This paper presents the justification for the approach, details and results of the Main Generator Seal Oil System reliability enhancements on the San Onofre Nuclear Generating Station, SONGS. The SONGS, Unit 3 experienced substantial turbine damage in early 2001 after the turbine bearings lubrication oil supply failed. During a loss of off-site power incident, power was lost to the two AC powered turbine lubrication oil pumps due to a breaker failure in the switchgear and the DC powered emergency bearing lubricating oil pump failed to start due to a breaker trip. The SONGS turbine generators coasted down from full speed to a full stop without lubricating oil. This resulted in significant bearing, journal and steam path damage that required a four-month duration repair outage during a time period where electricity was in short supply in the State of California. The generator hydrogen sealing system remained operable during this event, however it was recognized during the event follow up investigation that this system had vulnerabilities to failure similar to the bearing lubrication system. In order to prevent a reoccurrence of this extremely costly event, SONGS has taken actions to modify both of these critical turbine generator systems by adding additional, continuously operating pumps with a new, independent power source and independently routed cables. The main challenge was to integrate the additional equipment into the existing lubrication and seal oil systems. The lubrication Oil System was the first system to be retrofitted and these results already have been presented. Reference 2. This paper provides the result of the reliability enhancements for the Main Generator Seal Oil System, which concludes the turbine/generator critical oil systems reliability improvements, performed by SONGS. It is worth noting that the design team discovered and corrected a number of other significant operational issues, which had been present from the early days and also learned a great deal of detailed information about this vital system during the project. The SONGS approach and findings are discussed in this paper, as well as a summary of the work performed. This technical paper will be of interest to utilities with a need to improve turbine generator reliability issues.

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