ABB RELION 611 Series Technical Manual page 128

Section 4
Protection functions
122
GUID-1A2BD0AD-B217-46F4-A6B4-6FC6E6256EB3 V2 EN
Figure 56: Overcurrent protection of parallel lines using directional protection
relays
In these applications, there is a possibility that the fault current can also be fed from
the LV-side up to the HV-side. Therefore, the transformer is also equipped with
directional overcurrent protection.
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Figure 57: Overcurrent protection of parallel operating transformers
Closed ring network topology
The closed ring network topology is used in applications where electricity distribution
for the consumers is secured during network fault situations. The power is fed at least
from two directions which means that the current direction can be varied. The time
grading between the network level stages is challenging without unnecessary delays
in the time settings. In this case, it is practical to use the directional overcurrent
protection relays to achieve a selective protection scheme. Directional overcurrent
functions can be used in closed ring applications. The arrows define the operating
direction of the directional functionality. The double arrows define the non-
directional functionality where faults can be detected in both directions.
1MRS757454 D
611 series
Technical Manual