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Chapter 7 - Restricted Earth Fault Protection
3
RESTRICTED EARTH FAULT PROTECTION IMPLEMENTATION
3.1
RESTRICTED EARTH FAULT PROTECTION SETTINGS
Restricted Earth Fault Protection is implemented in the Restricted E/F column of the relevant settings group. It is
here that the constants and bias currents are set.
The REF protection may be configured to operate as either a high impedance or biased element.
3.2
LOW IMPEDANCE REF
3.2.1
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 88: REF bias characteristic
172
)
+
I
K I
C
N
}
+
I
,
I
,
I
K I
A
B
C
N
Idiff
Operate region
K1
Is1
Is1/K1
K2
Restraint region
Is2
Ibias
P24xM
P24xM-TM-EN-2.1
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