Conference Agenda

One of the advantages of the Roebel bar type stator winding is that in-service failures of the machine winding can be repaired by replacing the failed bars with spares. On large hydro-generators, access to the core face to allow replacement of the failed bars can be accomplished by either removing 2-3 of the rotor poles or by removing the rotor from the stator. This paper will describe an unusual situation in which neither of the normal repair access options were feasible, and how the specific winding failure location and Roebel bar design allowed for a novel approach to repair the winding. The paper will describe the initial testing and inspections performed, the reasoning that led to the novel repair approach, the repair work and in-process testing, the final testing, and observations made when returning the unit to service.

Failure of the stator winding and core happens more often in service than it can be predicted during offline and online diagnostics. Failures in stator assembly can be caused by various factors mainly affecting its insulation system, and include electrical stress, mechanical stress, and temperature exposure. On large machines above 100MVA, online stator failures may result in significant equipment downtime and repair costs.

This paper outlines the methodology for effective investigation, repair and testing of the stator failures to minimize the economic losses and extend useful life of the equipment following the failure. Moreover, asset management strategy is described to assist generation owner/operator with full lifecycle management of the stator assembly.

Abstract — High Voltage Rotating Machines (HVRM), generator stator cores, are susceptible to various operational issues that require thorough investigation and analysis. This paper focuses on different problems that can occur during operation: overfluxing, overheating of the end regions and spark damage. While these issues arise from fundamentally different causes, their visual symptoms can sometimes appear similar, making accurate diagnosis essential. This paper provides a comparison of these two failure mechanisms, clarifying their distinguishing characteristics with real-world examples. The specific case of 22 kV, 200 MW generator stator core overheating is presented, including failure analysis, root cause identification, repair methodology, and post-repair evaluation. To ensure the long-term reliability of the generator, computational analysis was performed to validate the effectiveness of the repair solution. The results demonstrate that the chosen approach is both feasible and capable of extending the operational life of the machine.