ABB Relion 611 Series, Technical Manual

Page: 223 / 536

GUID-AE532349-F4F6-4FF9-8A98-0C862162E208 V1 EN
Figure 101: 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
GUID-CB3CACF7-A2D5-4D76-83B3-44F6AD6D2F9B V1 EN
Figure 102: Equivalent circuit in case of in-zone fault
1MRS757454 D
Section 4
Protection functions
611 series 217
Technical Manual