Chapter 7 - Restricted Earth Fault Protection
V00671
Figure 77: High Impedance REF principle
When subjected to heavy through faults the line current transformer may enter saturation unevenly, resulting in
imbalance. To ensure stability under these conditions a series connected external resistor is required, so that most
of the unbalanced current will flow through the saturated CT. As a result, the current flowing through the device
will be less than the setting, therefore maintaining stability during external faults.
Voltage across REF element V
Stabilising resistor R
= V
ST
where:
I
= maximum secondary through fault current
●
F
R
= device burden
●
R
R
= CT secondary winding resistance
●
CT
R
and R
= Resistances of leads from the device to the current transformer
●
L2
L3
R
= Stabilising resistor
●
ST
High Impedance REF can be used for either delta windings or star windings in both solidly grounded and
resistance grounded systems. The connection to a modern IED are as follows:
V00680
Figure 78: High Impedance REF Connection
150
Healthy CT
Z
m1
R
CT1
R
L1
R
L2
= I
(R
+ R
+ R
s
F
CT2
L3
/I
–R
s
s
R
I
Phase A
I
Phase B
I
Phase C
R
STAB
I
Neutral
IED
Connecting IED to star winding for High
Impedance REF
Protected
circuit
I = I
+ I
s
F
R
CT2
I
I
F
I
R
S
L3
R
V
ST
s
R
R
L4
)
L4
Phase A
Phase B
Phase C
Connecting IED to delta winding for High
Saturated CT
A-G
Z
m2
Phase A
Phase B
Phase C
I
Neutral
R
STAB
IED
Impedance REF
P14D
P14D-TM-EN-8