I
d
R
s
R
m1
R
in1
R
in2
R
m2
E
1
V
E
2
V
E
1
E
2
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
s
+ relay) and
the other through the saturated CT (R
m
+ R
in2
).
The protection relay must not operate during the saturation. This is achieved by increasing
the relay impedance by using the stabilizing resistor (R
s
) which forces the majority of the
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
s
is the basis of all calculations.
Section 4 1MAC059074-MB A
Protection functions
556 615 series ANSI
Technical Manual
To Next Page
Loading...
