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Chapter 8 - Restricted Earth Fault Protection
If a CT is assigned to more than one winding, then an alarm is issued (CT Selection Alm). When this DDB signal is
asserted, the protection is also blocked.
3.3
LOW IMPEDANCE REF
3.3.1
SETTING THE BIAS CHARACTERISTIC
Low impedance REF uses a bias charactersitic for increasing sensitivity and stabilising for through faults. The
current required to trip the differential IED is called the Operate current. This Operate current is a function of the
differential current and the bias current according to the bias characteristic.
The differential current is defined as follows:
I
I
I
(
=
+
+
A
B
diff
The bias current is as follows:
1
{
I
max
=
bias
2
where:
K = Neutral CT ratio / Line CT ratio
●
IN = current measured by the neutral CT
●
The allowable range for K is:
0.05 < K < 15 for standard CTs
The operate current is calculated according to the following characteristic:
V00678
Figure 78: REF bias characteristic
The following settings are provided to define this bias characteristic:
IREF> Is1: sets the minimum trip threshold
●
IREF> Is2: sets the bias current kneepoint whereby the required trip current starts increasing
●
IREF> k1: defines the first slope (often set to 0%)
●
●
IREF> k2: defines the second slope
174
I
K I
)
+
C
N
}
I
,
I
,
I
K I
+
A
B
C
N
Idiff
Operate region
Is1
Is1/K1
K2
K1
Restraint region
Is2
Ibias
P64x
P64x-TM-EN-1.3
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