ABB RELION 620 Series Technical Manual page 485

1MAC504801-IB E
620 series ANSI
Technical Manual
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Figure 227: High-impedance busbar differential protection with different CT burden
value on each feeder
First, the stabilizing voltage, that is, the voltage appearing across the measuring branch
during the 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 the formula:
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 during
kmax
the out-of-zone fault.
n the turns ratio of the CT
R the secondary winding resistance of the CT in ohms
in
Section 4
Protection functions
(Equation 55)
479