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MEOL Oral Prep Questions

Discussion in 'Technical Discussion' started by Globetrotter76, May 13, 2010.

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  1. Globetrotter76

    Globetrotter76 New Member

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    Oct 20, 2004
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    Location:
    Cape Town - South Africa
    Hi Ya Folks,

    I am prepping for my MEOL Oral and am at the Electrical Chapter and would like some advice from you more Experienced Engineers out there. I have the notes/done the Oral Prep Course and googling like crazy, but just need clarification on a few things.

    My First Question is on the Reverse Power Trip.

    (I know that this can only happen when Generators are synchronized and running in Parallel and that the Reverse Power Trip is a Safety Device)

    From what I can gather it will trip the breaker at the Main Busbar on the generator that is shutting down unexpectedly due to High Coolant Temp/Low Oil Pressure or Fuel Starvation or other reasons. This is safety trip due to the Generators abilty to start acting like a motor and drawing current from the Switchboard which can cause damage to the prime mover and Generator itself which would possibly overload and knock off the second generator running in parallel and blacking out the entire ship) Please correct me if I am wrong??

    How does one test Reverse Power Trip onboard your vessel? What would you do if the trip failed to work? Is this test in your safety checklists?

    Could I just close the battery switch to activate the fuel shutoff solenoid??

    Thanks

    Globe
  2. C4ENG

    C4ENG Senior Member

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    Location:
    Ft Lauderdale
    I think you have a good understanding of the reverse trip.

    The way I have tested reverse trip in the past is to go to the breaker of the generator that is not on line and not running. Try manually flipping the non running gen breaker in the ON position, just as if you where trying to parrel it to the main live bus. The breaker should trip as the power from the live main bus is trying to rush into the breaker from the wrong direction. If the breaker does not trip, then immediately shut it off and figure out what the problem is.

    I have done this test with the ABS surveyors for vessel inspections. I would be curious to know other ways the reverse trip might be tested,, if there is any.
  3. Globetrotter76

    Globetrotter76 New Member

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    Thanks there C4ENG, How about when you are running them both online and Synced and operate the Switch(Governor Control) and unload the the Kw(Load) on the Genset and as it unloads the other generator will start to take the load and as the unloading generator starts going into reverse power it will eventually trip out on Reverse Power Trip(That is if it is working)

    What you say does make sense but like you said it would be nice to hear more folks experiences on this. Also if the Trip Failed, I would manually open up the circuit breaker taking it off the Bus Bar and then get the electricians in to resolve the issue.

    I would like to extend this topic further.

    I am all clear on Synchronising two Gensets using Syncroscope and all but what safety is there to prevent a person manually closing the breaker of incoming Genset 180degree's out of phase????(Of course there is one as paralleling out of phase would could serious damage to Generators and Prime Movers as they tried to compensate and Torque to get in Phase.

    I will keep extending this topic as I go and hope that other can learn from this as I learn as well.

    Thanks

    Globe
  4. Globetrotter76

    Globetrotter76 New Member

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    I do not want to Confuse things here but what you say C4ENG sounds like the following which I got directly from the notes in this chapter.

    "Under-voltage Trip

    Closure by mistake of an alternator breaker when the machine is dead is prevented by an under-voltage trip. This protective measure is fitted when alternators are arranged for parallel operation. Instantaneous operation of the trip is necessary to prevent closure of the breaker. However, an under-voltage trip also gives protection against loss of voltage while the machine is connected to the switchboard. Tripping in this case must be delayed for discrimination purposes, so that if the volt drop is caused by a fault then time is allowed for the appropriate fuse or breaker to operate and the voltage to receiver without loss of the power supply."
  5. FullaFlava

    FullaFlava New Member

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    Dec 4, 2009
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    Location:
    Fort Lauderdale
    Hiya,

    To test the reverse power trip on our load sharing modules (GAC LSM672) we reverse the polarity of the sensing circuit to the module and apply a normal load to simulate reverse power conditions. Adjustments can be made to the sensitivity at this time and on completion the polarity is returned to normal.

    To answer how do you prevent closing the supply breaker 180 degrees out of phase manually? It is done by training at the stage you are at; as far as I am aware there is no way to prevent inadvertant manual operation.
  6. Globetrotter76

    Globetrotter76 New Member

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    Okay I passed, These were just the things I needed clarification on.

    Anyways See you around

    Globe76
  7. gareth.griggs

    gareth.griggs New Member

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    Re:Globetrotter, Fullaflava
    Regarding a safety mech to prevent the operator manually bringing an incoming alternator online 180 degrees out of synch,
    There is a device called a 'check synchroniser' which uses an electronic circuit to compare the live busbars voltage with the phase angle, speed and voltage of the incoming alternator and if the check synch recognises the incoming phase angle and voltage are within acceptable limits the mech closes a switch pre the main breaker and busbar. The closing of the incoming alternator is still manually controlled by the operator however will trip if incorrect voltage or out of phase (due to check sync) but will stay closed if check sync is closed. safeguarding human error and protecting prime mover and alt.
    if the alternator is to run on a 'dead board' the check synch shall be turned off.