ABB RELION 620 Series Technical Manual page 479

1MAC504801-IB E
620 series ANSI
Technical Manual
R
m1
R
in1
U
E
I
1
d
U = I x (R
E
1
GUID-137DF2DA-9768-4430-8A8E-1986CA9EFD03 V1 EN
Figure 221: 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-D8F15382-5E3F-4371-B2AD-936D72941803 V1 EN
Figure 222: Secondary waveform of a saturated CT
The secondary circuit voltage can easily exceed the isolation voltage of the CTs,
connection wires and the protection relay because of the stabilizing resistance and CT
saturation. A voltage dependent resistor (VDR, R
Figure 218.
R
m2
R
in2
R
s
E
2
+ R )
m2 in2
E ≈ 0
2
) is used to limit the voltage as shown in
u
Section 4
Protection functions
Saturated
CT
473