Figure 92: High Impedance Ref Principle - GE MiCOM P40 Agile Technical Manual

P14x
V00671

Figure 92: 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:
P14xEd1-TM-EN-1
Healthy CT
Z
m1
R
CT1
R
L1
R
L2
= I (R + R + R
s F CT2 L3
/I –R
s s R
Chapter 7 - Restricted Earth Fault Protection
Protected
circuit
I = I + I
s F
I
I
F
I
R
S
L3
R
V
ST
s
R
R
L4
)
L4
Saturated CT
A-G
Z
m2
R
CT2
169