GE P24DM - Figure 87: High Impedance REF Connection

When subjected to heavy through faults the line current transformer may enter saturation unevenly, resulting in
imbalance. T
o 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
s
= I
F
(R
CT2
+ R
L3
+ R
L4
)
Stabilising resistor R
ST
= V
s
/I
s
–R
R
where:
● I
F
= maximum secondary through fault current/motor starting current
● R
R
= device burden
● R
CT
= CT secondary winding resistance
● R
L2
and R
L3
= Resistances of leads from the device to the current transformer
● R
ST
= Stabilising resistor
High Impedance REF connection is shown below:
R
ST
IED
M
E00798
Figure 87: High impedance REF connection
P24xM Chapter 7 - Restricted Earth Fault Protection
P24xM-TM-EN-2.1 171