ABB Relion 620 Series, Technical Manual

Page: 573 / 1224

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
Figure 298: 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 299
.
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
Figure 299: 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
Figure 298
.
1MRS757644 H Protection functions
620 series
Technical Manual
573