ABB 615 series Technical Manual page 562

Section 4
Protection functions
556
R
m1
R
in1
V
E
1
E
1
GUID-DDE84F0A-E45F-489B-A862-374769A886BD V1 EN
Figure 280: Equivalent circuit in case of in-zone fault
Figure 281 shows CT saturation at a through-fault, that is, out-of-zone, situation. The
magnetization impedance of a saturated CT is almost zero. The saturated CT winding can
be presented as a short circuit. When one CT is saturated, the current of the non-saturated
CT follows two paths, one through the protection relay measuring branch (R
the other through the saturated CT (R
The protection relay must not operate during the saturation. This is achieved by increasing
the relay impedance by using the stabilizing resistor (R
differential current to flow through the saturated CT. As a result, the relay operation is
avoided, that is, the relay operation is stabilized against the CT saturation at through-fault
current. The stabilizing voltage V
R
m2
R
R
s
E
I
2
d
V
+ R ).
m in2
is the basis of all calculations.
s
1MAC059074-MB A
in2
E
2
) which forces the majority of the
s
Technical Manual
+ relay) and
s
615 series ANSI