I
d
R
s
R
m1
R
in1
R
in2
R
m2
E
1
V
E
2
V = I x (R
m2
+ R
in2
)
E
1
E
2
≈ 0
Saturated
CT
GUID-6F73A8DE-A7A5-4D41-8B6B-FFFC93168D8A V1 EN
Figure 281: 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 282: 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
u
) is used to limit the voltage as shown in
Figure 278.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 557
Technical Manual
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