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Coordination Procedure

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    Step 1 – The Generator Owner determines the proper undervoltage trip set point for his machine. This should be based on the manufacturer’s recommendation or protection application circumstances for the generating station.

    Step 2 – The Transmission Owner determines the local or remote backup clearing times for all transmission elements connected to the high-side bus.

    Step 3 – The Generator Owner and Transmission Owner collaboratively analyze the settings to determine if they are coordinated. The time delay of the

    undervoltage function trip must be longer than the greater of the local or remote backup clearing times for all transmission elements connected to the high-side bus,
    but not less than 10 seconds.

    Alarm Only — Preferred Method

    IEEE Std. C37.102–2006 does not recommend use of the 27 function for tripping, but only to alarm to alert operators to take necessary actions.
    Undervoltage function (27) calculation:
    27 V = 90% of Vnominal = 0.9 x 120 V = 108 V with a 10-second time delay to prevent nuisance alarms
    (per IEEE Std. C37.102–2006).

     

    Tripping Used (Not Recommended)


    CAUTION: If the Generator Owner uses the 27 function for tripping, the following conditions must be met at a minimum: Time delay of the undervoltage function trip must be longer than the greater of the local or remote backup clearing times for all transmission elements connected to the high-side bus, but not less than 10 seconds.

     

    Undervoltage function (27) calculation:


    27 V = 87% of Vnominal = 0.87 x 120 V = 104 V with a coordinated time delay

     

    Note: An 87 percent set point was chosen because the power plant is not capable of continued operation at this voltage level, and it allows for a reasonable margin for extreme system contingencies.

     

    Examples


    Proper Coordination


    If the undervoltage function is set to trip the generator, a threshold setting below 90 percent voltage at the generator terminals and an adequate time delay is necessary to allow system recovery above this level.

     

    Improper Coordination


    If the undervoltage function is set to trip the generator, a threshold setting higher than 90 percent voltage at the generator terminals and/or an inadequate time delay.


    There is no improper coordination for an alarm-only function.

     

    Reference:

    1. NERC
    2. Schneider

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