ABB RELION REX640 Technical Manual page 732

Section 4
Protection functions
726
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Figure 404: High-impedance differential protection with different CT burden value
on each branch
The stabilizing voltage, that is the voltage appearing across the measuring branch
during an out-of-zone fault, is calculated assuming that one of the CTs connected in
parallel is fully saturated. The stabilizing voltage can be calculated using
166.
I
k max
U R R
( )
= ⋅ +
s in m
n
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I The highest through-fault current in primary Amps. The highest earth fault or short circuit current
kmax
during out-of-zone fault
n The turns ratio of the CT
R The secondary winding resistance of CT in ohms
in
R The resistance (maximum of R
m
The current transformers must be able to force enough current to operate the relay
through the differential circuit during a fault condition inside the protection zone. To
ensure this, the knee point voltage U
stabilizing voltage U .
s
The required knee point voltage U
Equation 167.
+ R ) of the CT secondary circuit in ohms
in m
should be at least twice higher than the
kn
of the current transformer is calculated using
kn
1MRS759142 C
Equation
(Equation 166)
REX640
Technical Manual