Siemens 7SR18 Solkor - Page 376

7SR18 Applications Guide
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63
Trajectory of differnetial current due to
transient CT saturation. for a Through Fault on
the Feeder
DifferentialCurrent(xI
N
)
Restraint Current (x I
N
)
MAGNITUDE
COMPARITOR
STABLE
Bias Break Point Settings
0.5 1.0 2.0(defautl)
MAGNITUDE
COMPARITOR
OPERATION
CT Requirements for Various Bias Break Point Settings
B
i
a
s
S
l
o
p
e
2
=
1
5
0
%
Fault cleared by downstream
protection and CB.
Figure 4.4-1 Relay Amplitude for CT Saturation for an external Phase Fault
The above demonstrates that decreasing the Bias Break Point (B
2
) setting has the effect of lowering the Minimum
CT requirements. The reduction in Bias Break Point setting must be balanced against making sure the relay will
operate for load bias due to arc resistance and non-effectively earth systems for single-end fed internal faults.
For example if the system was resistance earthed and an earth-fault occurred on a cable at a very high load a
Bias Break Point of 0.5 may not be suitable. A typical example for setting the relay with a resistance earthed
power system is given later.