ABB RELION REX640 Technical Manual page 698

Section 4
Protection functions
692
GUID-1FE64A73-2018-487B-AAE4-837085F24127 V1 EN
Figure 382: Equivalent circuit when there is no fault or CT saturation
When there is no fault, the CT secondary currents and their emf voltages, E
are opposite and the protection relay measuring branch has no voltage or current. If an
in-zone fault occurs, the secondary currents have the same direction. The relay
measures the sum of the currents as a differential and trips the circuit breaker. If the
fault current goes through only one CT, its secondary emf magnetizes the opposite CT,
that is, E ≈ E .
1 2
R
m1
R
in1
U
E
1
E
1
GUID-354E45A8-D116-4536-BD44-49162ECA1E65 V1 EN
Figure 383: Equivalent circuit in case of in-zone fault
Figure 384 shows CT saturation at a through-fault, that is, out-of-zone, situation. The
magnetization impedance of a saturated CT is almost zero. The saturated CT winding
R
m2
R
R
s
E
I
2
d
U
1MRS759142 C
in2
E
2
Technical Manual
and E ,
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REX640