ABB RELION 620 Series Technical Manual page 568

Section 4
Protection functions
562
CT
1
R
in1
R /2
m1
R
R /2
m1
GUID-80DC5CFE-118C-4C5C-A15F-13DCB1708C0E V1 EN
Figure 288: 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
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
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.
I
Saturated part
GUID-B4CBEF48-1C9C-410B-997F-440CB10486BD V1 EN
Figure 289: Secondary waveform of a saturated CT
R /2
m2
s R
u
I
d
R /2
m2
Non-saturated part
1MRS757644 F
CT
2
R
in2
) and lead resistance (R
in
Figure 289.
Technical Manual
).
m
620 series