The protection relay must not operate during the saturation. This is achieved by
increasing the relay impedance by using the stabilizing resistor (R
s
) which forces
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
s
is the basis of all
calculations.
I
d
R
s
R
m1
R
in1
R
in2
R
m2
E
1
U
E
2
U = I x (R
m2
+ R
in2
)
E
1
E
2
≈
0
Saturated
CT
Figure 307: Equivalent circuit in case of the CT saturation at through-fault
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 308: 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
u
) is used to limit the voltage as
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
Protection functions
1MRS757644 H
588
620 series
Technical Manual
