Restricted Earth Fault Implementation; Application Note; High Impedance Ref - GE P50 Agile P253 Technical Manual

6 Protection Functions
Figure 8: REF protection for star connected winding
The P253 relay uses High Impedance technique for REF. The high impedance technique ensures that
the circuit is of sufficiently high impedance such that the differential voltage that may occur under
external fault conditions is less than that required to drive setting current through the device.
2.9.1

Restricted Earth Fault Implementation

There is no dedicated setting for REF. When the P253 relay is ordered with SEF option, IN1 element
should be used for REF function.
Note: When using the REF function SEF cannot be used, and vice versa.
2.9.2

Application Note

2.9.2.1

High Impedance REF

The high impedance principle is best explained by considering a differential scheme where one CT is
saturated for an external fault, as shown below.
V00671
Figure 9: High Impedance principle
6-22
Healthy CT
Z
m1
R
CT1
R
L1
R
L2
CB
Protected
circuit
I = I + I
s F
I
I
F
I
R
S
L3
R
V
ST
s
R
R
L4
P50 Agile P253
Saturated CT
Ph-G
Z
m2
R
CT2
P253/EN M/C