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Crack Growth Analysis Averts Failure in Pumped-Storage Hydroelectric Generator Rotors

Abstract

Hydroelectric generators in pumped-storage service experience more loading cycles than their counterparts in continuous-flow service. As a result, generator rotors in pumped-storage service are more susceptible to fatigue cracking, especially in high-stress locations such as the field-pole attachment slots. Failure of a pole attachment during operation has catastrophic consequences. Pacific Gas and Electric Company, owner, and operator of the Helms Pumped-Storage Facility in California, sought assistance in performing stress and fatigue crack growth analyses of the original Helms rotor design to assess the potential for fatigue cracking in the pole-attachment dovetails. Informed by results from a preliminary analysis, the company decided to inspect the Helms pole attachments. Inspections revealed the presence of cracking in several pole-attachment dovetail slots. Cracks were removed by local grinding as a temporary repair to allow limited additional operation while replacement rotors were fabricated. Detailed analysis of the repaired rotor was performed to evaluate remaining strength based on conservative assumptions. This paper describes analyses that allowed the company to make important and informed decisions regarding the integrity and reliability of the Helms Facility.

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Notes

  1. 1.

    NASCRAC is a trademark of Exponent, Inc.

  2. 2.

    The term margin against failure in this context is taken to be the multiple of either the normal operating speed (360 rpm) or the overspeed limit (475 rpm) that is needed to cause failure by plastic collapse. Thus, a calculated value of 1.0 would mean that there is no margin against failure by plastic collapse.

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Correspondence to Robert Sire.

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Sire, R., Ames, N. & Hanson, E. Crack Growth Analysis Averts Failure in Pumped-Storage Hydroelectric Generator Rotors. J Fail. Anal. and Preven. (2021). https://doi.org/10.1007/s11668-021-01238-z

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Keywords

  • Fatigue crack growth
  • Hydroelectric generator
  • Pumped storage