ABB RELION REX640 Technical Manual page 699

1MRS759142 C
REX640
Technical Manual
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-76439B10-74FF-4568-8D37-6F2D54334F35 V1 EN
Figure 384: 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.
GUID-6A88D048-A1B1-4E67-BD12-1CC8B9F956B0 V1 EN
Figure 385: Secondary waveform of a saturated CT
R
m2
R
in2
R
s
E
2
d
+ R )
m2 in2
Section 4
Protection functions
+ R ).
m in2
) which forces the
s
is the basis of all
s
Saturated
CT
E ≈ 0
2
693