ABB RELION REX640 Technical Manual page 763

1MRS759142 F
Figure 434: 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
(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 435
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
Figure 435: Secondary waveform of a saturated CT
At internal fault, the secondary circuit voltage can easily exceed the isolation voltage
of the CTs, connection wires and IED. To limit this voltage, a voltage-dependent
resistor VDR is used as shown in
REX640
Technical Manual
CT
1
R
in1
R /2
m1
R
s
I
d
R /2
m1
).
.
Saturated part
R /2
m2
R
u
R /2
m2
Non-saturated part
Figure 434
.
Protection functions
CT
2
R
in2
) and lead resistance
in
763