GE MiCOM P145 Agile - Restricted Earth Fault Protection Implementation; Low Impedance REF; Setting the Bias Characteristic

3 RESTRICTED EARTH FAULT PROTECTION IMPLEMENTATION
3.1 RESTRICTED EARTH FAULT PROTECTION IMPLEMENTATION
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 curr
ents 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
curr
ent r
equired 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 I KI
diff
A B C N
= + +
( )
+
The bias current is as follows:
I I I I K I
bias A B C N
= 



+
{ }
1
2
max , ,
where:
● K = Neutral CT ratio / Line CT ratio
● IN = current measur
ed by the neutral CT
The allowable range for K is:
0.05 < K < 15 for standard CTs
0.05 < K < 20 for sensitive CTs
The operate current is calculated according to the following characteristic:
P14x Chapter 7 - Restricted Earth Fault Protection
P14xEd1-TM-EN-1 171