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View Diary: Tree falls on transmission line, shuts down Watts Bar Nuclear Plant (17 comments)

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  •  Eh. (7+ / 0-)

    It fell on the transmission lines; that doesn't endanger the reactor itself, or mean that the plant can't power itself.  (It does endanger the generators that're powered by the steam produced by the reactor; if they're producing power that has nowhere to go it can cause merry hell with all sorts of systems and burn them out.)  In fact -- while the article doesn't state it -- I doubt that the reactors were 'shut down' so much as they were placed in a 'standby' state since there was no reason to generate power.  

    •  I'm with Praxical on this (3+ / 0-)
      Recommended by:
      Praxical, Sandy on Signal, alain2112

      Here is a possible scenario (and a quick tutorial on how one type of protective relay works):

      Generator A feeds transmission line A, which in turn feeds substations B and C and lines B and C.  A tree falls on line B.  The ground detection relay monitoring line B senses a fault on line B, and so does the ground relay monitoring transmission line A.  Here is where it gets tricky.  If the relays are set correctly and work as intended, line B relay will trip the breaker to line B, the ground relay on transmission line A will show the fault cleared and power will continue on its merry way to substation C.  If line B relay fails to trip, either because it is faulty or because its timing is set SLOWER than transmission line A, then the line A relay will trip the transmission line.  If the transmission line trips, the generator feeding line A will suddenly have no load and go into an "overspeed" condition (the generator will have a sudden, and dangerous, increase in its rpm).  This is not the only thing that can happen, but let's keep it simple.  In most cases, the generator should shut itself down (stop) in order to protect itself.  This is likely what happened and what the tongue-tied spokesman meant when he said, "automatic shutdown procedures began".  It had little to do with the reactors, which are only a complicated means of boiling water to produce steam to turn the generators.  All steam turbine generators operate in exactly the same way, the only difference is how the hot water is produced (nuclear, gas, or coal).  

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