A case study of condition-based maintenance of a 202-MVA hydro-generator

Over the last few decades, partial discharge (PD) measurement has proven to be one of the most useful tools to identify degradation mechanisms of stator windings in air-cooled generators. Under normal operating conditions, PD levels and, more importantly, PD trending can indicate the presence of not only electrical but also mechanical, thermal or ambient (contamination) degradation processes. Hydro-Québec uses a 2D PDA (Partial Discharge Analysis) technique as its first line of diagnostics during yearly measurements on more than 120 large generators in its fleet. When a PDA measurement reveals a problem, such as high PD levels or sudden increase in level, the Phase Resolved Partial Discharge (PRPD) technique is used to improve the identification of the nature of the active PD sources. This paper presents results of on-line PD measurements from both the PDA and the PRPD techniques on the case study of a 13.8-kV 202-MVA generator. Identification of slot discharge activity in the PRPD patterns in this generator triggered a more detailed diagnostic. In-situ off-line measurement of the electrical contact between individual bars and stator core was performed on selected bars in order to confirm the presence and extent of a semi-conductive coating problem.