magnitude pulses generated by exciters in the generating station. A commercial instrument analyzed the PD pulses in terms of magnitude and relationship to the phase of the line voltage. Measurements have been repeated several times. This paper presents the the results obtained so far. Separate off-line tests on a similar motor showed that the HFCT method is at least as sensitive as the conventional OPG method, which uses a capacitive coupler, a wide-band filter, and an oscilloscope