Post-mortem dissection of stator bars and coils

In order to improve our understanding of failure mechanisms and their relative risk, dissection was performed on stator bars (coils) extracted from a generator after a failure. In combination with other non-destructive diagnostic tools such as partial discharge and dielectric response measurements, the information obtained from dissection reveals many symptoms which can be used to identify the root cause of the failure and to assess the progression rate of the different insulation degradation mechanisms. As the failed bar or coil is often jumped to resume operation, these key results are essential to decide on the best maintenance actions for this generator and other units of the same design to reduce further future failure risks in the plant. Dissection has been performed for years, and everyone has built his own expertise and defined criteria for what is normal or not for every aspect of the bar (coil) construction. This paper describes a procedure that relies on Hydro-Québec´s own experience in the dissection of stator bars (coils). A case study of in-service failure illustrates the correlation between dissection and other diagnostic test results. Examples of insulation degradation symptoms are presented and a quantification of some of them is proposed.