ABB RELION REX640 Technical Manual page 512

Protection functions
from origin to include some margin for the admittance operation point due to
CT/VT-errors, when fault is located outside the feeder.
Conductance forward : 15 A/(15 kV/sqrt(3)) * 0.2 = +0.35 mS corresponding to 3.0
A (at 15 kV). The selected value provides margin considering also the effect of
CT/VT-errors in case of outside faults.
In case of a lower rated value of the parallel resistor, for example, 5 A (at 15 kV), the
recommended security margin should be higher, for example 0.7, so that sufficient
margin for CT/VT-errors can be achieved.
Susceptance forward
By default, this setting should be based on the minimum operate current of 1 A.
Susceptance forward : 1 A/(15 kV/sqrt(3)) = +0.1 mS
Susceptance reverse
This setting should be set based on the value of the maximum earth-fault current
produced by the feeder (considering possible feeder topology changes) with a
security margin. This ensures that the admittance operating point stays inside
the "Box"-characteristics during outside fault. The recommended security margin
should not be lower than 1.5.
Susceptance reverse : - (10 A * 1.5)/ (15 kV/sqrt(3)) = -1.73 mS
Conductance reverse
This setting is used to complete the non-directional characteristics by closing the
"Box"-characteristic. In order to keep the shape of the characteristic reasonable and
to allow sufficient margin for the admittance operating point during outside fault, it
is recommended to use the same value as for setting Susceptance reverse.
Conductance reverse = -1.73 mS
Figure 295: Admittances of the example
4.2.4.8 Signals
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1MRS759142 F
REX640
Technical Manual