# Parameter Calculations; Figure 97: Low Impedance Ref Scaling Factor - GE MiCOM P40 Agile Technical Manual

Feeder management ied.

Chapter 7 - Restricted Earth Fault Protection
Neutral CT 200:1
V00683

### Figure 97: Low Impedance REF Scaling Factor

Another advantage of Low Impedance REF protection is that you can use a neutral CT with a lower ratio than the
line CTs in order to provide better earth fault sensitivity. In the bias calculation, the device applies a scaling factor
to the neutral current. This scaling factor is as follows:
Scaling factor = K = Neutral CT ratio / Line CT ratio
This results in the following differential and bias current equations:
I
(
I
I
=
+
+
A
B
diff
{
1
=
 
max
I
bias
2
4.2.3

## PARAMETER CALCULATIONS

Consider a solidly earthed 90 MVA transformer with a REF-protected star winding. Assume line CTS with a ratio of
400:1.
Is1 is set to 10% of the winding nominal current:
6
= (0.1 x 90 x 10
= 39 Amps primary
= 39/400 = 0.0975 Amps secondary (approx 0.1 A)
Is2 is set to the rated current of the transformer:
6
Ö 3 x 132 x 10
= 90 x 10
/ (
= 390 Amps primary
= 390/400 = 0.975 Amps secondary (approx 1 A)
Set K1 to 0% and K2 to 150%
176
Line CTs 1000:1
I
)
K I
+
C
N
}
  +
,
,
I
I
I
K I
A
B
C
N
Ö 3 x 132 x 10
3
) / (
)
3
)
I
Phase A
I
Phase B
I
Phase C
I
Neutral
IED
I
N
P14x
Phase A
Phase B
Phase C
P14xEd1-TM-EN-1  