GE P24DM - Restricted Earth Fault Protection Implementation; Restricted Earth Fault Protection Settings; Low Impedance REF; Setting the Bias Characteristic

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
her
e 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
curr
ent 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 I K I
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 = curr
ent 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:
K1
Operate region
Is1
Ibias
Idiff
V00678
Restraint region
K2
Is1/K1 Is2
Figure 88: REF bias characteristic
Chapter 7 - Restricted Earth Fault Protection P24xM
172 P24xM-TM-EN-2.1