7.1.4.1 Configuration
The configuration is done in the Application Configuration tool.
7.1.4.2 Settings of protection function
Operation: The operation of the high impedance differential function can be switched
Enabled or Disabled.
AlarmPickup: Set the alarm level. The sensitivity can roughly be calculated as a certain
percentage of the selected Trip level. A typical setting is 10% of TripPickup. This alarm
stage can be used for scheme CT supervision.
tAlarm: Set the time delay for the alarm. A typical setting is 2-3 seconds.
TripPickup: Set the trip level according to the calculations (see examples below for a
guidance). The level is selected with margin to the calculated required voltage to achieve
stability. Values can be within 20V - 400V range dependent on the application.
R series: Set the value of the used stabilizing series resistor. Calculate the value according
to the examples for each application. Adjust the resistor as close as possible to the
calculated value. Measure the value achieved and set this value for this parameter.
The value shall always be high impedance. This means for example, for
1A circuits say bigger than 400 ohms (400 VA) and for 5 A circuits say
bigger than 100 ohms (2500 VA). This ensures that the current will
circulate and not go through the differential circuit at through faults.
That the settings of U>Alarm, U>Trip and SeriesResistor must be chosen
such that both U>Alarm/SeriesResistor and U>Trip/SeriesResistor are
>4% of IRated of the used current input. Normally the settings shall also
be such that U>Alarm/SeriesResistor and U>Trip/SeriesResistor both
gives a value <4*IRated of the used current input. If not, the limitation in
how long time the actual current is allowed to persist not to overload the
current input must be considered especially during the secondary testing.
7.1.4.3 T-feeder protection
In many busbar arrangements such as breaker-and-a-half, ring breaker, mesh corner, there
will be a T-feeder from the current transformer at the breakers up to the current
transformers in the feeder circuit (for example, in the transformer bushings). It is often
required to separate the protection zones that the feeder is protected with one scheme
1MRK 506 369-UUS - Section 7
Differential protection
Line distance protection REL670 2.2 ANSI 145
Application manual
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