The need for PD quantification based on the type of discharge sources

Over the past 25 years, Hydro-Quebec has built a significant Partial Discharge (PD) database including PDA and PRPD measurements of hydro-generators. The analysis of the database suggests that quantification with respect to the type of PD sources is essential to better evaluate the risk associated with each type of discharge activity. For instance, at given amplitude, internal PD activity will not represent the same risk than slot PD activity. Obviously, many parameters must be taken into account to do such quantification and determine specific limit per discharge type. The frequency response of the measurement instrument and sensors as well as signals propagation, attenuation and degradation rate which are all different depending on the active PD sources, make such an analysis not trivial. In order to understand the contribution of those parameters, field measurements on actual generators have to be analyzed in conjunction with laboratory studies. To that end, a laboratory test set-up has been put together in order to specifically evaluate the attenuation of gap type discharges and of corona discharges at the junction of the semi-conducting and stress grading coatings. This paper presents the laboratory results obtained for these two types of PD sources and compare them with PD activity obtained from actual field measurements.