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On April 23, 2007, at 1024 EDT, Unit 2 exp … On April 23, 2007, at 1024 EDT, Unit 2 experienced an automatic reactor trip from approximately 54% power during power ascension following completion of refueling outage 2R12. The main generator neutral overcurrent relay actuated resulting in a Generator/Turbine/Reactor trip. The post-trip investigation revealed that a ground fault had occurred on the main generator 'A' phase bushing. The bushing fault was a result of stator water leaking out of the tang area of the main generator 'A' phase bushing and saturating the bushing insulation. The wet insulation provided the ground fault path that caused the main generator neutral overcurrent relay to actuate.</br></br>Investigation determined that the primary cause of this event was the design of the main generator bushing tang assembly due to it being a cast copper component utilizing a braze material that is susceptible to crevice corrosion. Prior to restart, the leak was repaired in the main generator 'A' phase bushing tang assembly, a helium tracer gas test was performed on all three main generator tang assemblies, and the Generator Gas System operating procedure was revised to ensure main generator hydrogen pressure is maintained at a higher pressure than stator water.ed at a higher pressure than stator water. +
April 23, 2007 +
On April 23, 2007, at 1024 EDT, Unit 2 exp … On April 23, 2007, at 1024 EDT, Unit 2 experienced an automatic reactor trip from approximately 54% power during power ascension following completion of refueling outage 2R12. The main generator neutral overcurrent relay actuated resulting in a Generator/Turbine/Reactor trip. The post-trip investigation revealed that a ground fault had occurred on the main generator 'A' phase bushing. The bushing fault was a result of stator water leaking out of the tang area of the main generator 'A' phase bushing and saturating the bushing insulation. The wet insulation provided the ground fault path that caused the main generator neutral overcurrent relay to actuate.</br></br>Investigation determined that the primary cause of this event was the design of the main generator bushing tang assembly due to it being a cast copper component utilizing a braze material that is susceptible to crevice corrosion. Prior to restart, the leak was repaired in the main generator 'A' phase bushing tang assembly, a helium tracer gas test was performed on all three main generator tang assemblies, and the Generator Gas System operating procedure was revised to ensure main generator hydrogen pressure is maintained at a higher pressure than stator water.ed at a higher pressure than stator water. +
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April 23, 2007 +
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