GE P24DM - High Impedance REF Protection Application; Setting Guidelines for High Impedance Operation

Scaling factor = K = Neutral CT ratio / Line CT ratio
This r
esults in the following differential and bias current equations:
I I I I K I
diff
A B C N
= + +
( )
+
I I I I K I
bias A B C N
= 



+
{ }
1
2
max , ,
4.2 HIGH IMPEDANCE REF PROTECTION APPLICATION
4.2.1 SETTING GUIDELINES FOR HIGH IMPEDANCE OPERATION
For this configuration, settings must be modified in the RESTRIC
TED E/F column. The REF Options setting should be
set to Hi Z REF (High Impedance REF) protection.
To protect as much of the machine winding as possible, the differential current setting IREF>Is should be set to a
low setting. A setting of 5% of the machine's rated current is adequate, but this may need to be increased where
low accuracy class CTs are used to supply the protection. Make sure the primary operating current of the element
is less than the minimum fault current for which the protection should operate.
Chapter 7 - Restricted Earth Fault Protection P24xM
176 P24xM-TM-EN-2.1