ABB RELION REX640 Technical Manual page 777

1MRS759142 F
Figure 441: 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.
2
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
Figure 442: Equivalent circuit in case of in-zone fault
Figure 443
The magnetization impedance of a saturated CT is almost zero. The saturated
CT winding can be presented as a short circuit. When one CT is saturated, the
current of the non-saturated CT follows two paths, one through the protection relay
measuring branch (R
REX640
Technical Manual
≈ E .
1 2
R
m1
R
in1
U
E
I
1
d
U
shows CT saturation at a through-fault, that is, out-of-zone, situation.
+ relay) and the other through the saturated CT (R
s
R
m2
R
in2
R
s
E
2
Protection functions
E
2
and E
1
+ R ).
m in2
777