Dissection Techniques used to Assess the Root Cause of a Hydro-Generator Stator Failure

When a premature failure occurs on a stator winding during its expected normal life, root cause analysis needs to be performed to assess the condition of the entire winding and evaluate the probability that a second failure occurs on the same machine or on other machines of the same design. The evaluation includes operation condition analysis, diagnostic tests on bars removed from the machine and on spare bars, and dissection. This paper presents a case study of a 370 MVA, 13.8 kV hydro-generator stator winding failure after 29 years of service. The paper describes the dissection techniques used to assess the insulation system degradation and to perform the root cause analysis following the phase-to-ground fault that occurred on a bottom bar. Operating conditions were evaluated which included the number of starts and stops, the stator temperature and the operating power through the years. Also, diagnostic tests were performed on the bars removed from the machine and compared to diagnostic tests performed on the spare bars.