I
d
CT
1
R
m1
/2
R
m2
/2
R
u
R
s
R
m1
/2 R
m2
/2
R
in1
R
in2
CT
2
GUID-80DC5CFE-118C-4C5C-A15F-13DCB1708C0E V1 EN
Figure 234: High-impedance principle
The stability of the protection is based on the use of the stabilizing resistor (Rs) and
the fact that the impedance of the CT secondary quickly decreases as the CT
saturates. The magnetization reactance of a fully saturated CT goes to zero and the
impedance is formed only by the resistance of the winding (R
in
) and lead resistance
(R
m
).
The CT saturation causes a differential current which now has two paths to flow:
through the saturated CT because of the near-zero magnetizing reactance and
through the measuring branch. The stabilizing resistor is selected as such that the
current in the measuring branch is below the relay operating current during out-of-
zone faults. As a result, the operation is stable during the saturation and can still be
sensitive at the non-saturated parts of the current waveform as shown in
Figure 235.
In case of an internal fault, the fault current cannot circulate through the CTs but it
flows through the measuring branch and the protection operates. Partial CT
saturation can occur in case of an internal fault, but the non-saturated part of the
current waveform causes the protection to operate.
Saturated part Non-saturated part
I
GUID-B4CBEF48-1C9C-410B-997F-440CB10486BD V1 EN
Figure 235: Secondary waveform of a saturated CT
1YHT530004D05 D Section 4
Protection functions
615 series 435
Technical Manual
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