ABB RELION REX640 Technical Manual page 778

Protection functions
The protection relay must not operate during the saturation. This is achieved by
increasing the relay impedance by using the stabilizing resistor (R
the majority of the differential current to flow through the saturated CT. As a result,
the relay operation is avoided, that is, the relay operation is stabilized against the
CT saturation at through-fault current. The stabilizing voltage U
calculations.
E
1
Figure 443: Equivalent circuit in case of the CT saturation at through-fault
Figure 444: 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
shown in
Busbar protection scheme
The basic concept for any bus differential protection relay is a direct use of
Kirchoff's first law that the sum of all currents connected to one differential
protection zone is zero. If the sum is not zero, an internal fault has occurred. In other
words, as seen by the busbar differential protection, the sum of all currents that
778
R
m1
R
in1
U
E
I
1
d
U = I x (R
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.
Figure 440
.
R
m2
R
in2
R
s
E
2
+ R )
m2 in2
E ≈ 0
2
1MRS759142 F
) which forces
s
is the basis of all
s
Saturated
CT
) is used to limit the voltage as
u
Technical Manual
REX640