Figure 305: Equivalent circuit when there is no fault or CT saturation
When there is no fault, the CT secondary currents and their emf voltages, E
1
and E
2
, 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
1
≈ E
2
.
I
d
R
s
R
m1
R
in1
R
in2
R
m2
E
1
U
E
2
U
E
1
E
2
Figure 306: Equivalent circuit in case of in-zone fault
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 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
s
+ relay) and the other through the saturated CT (R
m
+ R
in2
).
1MRS757644 H
Protection functions
620 series
Technical Manual
587
