ABB RELION 611 Series Technical Manual page 224

Section 4
Protection functions
218
Figure 103 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 + relay) and the other through the saturated CT (R
s
The protection relay must not operate during the saturation. This is achieved by
increasing the relay impedance by using the stabilizing resistor (R
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 U
calculations.
R
m1
R
in1
U
E
I
1
U = I x (R
E
1
GUID-137DF2DA-9768-4430-8A8E-1986CA9EFD03 V1 EN
Figure 103: Equivalent circuit in case of the CT saturation at through-fault
The CT saturation happens most likely in the case of an in-zone fault.
This is not a problem, because although the operation remains stable
(non-operative) during the saturated parts of the CT secondary current
waveform, the non-saturated part of the current waveform causes the
protection to operate.
R
m2
R
in2
R
s
E
2
d
+ R )
m2 in2
1MRS757454 D
+ R ).
m in2
) which forces the
s
is the basis of all
s
Saturated
CT
E ≈ 0
2
Technical Manual
611 series