ABB RELION 611 Series Applications Manual page 64

Section 4
Requirements for measurement transformers
58
Phase A
Phase B
Phase C
P
2
P
1
Outgoing feeder
GUID-125D0534-32F1-4E09-8A34-CBF36D547735 V2 EN
Figure 54: 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
kmax
during the out-of-zone fault.
n the turns ratio of the CT
R the secondary winding resistance of the CT in ohms
in
R the total resistance of the secondary circuit wires in ohms
m
Incoming feeder
P S
1
1
R
in1
P S
2 2
S
R R /2
2
in2 m2
S
1
I
d
R /2
m2
3x required for
three phases
1MRS757455 E
Busbar
R = 1.2Ω
in1
R = 0.8Ω
in2
R = 2.0Ω
m1
R = 1.9Ω
m2
R R = 3.2Ω
in1 + m1
R R = 2.7Ω
in2 + m2
R
u
(Equation 1)
Application Manual
REB611