Figure 76: High Impedance Ref Principle; Figure 77: High Impedance Ref Connection - GE P642 Technical Manual

Chapter 8 - Restricted Earth Fault Protection
V00671

Figure 76: 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 77: High Impedance REF Connection

172
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
P64x
P64x-TM-EN-1.3