ABB RELION 620 Series Technical Manual page 477

1MAC504801-IB E
620 series ANSI
Technical Manual
GUID-AE532349-F4F6-4FF9-8A98-0C862162E208 V1 EN
Figure 219: Equivalent circuit when there is no fault or CT saturation
When there is no fault, the CT secondary currents and their emf voltages, E
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
Section 4
Protection functions
and E , are
1 2
≈ E
1 2
.
471