ABB 615 Series - Page 561

The lower part of Figure 279 shows the voltage balance when there is no fault in the system
and no CT saturation.
I
d
R
s
R
m1
R
in1
R
in2
R
m2
E
1
V
E
2
V ≈ 0
E
1
E
2
GUID-FC2F69DB-84BE-4597-B6E6-21860755CC50 V1 EN
Figure 279: 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
.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 555
Technical Manual